Switch statement fallthrough in C

Question

Switch statement fallthrough is one of my personal major reasons for loving switch vs  if else if constructs  An example is in order here   static string NumberToWords int number        string   numbers   new string                   one    two    three    four    five               six    seven    eight    nine         string   tens   new string                       twenty    thirty    forty    fifty               sixty    seventy    eighty    ninety         string   teens   new string              ten    eleven    twelve    thirteen    fourteen    fifteen              sixteen    seventeen    eighteen    nineteen          string ans           switch  number ToString   Length                case 3              ans    string Format   0  hundred and    numbers number   100            case 2              int t    number   10    10              if  t    1                                ans    teens number   10                   break                            else if  t  gt  1                  ans    string Format   0 -   tens t            case 1              int o   number   10              ans    numbers o                break          default              throw new ArgumentException  number              return ans      The smart people are cringing because the string  s should be declared outside the function  well  they are  this is just an example   The compiler fails with the following error    Control cannot fall through from one case label   case 3    to another Control cannot fall through from one case label   case 2    to another   Why  And is there any way to get this sort of behaviour without having three ifs

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Switch fallthrough is historically one of the major source of bugs in modern softwares  The language designer decided to make it mandatory to jump at the end of the case  unless you are defaulting to the next case directly without processing   switch value        case 1    this is still legal     case 2

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You can  goto case label  http   www blackwasp co uk CSharpGoto aspx     The goto statement is a simple command that unconditionally transfers the control of the program to another statement  The command is often criticised with some developers advocating its removal from all high-level programming languages because it can lead to spaghetti code  This occurs when there are so many goto statements or similar jump statements that the code becomes difficult to read and maintain  However  there are programmers who point out that the goto statement  when used carefully  provides an elegant solution to some problems

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You can  goto case label  http   www blackwasp co uk CSharpGoto aspx     The goto statement is a simple command that unconditionally transfers the control of the program to another statement  The command is often criticised with some developers advocating its removal from all high-level programming languages because it can lead to spaghetti code  This occurs when there are so many goto statements or similar jump statements that the code becomes difficult to read and maintain  However  there are programmers who point out that the goto statement  when used carefully  provides an elegant solution to some problems

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Just a quick note to add that the compiler for Xamarin actually got this wrong and it allows fallthrough  It has supposedly been fixed  but has not been released  Discovered this in some code that actually was falling through and the compiler did not complain

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They changed the switch statement  from C Java C    behavior for c    I guess the reasoning was that people forgot about the fall through and errors were caused   One book I read said to use goto to simulate  but this doesn t sound like a good solution to me

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Copy paste of an answer I provided elsewhere   Falling through switch-cases can be achieved by having no code in a case  see case 0   or using the special goto case  see case 1  or goto default  see case 2  forms   switch                 case 0     shares the exact same code as case 1     case 1             do something         goto case 2      case 2             do something else         goto default      default             do something entirely different         break

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Just a quick note to add that the compiler for Xamarin actually got this wrong and it allows fallthrough  It has supposedly been fixed  but has not been released  Discovered this in some code that actually was falling through and the compiler did not complain

User · Answer

They changed the switch statement  from C Java C    behavior for c    I guess the reasoning was that people forgot about the fall through and errors were caused   One book I read said to use goto to simulate  but this doesn t sound like a good solution to me

User · Answer

Copy paste of an answer I provided elsewhere   Falling through switch-cases can be achieved by having no code in a case  see case 0   or using the special goto case  see case 1  or goto default  see case 2  forms   switch                 case 0     shares the exact same code as case 1     case 1             do something         goto case 2      case 2             do something else         goto default      default             do something entirely different         break

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After each case statement require break or goto statement even if it is a default case

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Switch fallthrough is historically one of the major source of bugs in modern softwares  The language designer decided to make it mandatory to jump at the end of the case  unless you are defaulting to the next case directly without processing   switch value        case 1    this is still legal     case 2

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The  why  is to avoid accidental fall-through  for which I m grateful  This is a not uncommon source of bugs in C and Java   The workaround is to use goto  e g   switch  number ToString   Length        case 3          ans    string Format   0  hundred and    numbers number   100            goto case 2      case 2         Etc     The general design of switch case is a little bit unfortunate in my view  It stuck too close to C - there are some useful changes which could be made in terms of scoping etc  Arguably a smarter switch which could do pattern matching etc would be helpful  but that s really changing from switch to  check a sequence of conditions  - at which point a different name would perhaps be called for

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You forgot to add the  break   statement into case 3  In case 2 you wrote it into the if block  Therefore try this   case 3                    ans    string Format   0  hundred and    numbers number   100        break      case 2                    int t    number   10    10                  if  t    1                                            ans    teens number   10                                         else if  t  gt  1                                ans    string Format   0 -   tens t                      break     case 1                    int o   number   10                  ans    numbers o                   break             default                    throw new ArgumentException  number

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After each case statement require break or goto statement even if it is a default case

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You forgot to add the  break   statement into case 3  In case 2 you wrote it into the if block  Therefore try this   case 3                    ans    string Format   0  hundred and    numbers number   100        break      case 2                    int t    number   10    10                  if  t    1                                            ans    teens number   10                                         else if  t  gt  1                                ans    string Format   0 -   tens t                      break     case 1                    int o   number   10                  ans    numbers o                   break             default                    throw new ArgumentException  number

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A jump statement such as a break is   required after each case block    including the last block whether it is   a case statement or a default   statement  With one exception   unlike   the C   switch statement   C  does not   support an implicit fall through from   one case label to another  The one   exception is if a case statement has   no code    -- C  switch   documentation

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Copy paste of an answer I provided elsewhere   Falling through switch-cases can be achieved by having no code in a case  see case 0   or using the special goto case  see case 1  or goto default  see case 2  forms   switch                 case 0     shares the exact same code as case 1     case 1             do something         goto case 2      case 2             do something else         goto default      default             do something entirely different         break

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They left out this behaviour by design to avoid when it was not used by will but caused problems   It can be used only if there is no statement in the case part  like   switch  whatever        case 1      case 2      case 3  boo  break

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A jump statement such as a break is   required after each case block    including the last block whether it is   a case statement or a default   statement  With one exception   unlike   the C   switch statement   C  does not   support an implicit fall through from   one case label to another  The one   exception is if a case statement has   no code    -- C  switch   documentation

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They left out this behaviour by design to avoid when it was not used by will but caused problems   It can be used only if there is no statement in the case part  like   switch  whatever        case 1      case 2      case 3  boo  break

User · Answer

The  why  is to avoid accidental fall-through  for which I m grateful  This is a not uncommon source of bugs in C and Java   The workaround is to use goto  e g   switch  number ToString   Length        case 3          ans    string Format   0  hundred and    numbers number   100            goto case 2      case 2         Etc     The general design of switch case is a little bit unfortunate in my view  It stuck too close to C - there are some useful changes which could be made in terms of scoping etc  Arguably a smarter switch which could do pattern matching etc would be helpful  but that s really changing from switch to  check a sequence of conditions  - at which point a different name would perhaps be called for

User · Answer

Switch fallthrough is historically one of the major source of bugs in modern softwares  The language designer decided to make it mandatory to jump at the end of the case  unless you are defaulting to the next case directly without processing   switch value        case 1    this is still legal     case 2

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switch  C  Reference  says     C  requires the end of switch sections  including the final one     So you also need to add a break  to your default section  otherwise there will still will be a compiler error

User · Answer

Switch fallthrough is historically one of the major source of bugs in modern softwares  The language designer decided to make it mandatory to jump at the end of the case  unless you are defaulting to the next case directly without processing   switch value        case 1    this is still legal     case 2

User · Answer

Copy paste of an answer I provided elsewhere   Falling through switch-cases can be achieved by having no code in a case  see case 0   or using the special goto case  see case 1  or goto default  see case 2  forms   switch                 case 0     shares the exact same code as case 1     case 1             do something         goto case 2      case 2             do something else         goto default      default             do something entirely different         break

User · Answer

You can  goto case label  http   www blackwasp co uk CSharpGoto aspx     The goto statement is a simple command that unconditionally transfers the control of the program to another statement  The command is often criticised with some developers advocating its removal from all high-level programming languages because it can lead to spaghetti code  This occurs when there are so many goto statements or similar jump statements that the code becomes difficult to read and maintain  However  there are programmers who point out that the goto statement  when used carefully  provides an elegant solution to some problems

User · Answer

You can  goto case label  http   www blackwasp co uk CSharpGoto aspx     The goto statement is a simple command that unconditionally transfers the control of the program to another statement  The command is often criticised with some developers advocating its removal from all high-level programming languages because it can lead to spaghetti code  This occurs when there are so many goto statements or similar jump statements that the code becomes difficult to read and maintain  However  there are programmers who point out that the goto statement  when used carefully  provides an elegant solution to some problems

User · Answer

They changed the switch statement  from C Java C    behavior for c    I guess the reasoning was that people forgot about the fall through and errors were caused   One book I read said to use goto to simulate  but this doesn t sound like a good solution to me

User · Answer

The  why  is to avoid accidental fall-through  for which I m grateful  This is a not uncommon source of bugs in C and Java   The workaround is to use goto  e g   switch  number ToString   Length        case 3          ans    string Format   0  hundred and    numbers number   100            goto case 2      case 2         Etc     The general design of switch case is a little bit unfortunate in my view  It stuck too close to C - there are some useful changes which could be made in terms of scoping etc  Arguably a smarter switch which could do pattern matching etc would be helpful  but that s really changing from switch to  check a sequence of conditions  - at which point a different name would perhaps be called for

User · Answer

You can achieve fall through like c   by the goto keyword   EX   switch num       case 1        goto case 3     case 2        goto case 3     case 3          do something       break     case 4          do something else       break     case default        break

User · Answer

A jump statement such as a break is   required after each case block    including the last block whether it is   a case statement or a default   statement  With one exception   unlike   the C   switch statement   C  does not   support an implicit fall through from   one case label to another  The one   exception is if a case statement has   no code    -- C  switch   documentation

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To add to the answers here  I think it s worth considering the opposite question in conjunction with this  viz  why did C allow fall-through in the first place  Any programming language of course serves two goals   Provide instructions to the computer  Leave a record of the intentions of the programmer   The creation of any programming language is therefore a balance between how to best serve these two goals  On the one hand  the easier it is to turn into computer instructions  whether those are machine code  bytecode like IL  or the instructions are interpreted on execution  then more able that process of compilation or interpretation will be to be efficient  reliable and compact in output  Taken to its extreme  this goal results in our just writing in assembly  IL  or even raw op-codes  because the easiest compilation is where there is no compilation at all  Conversely  the more the language expresses the intention of the programmer  rather than the means taken to that end  the more understandable the program both when writing and during maintenance  Now  switch could always have been compiled by converting it into the equivalent chain of if-else blocks or similar  but it was designed as allowing compilation into a particular common assembly pattern where one takes a value  computes an offset from it  whether by looking up a table indexed by a perfect hash of the value  or by actual arithmetic on the value    It s worth noting at this point that today  C  compilation will sometimes turn switch into the equivalent if-else  and sometimes use a hash-based jump approach  and likewise with C  C    and other languages with comparable syntax   In this case there are two good reasons for allowing fall-through   It just happens naturally anyway  if you build a jump table into a set of instructions  and one of the earlier batches of instructions doesn t contain some sort of jump or return  then execution will just naturally progress into the next batch  Allowing fall-through was what would  quot just happen quot  if you turned the switch-using C into jump-table   using machine code   Coders who wrote in assembly were already used to the equivalent  when writing a jump table by hand in assembly  they would have to consider whether a given block of code would end with a return  a jump outside of the table  or just continue on to the next block  As such  having the coder add an explicit break when necessary was  quot natural quot  for the coder too    At the time therefore  it was a reasonable attempt to balance the two goals of a computer language as it relates to both the produced machine code  and the expressiveness of the source code  Four decades later though  things are not quite the same  for a few reasons   Coders in C today may have little or no assembly experience  Coders in many other C-style languages are even less likely to  especially Javascript    Any concept of  quot what people are used to from assembly quot  is no longer relevant  Improvements in optimisations mean that the likelihood of switch either being turned into if-else because it was deemed the approach likely to be most efficient  or else turned into a particularly esoteric variant of the jump-table approach are higher  The mapping between the higher- and lower-level approaches is not as strong as it once was  Experience has shown that fall-through tends to be the minority case rather than the norm  a study of Sun s compiler found 3  of switch blocks used a fall-through other than multiple labels on the same block  and it was thought that the use-case here meant that this 3  was in fact much higher than normal   So the language as studied make the unusual more readily catered-to than the common  Experience has shown that fall-through tends to be the source of problems both in cases where it is accidentally done  and also in cases where correct fall-through is missed by someone maintaining the code  This latter is a subtle addition to the bugs associated with fall-through  because even if your code is perfectly bug-free  your fall-through can still cause problems   Related to those last two points  consider the following quote from the current edition of K amp R   Falling through from one case to another is not robust  being prone to disintegration when the program is modified  With the exception of multiple labels for a single computation  fall-throughs should be used sparingly  and commented  As a matter of good form  put a break after the last case  the default here  even though it s logically unnecessary  Some day when another case gets added at the end  this bit of defensive programming will save you   So  from the horse s mouth  fall-through in C is problematic  It s considered good practice to always document fall-throughs with comments  which is an application of the general principle that one should document where one does something unusual  because that s what will trip later examination of the code and or make your code look like it has a novice s bug in it when it is in fact correct  And when you think about it  code like this  switch x      case 1     foo          FALLTHRU      case 2      bar        break     Is adding something to make the fall-through explicit in the code  it s just not something that can be detected  or whose absence can be detected  by the compiler  As such  the fact that on has to be explicit with fall-through in C  doesn t add any penalty to people who wrote well in other C-style languages anyway  since they would already be explicit in their fall-throughs     Finally  the use of goto here is already a norm from C and other such languages  switch x      case 0    case 1    case 2      foo        goto below six    case 3      bar        goto below six    case 4      baz           FALLTHRU      case 5    below six      qux        break    default      quux       In this sort of case where we want a block to be included in the code executed for a value other than just that which brings one to the preceding block  then we re already having to use goto   Of course  there are means and ways of avoiding this with different conditionals but that s true of just about everything relating to this question   As such C  built on the already normal way to deal with one situation where we want to hit more than one block of code in a switch  and just generalised it to cover fall-through as well  It also made both cases more convenient and self-documenting  since we have to add a new label in C but can use the case as a label in C   In C  we can get rid of the below six label and use goto case 5 which is clearer as to what we are doing   We d also have to add break for the default  which I left out just to make the above C code clearly not C  code   In summary therefore   C  no longer relates to unoptimised compiler output as directly as C code did 40 years ago  nor does C these days   which makes one of the inspirations of fall-through irrelevant  C  remains compatible with C in not just having implicit break  for easier learning of the language by those familiar with similar languages  and easier porting  C  removes a possible source of bugs or misunderstood code that has been well-documented as causing problems for the last four decades  C  makes existing best-practice with C  document fall through  enforceable by the compiler  C  makes the unusual case the one with more explicit code  the usual case the one with the code one just writes automatically  C  uses the same goto-based approach for hitting the same block from different case labels as is used in C  It just generalises it to some other cases  C  makes that goto-based approach more convenient  and clearer  than it is in C  by allowing case statements to act as labels   All in all  a pretty reasonable design decision   Some forms of BASIC would allow one to do the likes of GOTO  x AND 7    50   240 which while brittle and hence a particularly persuasive case for banning goto  does serve to show a higher-language equivalent of the sort of way that lower-level code can make a jump based on arithmetic upon a value  which is much more reasonable when it s the result of compilation rather than something that has to be maintained manually  Implementations of Duff s Device in particular lend themselves well to the equivalent machine code or IL because each block of instructions will often be the same length without needing the addition of nop fillers     Duff s Device comes up here again  as a reasonable exception  The fact that with that and similar patterns there s a repetition of operations serves to make the use of fall-through relatively clear even without an explicit comment to that effect

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You can achieve fall through like c   by the goto keyword   EX   switch num       case 1        goto case 3     case 2        goto case 3     case 3          do something       break     case 4          do something else       break     case default        break

User · Answer

The  why  is to avoid accidental fall-through  for which I m grateful  This is a not uncommon source of bugs in C and Java   The workaround is to use goto  e g   switch  number ToString   Length        case 3          ans    string Format   0  hundred and    numbers number   100            goto case 2      case 2         Etc     The general design of switch case is a little bit unfortunate in my view  It stuck too close to C - there are some useful changes which could be made in terms of scoping etc  Arguably a smarter switch which could do pattern matching etc would be helpful  but that s really changing from switch to  check a sequence of conditions  - at which point a different name would perhaps be called for

User · Answer

A jump statement such as a break is   required after each case block    including the last block whether it is   a case statement or a default   statement  With one exception   unlike   the C   switch statement   C  does not   support an implicit fall through from   one case label to another  The one   exception is if a case statement has   no code    -- C  switch   documentation

User · Answer

You forgot to add the  break   statement into case 3  In case 2 you wrote it into the if block  Therefore try this   case 3                    ans    string Format   0  hundred and    numbers number   100        break      case 2                    int t    number   10    10                  if  t    1                                            ans    teens number   10                                         else if  t  gt  1                                ans    string Format   0 -   tens t                      break     case 1                    int o   number   10                  ans    numbers o                   break             default                    throw new ArgumentException  number

User · Answer

They left out this behaviour by design to avoid when it was not used by will but caused problems   It can be used only if there is no statement in the case part  like   switch  whatever        case 1      case 2      case 3  boo  break

User · Answer

To add to the answers here  I think it s worth considering the opposite question in conjunction with this  viz  why did C allow fall-through in the first place  Any programming language of course serves two goals   Provide instructions to the computer  Leave a record of the intentions of the programmer   The creation of any programming language is therefore a balance between how to best serve these two goals  On the one hand  the easier it is to turn into computer instructions  whether those are machine code  bytecode like IL  or the instructions are interpreted on execution  then more able that process of compilation or interpretation will be to be efficient  reliable and compact in output  Taken to its extreme  this goal results in our just writing in assembly  IL  or even raw op-codes  because the easiest compilation is where there is no compilation at all  Conversely  the more the language expresses the intention of the programmer  rather than the means taken to that end  the more understandable the program both when writing and during maintenance  Now  switch could always have been compiled by converting it into the equivalent chain of if-else blocks or similar  but it was designed as allowing compilation into a particular common assembly pattern where one takes a value  computes an offset from it  whether by looking up a table indexed by a perfect hash of the value  or by actual arithmetic on the value    It s worth noting at this point that today  C  compilation will sometimes turn switch into the equivalent if-else  and sometimes use a hash-based jump approach  and likewise with C  C    and other languages with comparable syntax   In this case there are two good reasons for allowing fall-through   It just happens naturally anyway  if you build a jump table into a set of instructions  and one of the earlier batches of instructions doesn t contain some sort of jump or return  then execution will just naturally progress into the next batch  Allowing fall-through was what would  quot just happen quot  if you turned the switch-using C into jump-table   using machine code   Coders who wrote in assembly were already used to the equivalent  when writing a jump table by hand in assembly  they would have to consider whether a given block of code would end with a return  a jump outside of the table  or just continue on to the next block  As such  having the coder add an explicit break when necessary was  quot natural quot  for the coder too    At the time therefore  it was a reasonable attempt to balance the two goals of a computer language as it relates to both the produced machine code  and the expressiveness of the source code  Four decades later though  things are not quite the same  for a few reasons   Coders in C today may have little or no assembly experience  Coders in many other C-style languages are even less likely to  especially Javascript    Any concept of  quot what people are used to from assembly quot  is no longer relevant  Improvements in optimisations mean that the likelihood of switch either being turned into if-else because it was deemed the approach likely to be most efficient  or else turned into a particularly esoteric variant of the jump-table approach are higher  The mapping between the higher- and lower-level approaches is not as strong as it once was  Experience has shown that fall-through tends to be the minority case rather than the norm  a study of Sun s compiler found 3  of switch blocks used a fall-through other than multiple labels on the same block  and it was thought that the use-case here meant that this 3  was in fact much higher than normal   So the language as studied make the unusual more readily catered-to than the common  Experience has shown that fall-through tends to be the source of problems both in cases where it is accidentally done  and also in cases where correct fall-through is missed by someone maintaining the code  This latter is a subtle addition to the bugs associated with fall-through  because even if your code is perfectly bug-free  your fall-through can still cause problems   Related to those last two points  consider the following quote from the current edition of K amp R   Falling through from one case to another is not robust  being prone to disintegration when the program is modified  With the exception of multiple labels for a single computation  fall-throughs should be used sparingly  and commented  As a matter of good form  put a break after the last case  the default here  even though it s logically unnecessary  Some day when another case gets added at the end  this bit of defensive programming will save you   So  from the horse s mouth  fall-through in C is problematic  It s considered good practice to always document fall-throughs with comments  which is an application of the general principle that one should document where one does something unusual  because that s what will trip later examination of the code and or make your code look like it has a novice s bug in it when it is in fact correct  And when you think about it  code like this  switch x      case 1     foo          FALLTHRU      case 2      bar        break     Is adding something to make the fall-through explicit in the code  it s just not something that can be detected  or whose absence can be detected  by the compiler  As such  the fact that on has to be explicit with fall-through in C  doesn t add any penalty to people who wrote well in other C-style languages anyway  since they would already be explicit in their fall-throughs     Finally  the use of goto here is already a norm from C and other such languages  switch x      case 0    case 1    case 2      foo        goto below six    case 3      bar        goto below six    case 4      baz           FALLTHRU      case 5    below six      qux        break    default      quux       In this sort of case where we want a block to be included in the code executed for a value other than just that which brings one to the preceding block  then we re already having to use goto   Of course  there are means and ways of avoiding this with different conditionals but that s true of just about everything relating to this question   As such C  built on the already normal way to deal with one situation where we want to hit more than one block of code in a switch  and just generalised it to cover fall-through as well  It also made both cases more convenient and self-documenting  since we have to add a new label in C but can use the case as a label in C   In C  we can get rid of the below six label and use goto case 5 which is clearer as to what we are doing   We d also have to add break for the default  which I left out just to make the above C code clearly not C  code   In summary therefore   C  no longer relates to unoptimised compiler output as directly as C code did 40 years ago  nor does C these days   which makes one of the inspirations of fall-through irrelevant  C  remains compatible with C in not just having implicit break  for easier learning of the language by those familiar with similar languages  and easier porting  C  removes a possible source of bugs or misunderstood code that has been well-documented as causing problems for the last four decades  C  makes existing best-practice with C  document fall through  enforceable by the compiler  C  makes the unusual case the one with more explicit code  the usual case the one with the code one just writes automatically  C  uses the same goto-based approach for hitting the same block from different case labels as is used in C  It just generalises it to some other cases  C  makes that goto-based approach more convenient  and clearer  than it is in C  by allowing case statements to act as labels   All in all  a pretty reasonable design decision   Some forms of BASIC would allow one to do the likes of GOTO  x AND 7    50   240 which while brittle and hence a particularly persuasive case for banning goto  does serve to show a higher-language equivalent of the sort of way that lower-level code can make a jump based on arithmetic upon a value  which is much more reasonable when it s the result of compilation rather than something that has to be maintained manually  Implementations of Duff s Device in particular lend themselves well to the equivalent machine code or IL because each block of instructions will often be the same length without needing the addition of nop fillers     Duff s Device comes up here again  as a reasonable exception  The fact that with that and similar patterns there s a repetition of operations serves to make the use of fall-through relatively clear even without an explicit comment to that effect

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switch  C  Reference  says     C  requires the end of switch sections  including the final one     So you also need to add a break  to your default section  otherwise there will still will be a compiler error

User · Answer

You forgot to add the  break   statement into case 3  In case 2 you wrote it into the if block  Therefore try this   case 3                    ans    string Format   0  hundred and    numbers number   100        break      case 2                    int t    number   10    10                  if  t    1                                            ans    teens number   10                                         else if  t  gt  1                                ans    string Format   0 -   tens t                      break     case 1                    int o   number   10                  ans    numbers o                   break             default                    throw new ArgumentException  number

User · Answer

They changed the switch statement  from C Java C    behavior for c    I guess the reasoning was that people forgot about the fall through and errors were caused   One book I read said to use goto to simulate  but this doesn t sound like a good solution to me

User · Answer

They left out this behaviour by design to avoid when it was not used by will but caused problems   It can be used only if there is no statement in the case part  like   switch  whatever        case 1      case 2      case 3  boo  break

[c#] Switch statement fallthrough in C#?

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