How do I put two increment statements in a C for loop

Question

I would like to increment two variables in a for-loop condition instead of one   So something like   for  int i   0  i    5    i and   j       do something i  j     What is the syntax for this

User · Answer

Try not to do it   From http   www research att com  bs JSF-AV-rules pdf      AV Rule 199   The increment expression in a for loop will perform no action other than to change a single    loop parameter to the next value for the loop        Rationale  Readability

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A common idiom is to use the comma operator which evaluates both operands  and returns the second operand  Thus   for int i   0  i    5    i   j       do something i j     But is it really a comma operator   Now having wrote that  a commenter suggested it was actually some special syntactic sugar in the for statement  and not a comma operator at all  I checked that in GCC as follows   int i 0  int a 5  int x 0   for i  i lt 5  x i   a         printf  i  d a  d x  d n  i a x       I was expecting x to pick up the original value of a  so it should have displayed 5 6 7   for x  What I got was this  i 0 a 5 x 0 i 1 a 6 x 0 i 2 a 7 x 1 i 3 a 8 x 2 i 4 a 9 x 3   However  if I bracketed the expression to force the parser into really seeing a comma operator  I get this  int main        int i 0      int a 5      int x 0       for i 0  i lt 5  x  i   a              printf  i  d a  d x  d n  i a x            i 0 a 5 x 0 i 1 a 6 x 5 i 2 a 7 x 6 i 3 a 8 x 7 i 4 a 9 x 8   Initially I thought that this showed it wasn t behaving as a comma operator at all  but as it turns out  this is simply a precedence issue - the comma operator has the lowest possible precedence  so the expression x i   a   is effectively parsed as  x i    a    Thanks for all the comments  it was an interesting learning experience  and I ve been using C for many years

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Try this  for int i   0  i    5    i    j      do something i j

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for  int i   0  i    5    i    j       do something i  j

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Use Maths  If the two operations mathematically depend on the loop iteration  why not do the math   int i  j   That have some meaningful values in them  for  int counter   0  counter  lt  count max    counter       do something  counter i  counter j     Or  more specifically referring to the OP s example   for int i   0  i    5    i      do something i  j i     Especially if you re passing into a function by value  then you should get something that does exactly what you want

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int main        int i 0      int a 0      for i i lt 5 i   a             printf   d  d n  a i

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I agree with squelart  Incrementing two variables is bug prone  especially if you only test for one of them   This is the readable way to do this   int j   0  for int i   0  i  lt  5    i        do something i  j         j      For loops are meant for cases where your loop runs on one increasing decreasing variable  For any other variable  change it in the loop   If you need j to be tied to i  why not leave the original variable as is and add i   for int i   0  i  lt  5    i        do something i a i       If your logic is more complex  for example  you need to actually monitor more than one variable   I d use a while loop

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I came here to remind myself how to code a second index into the increment clause of a FOR loop  which I knew could be done mainly from observing it in a sample that I incorporated into another project  that written in C     Today  I am working in C   but I felt sure that it would obey the same rules in this regard  since the FOR statement is one of the oldest control structures in all of programming  Thankfully  I had recently spent several days precisely documenting the behavior of a FOR loop in one of my older C programs  and I quickly realized that those studies held lessons that applied to today s C  problem  in particular to the behavior of the second index variable   For the unwary  following is a summary of my observations  Everything I saw happening today  by carefully observing variables in the Locals window  confirmed my expectation that a C  FOR statement behaves exactly like a C or C   FOR statement    The first time a FOR loop executes  the increment clause  the 3rd of its three  is skipped  In Visual C and C    the increment is generated as three machine instructions in the middle of the block that implements the loop  so that the initial pass runs the initialization code once only  then jumps over the increment block to execute the termination test  This implements the feature that a FOR loop executes zero or more times  depending on the state of its index and limit variables  If the body of the loop executes  its last statement is a jump to the first of the three increment instructions that were skipped by the first iteration  After these execute  control falls naturally into the limit test code that implements the middle clause  The outcome of that test determines whether the body of the FOR loop executes  or whether control transfers to the next instruction past the jump at the bottom of its scope  Since control transfers from the bottom of the FOR loop block to the increment block  the index variable is incremented before the test is executed  Not only does this behavior explain why you must code your limit clauses the way you learned  but it affects any secondary increment that you add  via the comma operator  because it becomes part of the third clause  Hence  it is not changed on the first iteration  but it is on the last iteration  which never executes the body    If either of your index variables remains in scope when the loop ends  their value will be one higher than the threshold that stops the loop  in the case of the true index variable  Likewise  if  for example  the second variable is initialized to zero before the loop is entered  its value at the end will be the iteration count  assuming that it is an increment       not a decrement  and that nothing in the body of the loop changes its value

[c++] How do I put two increment statements in a C++ 'for' loop?

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