For vs while in C programming

Question

There are three loops in C  for  while  and do-while  What s the difference between them   For example  it seems nearly all while statements can be replaced by for statements  right  Then  what s the advantage using while

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They are pretty much same except for do-while loop  The for loop is good when you have a counter kind of variable  It makes it obvious  while loop makes sense in cases where a flag is being checked as show below    while   done       if  some condtion         done   true

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Remember  a for loop is essentially a fancy while loop  They re the same thing   while  lt some condition is true gt          do some stuff       possibly do something to change the condition     for   some var   lt some condition is true gt   increment var          The advantage of a for loop is that it s harder to accidentally do an infinite loop  Or rather  it s more obvious when you do one because you generally put the loop var in the initial statement    A while loop is more clear when you re not doing a standard incrementing pattern  For example   int x   1  while  x    10       if   some condition        x   10    else       x    5

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A for suggest a fixed iteration using an index or variants on this scheme   A while and do    while are constructions you use when there is a condition that must be checked each time  apart from some index-alike construction  see above   They differ in when the first execution of the condition check is performed   You can use either construct  but they have their advantages and disadvantages depending on your use case

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If there is a strong concern about speed and performance  the best approach is to verify the code produced by the compiler at the assembly level   For instance  the following code shows that the  do-while  is a bit faster  This because the  jmp  instruction is not used by the  do-while  loop   BTW  in this specific example  the worst case is given by the  for  loop       int main int argc  char  argv          int i      char x 100            FOR  LOOP      for  i 0  i lt 100  i                   x i    0                 WHILE  LOOP      i   0      while  i lt 100                 x i      0                 DO-WHILE  LOOP      i   0      do               x i      0            while  i lt 100        return 0          FOR  LOOP       010013C8  mov         dword ptr  ebp-0Ch  0     010013CF  jmp         wmain 3Ah  10013DAh     for  i 0  i lt 100  i               x i    0      010013D1  mov         eax dword ptr  ebp-0Ch    lt  lt  lt  UPDATE i     010013D4  add         eax 1     010013D7  mov         dword ptr  ebp-0Ch  eax     010013DA  cmp         dword ptr  ebp-0Ch  64h   lt  lt  lt  TEST     010013DE  jge         wmain 4Ah  10013EAh       lt  lt  lt  COND JUMP     010013E0  mov         eax dword ptr  ebp-0Ch    lt  lt  lt  DO THE JOB       010013E3  mov         byte ptr  ebp eax-78h  0     010013E8  jmp         wmain 31h  10013D1h       lt  lt  lt  UNCOND JUMP           WHILE  LOOP     i   0    010013EA  mov         dword ptr  ebp-0Ch  0   while  i lt 100             x i      0      010013F1  cmp         dword ptr  ebp-0Ch  64h    lt  lt  lt  TEST     010013F5  jge         wmain 6Ah  100140Ah        lt  lt  lt  COND JUMP     010013F7  mov         eax dword ptr  ebp-0Ch     lt  lt  lt  DO THE JOB       010013FA  mov         byte ptr  ebp eax-78h  0     010013FF  mov         ecx dword ptr  ebp-0Ch     lt  lt  lt  UPDATE i     01001402  add         ecx 1     01001405  mov         dword ptr  ebp-0Ch  ecx     01001408  jmp         wmain 51h  10013F1h        lt  lt  lt  UNCOND JUMP           DO-WHILE  LOOP   i   0     0100140A  mov         dword ptr  ebp-0Ch  0   do           x i      0      01001411  mov         eax dword ptr  ebp-0Ch     lt  lt  lt  DO THE JOB       01001414  mov         byte ptr  ebp eax-78h  0     01001419  mov         ecx dword ptr  ebp-0Ch     lt  lt  lt  UPDATE i     0100141C  add         ecx 1     0100141F  mov         dword ptr  ebp-0Ch  ecx     01001422  cmp         dword ptr  ebp-0Ch  64h    lt  lt  lt  TEST     01001426  jl          wmain 71h  1001411h        lt  lt  lt  COND JUMP       while  i lt 100

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For the sake of readability

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One peculiarity of the do while is that you need a semi-colon after the while to complete  It is often  used in macro definitions to execute several statements only once while constraining the impact of the macro  If macros where defined as blocks  some parsing errors may occur   One explanation among others

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They are all the same in the work they do  You can do the same things using any of them  But for readability  usability  convenience etc   they differ

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If you want a loop to execute while a condition is true  and not for a certain number of iterations  it is much easier for someone else to understand   while  cond true    than something like this   for    cond true

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I noticed some time ago that a For loop typically generates several more machine instructions than a while loop  However  if you look closely at the examples  which mirror my observations  the difference is two or three machine instructions  hardly worth much consideration   Note  too  that the initializer for a WHILE loop can be eliminated by baking it into the code  e  g    static int intStartWith   100    The static modifier bakes the initial value into the code  saving  drum roll  one MOV instruction  Of greater significance  marking a variable as static moves it outside the stack frame  Variable alignment permitting  it may also produce slightly smaller code  too  since the MOV instruction and its operands take more room than  for example an integer  Boolean  or character value  either ANSI or Unicode    However  if variables are aligned on 8 byte boundaries  a common default setting  an int  bool  or TCHAR baked into code costs the same number of bytes as a MOV instruction

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You should use such a loop  that most fully conforms to your needs  For example   for int i   0  i  lt  10  i          print i        or  int i   0  while i  lt  10        print i       i        Obviously  in such situation   for  looks better  than  while   And  do while  shoud be used when some operations must be done already before the moment when condition of your loop will be checked   Sorry for my bad english

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They re all interchangeable  you could pick one type and use nothing but that forever  but usually one is more convenient for a given task  It s like saying  why have switch  you can just use a bunch of if statements  -- true  but if it s a common pattern to check a variable for a set of values  it s convenient and much easier to read if there s a language feature to do that

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One common misunderstanding withwhile for loops I ve seen is that their efficiency differs  While loops and for loops are equally efficient  I remember my computer teacher from highschool told me that for loops are more efficient for iteration when you have to increment a number  That is not the case   For loops are simply syntactically sugared while loops  and make iteration code faster to write   When the compiler takes your code and compiles it  it is translating it into a form that is easier for the computer to understand and execute on a lower level  assembly   During this translation  the subtle differences between the while and for syntaxes are lost  and they become exactly the same

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For loops  at least considering C99  are superior to while loops because they limit the scope of the incremented variable s    Do while loops are useful when the condition is dependant on some inputs  They are the most seldom used of the three loop types

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A difference between while and do-while is that while checks the loop condition and if this is true  the body is executed and the condition checked again  The do-while checks the condition after execution of the body  so with do-while the body is executed at least one time   Of course you can write a while loop as a do-while and vv  but this usually requires some code duplication

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while loop  5 bucks  1 chocolate   1 bucks  while my money is greater than 1 bucks    select chocolate   pay 1 bucks to the shopkeeper   money   money - 1 end   come to home and cant go to while shop because my money   0 bucks      include lt stdio h gt  int main      int money   5     while  money  gt   1          printf  inside the shopk and selecting chocolate n        printf  after selecting chocolate paying 1 bucks n        money   money - 1         printf  my remaining moeny    d n   money       printf   n n           printf  dont have money cant go inside the shop  money    d    money      return 0       infinite money  while  codition       condition will always true     infinite loop   statement s      please visit this video for better understanding https   www youtube com watch v eqDv2wxDMJ8 amp t 25s     for loop  5 bucks  for my money is greater than equal to 1 bucks   0      money  gt   1   select chocolate   pay 1 bucks to the shopkeeper   money   money - 1  1-1   gt  0 end        include lt stdio h gt  int main       int money   5    for    money  gt   1         0 gt  1  false      printf  select chocolate  n        printf  paying 1 bucks to the shopkeeper n        money   money - 1     1-1   0     printf   remaining money   d n   money       printf   n n           return 0      For better understanding please visit https   www youtube com watch v  vdvyzzp-R4 amp t 25s

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while and for statements can both be used for looping in programming  It will depend on the programmer as to whether the while loop or for loop is used  Some are comfortable using while loop and some are with for loop    Use any loop you like  However  the do   while loop can be somewhat tricky in C programming

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Between for and while  while does not need initialization nor update statement  so it may look better  more elegant  for can have statements missing  one two or all  so it is the most flexible and obvious if you need initialization  looping condition and  update  before looping  If you need only loop condition  tested at the beginning of the loop  then while is more elegant   Between for while and do-while  in do-while the condition is evaluated at the end of the loop  More confortable if the loop must be executed at least once

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WHILE is more flexible  FOR is more concise in those instances in which it applies   FOR is great for loops which have a counter of some kind  like  for  int n 0  n lt max    n    You can accomplish the same thing with a WHILE  of course  as others have pointed out  but now the initialization  test  and increment are broken across three lines  Possibly three widely-separated lines if the body of the loop is large  This makes it harder for the reader to see what you re doing  After all  while    n  is a very common third piece of the FOR  it s certainly not the only possibility  I ve written many loops where I write  n  increment  or some more complex expression   FOR can also work nicely with things other than a counter  of course  Like  for  int n getFirstElementFromList    listHasMoreElements    n getNextElementFromList      Etc   But FOR breaks down when the  next time through the loop  logic gets more complicated  Consider   initializeList    while  listHasMoreElements        n getCurrentElement      int status processElement n     if  status gt 0          skipElements status       advanceElementPointer          else         n -status      findElement n           That is  if the process of advancing may be different depending on conditions encountered while processing  a FOR statement is impractical  Yes  sometimes you could make it work with a complicated enough expressions  use of the ternary    operator  etc  but that usually makes the code less readable rather than more readable   In practice  most of my loops are either stepping through an array or structure of some kind  in which case I use a FOR loop  or are reading a file or a result set from a database  in which case I use a WHILE loop   while   eof     or something of that sort

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A while loop will always evaluate the condition first  while  condition        gets executed after condition is checked    A do while loop will always execute the code in the do   block first and then evaluate the condition  do       gets executed at least once   while  condition     A for loop allows you to initiate a counter variable  a check condition  and a way to increment your counter all in one line  for  int x   0  x  lt  100  x           executed until x  gt   100    At the end of the day  they are all still loops  but they offer some flexibility as to how they are executed  Here is a great explanation of the reasoning behind the use of each different type of loop that may help clear things up   Thanks clyfe  The main difference between the for s and the while s  is a matter of pragmatics  we usually use for when there is a known number of iterations  and use while constructs when the number of iterations in not known in advance  The while vs do     while issue is also of pragmatics  the second executes the instructions once at start  and afterwards it behaves just like the simple while    For loops are especially nice because they are concise   In order for this for loop  for  int x   0  x  lt  100  x           executed until x  gt   100    to be written as a while loop  you d have to do the following  int count   0  while  count  lt  100        do stuff   count       In this case  there s just more stuff to keep up with and the count    could get lost in the logic   This could end up being troublesome depending on where count gets incremented  and whether or not it should get incremented before or after the loop s logic   With a for loop  your counter variable is always incremented before the next iteration of the loop  which adds some uniformity to your code   For the sake of completeness  it s probably meaningful to talk about break and continue statements here which come in handy when doing loop processing  break will instantly terminate the current loop and no more iterations will be executed    will only run  quot do stuff quot  twice for  int x   0  x  lt  100  x        if  x    2        break          do stuff    continue will terminate the current iteration and move on to the next one    will run  quot do stuff quot  until x  gt   100 except for when x   2 for  int x   0  x  lt  100  x        if  x    2        continue          do stuff    Note that in a for loop  continue evaluates the part3 expression of for  part1  part2  part3   in contrast  in a while loop  it just jumps to re-evaluate the loop condition

[c] For vs. while in C programming?

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