Multiple conditions in a C for loop

Question

I came across this piece of code  I generally use   amp  amp   or      to separate multiple conditions in a for loop  but this code uses commas to do that   Surprisingly  if I change the order of the conditions the output varies    include lt stdio h gt   int main         int i  j 2       for i 0  j gt  0 i lt  5  i                   printf   d    i j            j--            return 0      Output   2 2 2 2 2 2   include lt stdio h gt   int main        int i  j 2       for i 0  i lt  5 j gt  0  i                   printf   d    i j            j--            return 0      Output   2 2 2  Can somebody explain the reason  It seems to be checking only the last comma-separated condition

User · Answer

Do not use this code  whoever wrote it clearly has a fundamental misunderstanding of the language and is not trustworthy  The expression   j  gt   0  i  lt   5   evaluates  j    0   then throws it away and does nothing with it  Then it evaluates  i  lt   5  and uses that  and only that  as the condition for ending the loop  The comma operator can be used meaningfully in a loop condition when the left operand has side effects  you ll often see things like   for  i   0  j   0  i  lt  10    i    j          in which the comma is used to sneak in extra initialization and increment statements  But the code shown is not doing that  or anything else meaningful

User · Answer

Completing Mr  Crocker s answer  be careful about    or -- operators or I don t know maybe other operators  They can affect the loop  for example I saw a code similar to this one in a course   for int i 0  i   i lt -1  i lt 3  printf    d   i      The result would be   1 2   So the first statement has affected the loop while the outcome of the following is lots of zeros   for int i 0  i lt 3  printf    d   i

User · Answer

Of course it is right what you say at the beginning  and C logical operator  amp  amp  and    are what you usually use to  connect  conditions  expressions that can be evaluated as true or false   the comma operator is not a logical operator and its use in that example makes no sense  as explained by other users  You can use it e g  to  concatenate  statements in the for itself  you can initialize and update j altogether with i  or use the comma operator in other ways   include  lt stdio h gt   int main void      as std wants     int i  j        init both i and j  condition  we suppose  amp  amp  is the  original       intention  update i and j   for i 0  j 2  j gt  0  amp  amp  i lt  5  i    j--             printf   d    i j         return 0

User · Answer

There is an operator in C called the comma operator   It executes each expression in order and returns the value of the last statement   It s also a sequence point  meaning each expression is guaranteed to execute in completely and in order before the next expression in the series executes  similar to  amp  amp  or

User · Answer

The comma operator evaluates all its operands and yields the value of the last one  So basically whichever condition you write first  it will be disregarded  and the second one will be significant only   for  i   0  j  gt   0  i  lt   5  i      is thus equivalent with  for  i   0  i  lt   5  i      which may or may not be what the author of the code intended  depending on his intents - I hope this is not production code  because if the programmer having written this wanted to express an AND relation between the conditions  then this is incorrect and the  amp  amp  operator should have been used instead

User · Answer

Wikipedia tells what comma operator does    In the C and C   programming languages  the comma operator  represented by the token    is a binary operator that evaluates its first operand and discards the result  and then evaluates the second operand and returns this value  and type

User · Answer

The comma expression takes on the value of the last  eg  right-most  expression   So in your first loop  the only controlling expression is i lt  5  and j gt  0 is ignored   In the second loop  j gt  0 controls the loop  and i lt  5 is ignored     As for a reason    there is no reason   This code is just wrong   The first part of the comma-expressions does nothing except confuse programmers   If a serious programmer wrote this  they should be ashamed of themselves and have their keyboard revoked

[c] Multiple conditions in a C 'for' loop

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