CODEWITHSUNDEEP
cfor-loopconditional-statements
RobertS supports Monica Cellio
RobertS supports Monica Cellio

Reputation: 15062

How do Comma-separated expressions as condition in a for-loop work?

I´ve seen a a few times already in answers and questions here on Stack Overflow, a comma-separated list as loop condition for a for-loop like : for(int i = 0, int j = 0 ; i < 2, j < 4; i++, j++) { ... } . How does this work?

How does a comma-separated condition work in a for-loop in C? Like: for(int i = 0, int j = 1 ; i < 5, j < 4; i++, j++) { ... }

Isn´t this invalid because the condition needs to be an evaluated expression? How do the multiple comma-separated expressions get evaluated to one expression in the for-loop?

The mentioned expressions are enclosed by semicolons ; i < 5, j < 4;, and this shall be the condition of the loop. Or am I wrong?

Upvotes: 3

Views: 965

Answers (2)

waldente
waldente

Reputation: 1434

All the expressions in the comma-separated list are evaluated, but only the last expression is used for the for loop condition. So in effect the i<5 condition is ignored, since it's not the last expression.

Upvotes: 1

Petr Skocik
Petr Skocik

Reputation: 60107

The syntax for a for loop with a declaration is (6.8.5):

for ( declaration expression_opt ; expression_opt ) statement

This

for(int i = 0, int j = 1 ; i < 5, j < 4; i++, j++) { }

doesn't parse because

int i = 0, int j = 1 ; isn't syntactically valid as a declaration (or any sentential form in C for that matter) but 

for(int i = 0, j = 1; i < 5, j < 4; i++, j++) { }

would parse, because 

int i = 0, j = 1;

is a valid declaration (with multiple, initializer-containing declarators) and

i < 5, j < 4

is a valid comma-operator-containing expression, as is

i++, j++

.

(Note that i < 5, j < 4 has the same effect as just j < 4 because comma expressions are evaluated left to right and the value of the final part is used, and since i < 5 has no side-effect here, the whole expression is equivalent to just j < 4. You might have wanted i < 5 && j < 4 instead.)

In the original form of the for loop (without a declaration) you have (6.8.5)

 for ( expression_opt ; expression_opt ; expression_opt ) statement

and the semicolons are used to separate the 3 (optional) expressions.

In the declaration-containing form, the first semicolon is parsed as part of the declaration

for ( declaration expression_opt ; expression_opt ) statement

.

Upvotes: 8

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