do while vs while

Question

Possible Duplicates     While vs  Do While    When should I use do-while instead of while loops      I ve been programming for a while now  2 years work   4 5 years degree   1 year pre-college   and I ve never used a do-while loop short of being forced to in the Introduction to Programming course  I have a growing feeling that I m doing programming wrong if I never run into something so fundamental   Could it be that I just haven t run into the correct circumstances   What are some examples where it would be necessary to use a do-while instead of a while    My schooling was almost all in C C   and my work is in C   so if there is another language where it absolutely makes sense because do-whiles work differently  then these questions don t really apply    To clarify   I know the difference between a while and a do-while  While checks the exit condition and then performs tasks  do-while performs tasks and then checks exit condition

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This is my personal opinion  but this question begs for an answer rooted in experience   I have been programming in C for 38 years  and I never use do   while loops in regular code   The only compelling use for this construct is in macros where it can wrap multiple statements into a single statement via a do   multiple statements   while  0   I have seen countless examples of do   while loops with bogus error detection or redundant function calls   My explanation for this observation is programmers tend to model problems incorrectly when they think in terms of do   while loops  They either miss an important ending condition or they miss the possible failure of the initial condition which they move to the end    For these reasons  I have come to believe that where there is a do   while loop  there is a bug  and I regularly challenge newbie programmers to show me a do   while loop where I cannot spot a bug nearby  This type of loop can be easily avoided  use a for              and add the necessary termination tests where they are appropriate   It is quite common that there need be more than one such test  Here is a classic example     skip the line    do       c   getc fp     while  c      n     This will fail if the file does not end with a newline   A trivial example of such a file is the empty file  A better version is this  int c      another classic bug is to define c as char  while   c   getc fp      EOF  amp  amp  c      n       continue   Alternately  this version also hides the c variable  for            int c   getc fp       if  c    EOF    c      n           break     Try searching for while  c      n    in any search engine  and you will find bugs such as this one  retrieved June 24  2017   In ftp   ftp dante de tex-archive biblio tib src streams c   function getword stream p ignore   has a do   while and sure enough at least 2 bugs   c is defined as a char and there is a potential infinite loop while  c    n   c getc stream    Conclusion  avoid do   while loops and look for bugs when you see one

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This is sort of an indirect answer  but this question got me thinking about the logic behind it  and I thought this might be worth sharing   As everyone else has said  you use a do     while loop when you want to execute the body at least once  But under what circumstances would you want to do that   Well  the most obvious class of situations I can think of would be when the initial   unprimed   value of the check condition is the same as when you want to exit  This means that you need to execute the loop body once to prime the condition to a non-exiting value  and then perform the actual repetition based on that condition  What with programmers being so lazy  someone decided to wrap this up in a control structure   So for example  reading characters from a serial port with a timeout might take the form  in Python    response buffer      char read   port read 1   while char read      response buffer append char read      char read   port read 1     When there s nothing to read after 1s  there is no more data  response      join response buffer    Note the duplication of code  char read   port read 1   If Python had a do     while loop  I might have used   do      char read   port read 1      response buffer append char read  while char read   The added benefit for languages that create a new scope for loops  char read does not pollute the function namespace  But note also that there is a better way to do this  and that is by using Python s None value   response buffer      char read   None  while char read            char read   port read 1      response buffer append char read   response      join response buffer    So here s the crux of my point  in languages with nullable types  the situation initial value    exit value arises far less frequently  and that may be why you do not encounter it  I m not saying it never happens  because there are still times when a function will return None to signify a valid condition  But in my hurried and briefly-considered opinion  this would happen a lot more if the languages you used did not allow for a value that signifies  this variable has not been initialised yet   This is not perfect reasoning  in reality  now that null-values are common  they simply form one more element of the set of valid values a variable can take  But practically  programmers have a way to distinguish between a variable being in sensible state  which may include the loop exit state  and it being in an uninitialised state

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I use do-while loops all the time when reading in files   I work with a lot of text files that include comments in the header     some comments   some more comments column1 column2   1 234   5 678   9 012   3 456                   i ll use a do-while loop to read up to the  column1 column2  line so that I can look for the column of interest   Here s the pseudocode   do       line   read line      while   line 0              parse line      Then I ll do a while loop to read through the rest of the file

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I can t imagine how you ve gone this long without using a do   while loop   There s one on another monitor right now and there are multiple such loops in that program   They re all of the form   do       GetProspectiveResult      while   ProspectIsGood

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One of the applications I have seen it is in Oracle when we look at result sets   Once you a have a result set  you first fetch from it  do  and from that point on   check if the fetch returns an element or not  while element found       The same might be applicable for any other  fetch-like  implementations

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I have used this in a TryDeleteDirectory function  It was something like this   do       try               DisableReadOnly directory           directory Delete true             catch  Exception                retryDeleteDirectoryCount            while  Directory Exists fullPath   amp  amp  retryDeleteDirectoryCount  lt  4

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Even though there are plenty of answers here is my take  It all comes down to optimalization  I ll show two examples where one is faster then the other  Case 1  while string fileName   string Empty  fullPath   string Empty   while  string IsNullOrEmpty fileName     File Exists fullPath         fileName   Guid NewGuid   ToString     fileExtension      fullPath   Path Combine uploadDirectory  fileName      Case 2  do while string fileName   string Empty  fullPath   string Empty   do       fileName   Guid NewGuid   ToString     fileExtension      fullPath   Path Combine uploadDirectory  fileName     while  File Exists fullPath     So there two will do the exact same things  But there is one fundamental difference and that is that the while requires an extra statement to enter the while  Which is ugly because let s say every possible scenario of the Guid class has already been taken except for one variant  This means I ll have to loop around 5 316 911 983 139 663 491 615 228 241 121 400 000 times  Every time I get to the end of my while statement I will need to do the string IsNullOrEmpty fileName  check  So this would take up a little bit  a tiny fraction of CPU work  But do this very small task times the possible combinations the Guid class has and we are talking about hours  days  months or extra time  Of course this is an extreme example because you probably wouldn t see this in production  But if we would think about the YouTube algorithm  it is very well possible that they would encounter the generation of an ID where some ID s have already been taken  So it comes down to big projects and optimalization

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Console WriteLine  hoeveel keer moet je de zin schrijven             int aantal   Convert ToInt32 Console ReadLine             int counter   0           while   counter  lt   aantal                        Console WriteLine  Ik mag geen stiften gooien                counter   counter   1

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A situation where you always need to run a piece of code once  and depending on its result  possibly more times   The same can be produced with a regular while loop as well   rc   get something    while  rc    wrong stuff        rc   get something       do       rc   get something      while  rc    wrong stuff

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I am programming about 12 years and only 3 months ago I have met a situation where it was really convenient to use do-while as one iteration was always necessary before checking a condition  So guess your big-time is ahead

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I ran across this while researching the proper loop to use for a situation I have   I believe this will fully satisfy a common situation where a do   while loop is a better implementation than a while loop  C  language  since you stated that is your primary for work    I am generating a list of strings based on the results of an SQL query   The returned object by my query is an SQLDataReader   This object has a function called Read   which advances the object to the next row of data  and returns true if there was another row   It will return false if there is not another row   Using this information  I want to return each row to a list  then stop when there is no more data to return   A Do    While loop works best in this situation as it ensures that adding an item to the list will happen BEFORE checking if there is another row   The reason this must be done BEFORE checking the while condition  is that when it checks  it also advances   Using a while loop in this situation would cause it to bypass the first row due to the nature of that particular function   In short   This won t work in my situation         This will skip the first row because Read   returns true after advancing      while   read NextResult                                list Add  read GetValue 0  ToString                      return list    This will         This will make sure the currently read row is added before advancing      do                       list Add  read GetValue 0  ToString                        while   read NextResult          return list

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Being a geezer programmer  many of my school programming projects used text menu driven interactions   Virtually all used something like the following logic for the main procedure   do     display options     get choice     perform action appropriate to choice while choice is something other than exit   Since school days  I have found that I use the while loop more frequently

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Any sort of console input works well with do-while because you prompt the first time  and re-prompt whenever the input validation fails

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Here s my theory why most people  including me  prefer while     loops to do  while    A while     loop can easily be adapted to perform like a do  while   loop while the opposite is not true  A while loop is in a certain way  more general   Also programmers like easy to grasp patterns  A while loop says right at start what its invariant is and this is a nice thing   Here s what I mean about the  more general  thing  Take this do  while loop    do    A   if  condition  INV false    B    while INV     Transforming this in to a while loop is straightforward    INV true  while INV     A   if  condition  INV false    B      Now  we take a model while loop   while INV         A       if  condition  INV false        B      And transform this into a do  while loop  yields this monstrosity   if  INV     do             A           if  condition  INV false            B      while INV      Now we have two checks on opposite ends and if the invariant changes you have to update it on two places  In a certain way do  while is like the specialized screwdrivers in the tool box which you never use  because the standard screwdriver does everything you need

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I used them a fair bit when I was in school  but not so much since    In theory they are useful when you want the loop body to execute once before the exit condition check  The problem is that for the few instances where I don t want the check first  typically I want the exit check in the middle of the loop body rather than at the very end  In that case  I prefer to use the well-known for      with an if  condition  exit  somewhere in the body   In fact  if I m a bit shaky on the loop exit condition  sometimes I find it useful to start writing the loop as a for         with an exit statement where needed  and then when I m done I can see if it can be  cleaned up  by moving initilizations  exit conditions  and or increment code inside the for s parentheses

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If you always want the loop to execute at least once  It s not common  but I do use it from time to time  One case where you might want to use it is trying to access a resource that could require a retry  e g   do      try to access resource       put up message box with retry option    while  user says retry

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while loops check the condition before the loop  do   while loops check the condition after the loop  This is useful is you want to base the condition on side effects from the loop running or  like other posters said  if you want the loop to run at least once   I understand where you re coming from  but the do-while is something that most use rarely  and I ve never used myself  You re not doing it wrong   You re not doing it wrong  That s like saying someone is doing it wrong because they ve never used the byte primitive  It s just not that commonly used

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I ve used a do while when I m reading a sentinel value at the beginning of a file  but other than that  I don t think it s abnormal that this structure isn t too commonly used--do-whiles are really situational   -- file -- 5 Joe Bob Jake Sarah Sue  -- code -- int MAX  int count   0  do   MAX   a readLine    k count    a readLine    count      while count  lt   MAX

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The answers so far summarize the general use for do-while  But the OP asked for an example  so here is one  Get user input  But the user s input may be invalid - so you ask for input  validate it  proceed if it s valid  otherwise repeat   With do-while  you get the input while the input is not valid  With a regular while-loop  you get the input once  but if it s invalid  you get it again and again until it is valid  It s not hard to see that the former is shorter  more elegant  and simpler to maintain if the body of the loop grows more complex

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I ve used it for a reader that reads the same structure multiple times   using IDataReader reader   connection ExecuteReader          do               while reader Read                            Read record                 while reader NextResult

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I like to understand these two as  while - gt   repeat until   do     while - gt   repeat if

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The most common scenario I run into where I use a do while loop is in a little console program that runs based on some input and will repeat as many times as the user likes  Obviously it makes no sense for a console program to run no times  but beyond the first time it s up to the user -- hence do while instead of just while   This allows the user to try out a bunch of different inputs if desired   do      int input   GetInt  Enter any integer          Do something with input    while  GetBool  Go again        I suspect that software developers use do while less and less these days  now that practically every program under the sun has a GUI of some sort  It makes more sense with console apps  as there is a need to continually refresh the output to provide instructions or prompt the user with new information  With a GUI  in contrast  the text providing that information to the user can just sit on a form and never need to be repeated programmatically

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Do while is useful for when you want to execute something at least once  As for a good example for using do while vs  while  lets say you want to make the following  A calculator   You could approach this by using a loop and checking after each calculation if the person wants to exit the program  Now you can probably assume that once the program is opened the person wants to do this at least once so you could do the following   do         do calculator logic here       prompt user for continue here   while cont  true    cont is short for continue

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do-while is better if the compiler isn t competent at optimization   do-while has only a single conditional jump  as opposed to for and while which have a conditional jump and an unconditional jump   For CPUs which are pipelined and don t do branch prediction  this can make a big difference in the performance of a tight loop   Also  since most compilers are smart enough to perform this optimization  all loops found in decompiled code will usually be do-while  if the decompiler even bothers to reconstruct loops from backward local gotos at all

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I  ve used it in a function that returned the next character position in an utf-8 string   char  next utf8 character const char  txt        if   txt     txt      0           return txt       do           txt          while    signed char   txt   lt  0  amp  amp     unsigned char   txt   amp  0xc0     0xc0       return  char   txt      Note that  this function is written from mind and not tested  The point is that you have to do the first step anyway and you have to do it before you can evaluate the condition

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do while is if you want to run the code block at least once  while on the other hand won t always run depending on the criteria specified

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It s as simple as that   precondition vs postcondition   while  cond        - precondition  it executes the code only after checking   do       while  cond  - postcondition  code is executed at least once    Now that you know the secret    use them wisely

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I see that this question has been adequately answered  but would like to add this very specific use case scenario   You might start using do   while more frequently   do            while  0    is often used for multi-line  defines   For example    define compute values          area   pi   r   r            volume   area   h   This works alright for   r   4  h   3  compute values    -but- there is a gotcha for   if  shape    circle   compute values    as this expands to   if  shape    circle  area   pi  r   r  volume   area   h    If you wrap it in a do     while 0  loop it properly expands to a single block   if  shape    circle    do         area   pi   r   r      volume   area   h      while  0

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It is a quite common structure in a server consumer   DOWHILE  no shutdown requested     determine timeout    wait for work timeout     IF  there is work        REPEAT           process       UNTIL wait for work 0 timeout  indicates no work        do what is supposed to be done at end of busy period     ENDIF ENDDO   the REPEAT UNTIL cond  being a do       while  cond   Sometimes the wait for work 0  can be cheaper CPU wise  even eliminating the timeout calculation might be an improvement with very high arrival rates   Moreover  there are many queuing theory results that make the number served in a busy period an important statistic   See for example Kleinrock - Vol 1    Similarly   DOWHILE  no shutdown requested     determine timeout    wait for work timeout     IF  there is work        set throttle       REPEAT           process       UNTIL --throttle lt 0   OR   wait for work 0 timeout  indicates no work     ENDIF    check for and do other  perhaps polled  work  ENDDO   where check for and do other work may be exorbitantly expensive to put in the main loop or perhaps a kernel that does not support an efficient waitany waitcontrol  n  type operation or perhaps a situation where a prioritized queue might starve the other work and throttle is used as starvation control   This type of balancing can seem like a hack  but it can be necessary  Blind use of thread pools would entirely defeat the performance benefits of the use of a caretaker thread with a private queue for a high updating rate complicated data structure as the use of a thread pool rather than a caretaker thread would require thread-safe implementation   I really don t want to get into a debate about the pseudo code  for example  whether shutdown requested should be tested in the UNTIL  or caretaker threads versus thread pools - this is just meant to give a flavor of a particular use case of the control flow structure

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