Post-increment and pre-increment within a for loop produce same output

Question

The following for loops produce identical results even though one uses post increment and the other pre-increment   Here is the code   for i 0  i lt 5  i          printf   d   i      for i 0  i lt 5    i        printf   d   i       I get the same output for both  for  loops   Am I missing something

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Yes  you ll get exactly same outputs for both  why do you think they should give you different outputs   Post-increment or pre-increment matters in situations like this   int j     i  int k   i    f i     g   i     where you provide some value  either by assigning or by passing an argument  You do neither in your for loops  It gets incremented only  Post- and pre- don t make sense there

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You could read Google answer for it here  http   google-styleguide googlecode com svn trunk cppguide xml Preincrement and Predecrement  So  main point is  what no difference for simple object  but for iterators and other template objects you should use preincrement   EDITED   There are no difference because you use simple type  so no side effects  and post- or preincrements executed after loop body  so no impact on value in loop body   You could check it with such a loop   for  int i   0  i  lt  5  cout  lt  lt   we still not incremented here     lt  lt  i  lt  lt  endl  i          cout  lt  lt   inside loop body     lt  lt  i  lt  lt  endl

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Well  this is simple  The above for loops are semantically equivalent to  int i   0  while i  lt  5        printf   d   i       i        and  int i   0  while i  lt  5        printf   d   i         i      Note that the lines i    and   i  have the same semantics FROM THE PERSPECTIVE OF THIS BLOCK OF CODE  They both have the same effect on the value of i  increment it by one  and therefore have the same effect on the behavior of these loops   Note that there would be a difference if the loop was rewritten as  int i   0  int j   i  while j  lt  5        printf   d   i       j     i     int i   0  int j   i  while j  lt  5        printf   d   i       j   i        This is because in first block of code j sees the value of i after the increment  i is incremented first  or pre-incremented  hence the name  and in the second block of code j sees the value of i before the increment

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If you wrote it like this then it would matter    for i 0  i lt 5  i j          printf   d  i       Would iterate once more than if written like this    for i 0  i lt 5  i   j        printf   d  i

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Compilers translate  for  a  b  c                to  a  while b             end      c      So in your case  post pre- increment  it doesn t matter   EDIT  continues are simply replaced by goto end

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There is a difference if   int main       for int i 0   i lt 2  printf  i   post increment in loop  d n   i             cout  lt  lt   inside post incement      lt  lt  i  lt  lt  endl          for int i 0   i lt 2  printf  i   pre increment in loop  d n    i           cout  lt  lt   inside pre incement      lt  lt  i  lt  lt  endl         return 0      The result   inside post incement   0  i   post increment in loop 0  inside post incement   1  i   post increment in loop 1  The second for loop    inside pre incement   0  i   pre increment in loop 1  inside pre incement   1  i   pre increment in loop 2

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After evaluating i   or   i  the new value of i will be the same in both cases   The difference between pre- and post-increment is in the result of evaluating the expression itself     i increments i and evaluates to the new value of i   i   evaluates to the old value of i  and increments i   The reason this doesn t matter in a for loop is that the flow of control works roughly like this    test the condition if it is false  terminate if it is true  execute the body execute the incrementation step   Because  1  and  4  are decoupled  either pre- or post-increment can be used

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Because in either case the increment is done after the body of the loop and thus doesn t affect any of the calculations of the loop   If the compiler is stupid  it might be slightly less efficient to use post-increment  because normally it needs to keep a copy of the pre value for later use   but I would expect any differences to be optimized away in this case   It might be handy to think of how the for loop is implemented  essentially translated into a set of assignments  tests  and branch instructions  In pseudo-code the pre-increment would look like         set i   0 test  if i  gt   5 goto done       call printf   d  i       set i   i   1       goto test done  nop   Post-increment would have at least another step  but it would be trivial to optimize away        set i   0 test  if i  gt   5 goto done       call printf   d  i       set j   i      store value of i for later increment       set i   j   1     oops  we re incrementing right-away       goto test done  nop

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The result of your code will be the same  The reason is that the two incrementation operations can be seen as two distinct function calls  Both functions cause an incrementation of the variable  and only their return values are different  In this case  the return value is just thrown away  which means that there s no distinguishable difference in the output   However  under the hood there s a difference  The post-incrementation i   needs to create a temporary variable to store the original value of i  then performs the incrementation and returns the temporary variable  The pre-incrementation   i doesn t create a temporary variable  Sure  any decent optimization setting should be able to optimize this away when the object is something simple like an int  but remember that the   -operators are overloaded in more complicated classes like iterators  Since the two overloaded methods might have different operations  one might want to output  Hey  I m pre-incremented   to stdout for example  the compiler can t tell whether the methods are equivalent when the return value isn t used  basically because such a compiler would solve the unsolvable halting problem   it needs to use the more expensive post-incrementation version if you write myiterator     Three reasons why you should pre-increment    You won t have to think about whether the variable object might have an overloaded post-incrementation method  for example in a template function  and treat it differently  or forget to treat it differently   Consistent code looks better  When someone asks you  Why do you pre-increment   you ll get the chance to teach them about the halting problem and theoretical limits of compiler optimization

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Both i   and   i is executed after printf   d   i  is executed at each time  so there s no difference

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This is one of my favorite interview questions   I ll explain the answer first  and then tell you why I like the question   Solution   The answer is that both snippets print the numbers from 0 to 4  inclusive   This is because a for   loop is generally equivalent to a while   loop   for  INITIALIZER  CONDITION  OPERATION        do stuff        Can be written   INITIALIZER  while CONDITION        do stuff        OPERATION      You can see that the OPERATION is always done at the bottom of the loop   In this form  it should be clear that i   and   i will have the same effect  they ll both increment i and ignore the result   The new value of i is not tested until the next iteration begins  at the top of the loop     Edit  Thanks to Jason for pointing out that this for   to while   equivalence does not hold if the loop contains control statements  such as continue  that would prevent OPERATION from being executed in a while   loop   OPERATION is always executed just before the next iteration of a for   loop     Why it s a Good Interview Question  First of all  it takes only a minute or two if a candidate tells the the correct answer immediately  so we can move right on to the next question   But surprisingly  to me   many candidates tell me the loop with the post-increment will print the numbers from 0 to 4  and the pre-increment loop will print 0 to 5  or 1 to 5   They usually explain the difference between pre- and post-incrementing correctly  but they misunderstand the mechanics of the for   loop   In that case  I ask them to rewrite the loop using while    and this really gives me a good idea of their thought processes   And that s why I ask the question in the first place  I want to know how they approach a problem  and how they proceed when I cast doubt on the way their world works   At this point  most candidates realize their error and find the correct answer   But I had one who insisted his original answer was right  then changed the way he translated the for   to the while     It made for a fascinating interview  but we didn t make an offer   Hope that helps

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The third statement in the for construct is only executed  but its evaluated value is discarded and not taken care of  When the evaluated value is discarded  pre and post increment are equal  They only differ if their value is taken

[c++] Post-increment and pre-increment within a 'for' loop produce same output

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