What is the full "for" loop syntax in C?

Solution 1:

The comma is not exclusive of for loops; it is the comma operator.

x = (a, b);

will do first a, then b, then set x to the value of b.

The for syntax is:

for (init; condition; increment)
    ...

Which is somewhat (ignoring continue and break for now) equivalent to:

init;
while (condition) {
    ...
    increment;
}

So your for loop example is (again ignoring continue and break) equivalent to

p=0;
while (p+=(a&1)*b,a!=1) {
    ...
    a>>=1,b<<=1;
}

Which acts as if it were (again ignoring continue and break):

p=0; 
while (true) {
    p+=(a&1)*b;
    if (a == 1) break;
    ...
    a>>=1;
    b<<=1;
}

Two extra details of the for loop which were not in the simplified conversion to a while loop above:

• If the condition is omitted, it is always true (resulting in an infinite loop unless a break, goto, or something else breaks the loop).
• A continue acts as if it were a goto to a label just before the increment, unlike a continue in the while loop which would skip the increment.

Also, an important detail about the comma operator: it is a sequence point, like && and || (which is why I can split it in separate statements and keep its meaning intact).

Changes in C99

The C99 standard introduces a couple of nuances not mentioned earlier in this explanation (which is very good for C89/C90).

First, all loops are blocks in their own right. Effectively,

for (...) { ... }

is itself wrapped in a pair of braces

{
for (...) { ... }
}

The standard sayeth:

ISO/IEC 9899:1999 §6.8.5 Iteration statements

¶5 An iteration statement is a block whose scope is a strict subset of the scope of its enclosing block. The loop body is also a block whose scope is a strict subset of the scope of the iteration statement.

This is also described in the Rationale in terms of the extra set of braces.

Secondly, the init portion in C99 can be a (single) declaration, as in

for (int i = 0; i < sizeof(something); i++) { ... }

Now the 'block wrapped around the loop' comes into its own; it explains why the variable i cannot be accessed outside the loop. You can declare more than one variable, but they must all be of the same type:

for (int i = 0, j = sizeof(something); i < j; i++, j--) { ... }

The standard sayeth:

ISO/IEC 9899:1999 §6.8.5.3 The for statement

The statement

for ( clause-1 ; expression-2 ; expression-3 ) statement

behaves as follows: The expression expression-2 is the controlling expression that is evaluated before each execution of the loop body. The expression expression-3 is evaluated as a void expression after each execution of the loop body. If clause-1 is a declaration, the scope of any variables it declares is the remainder of the declaration and the entire loop, including the other two expressions; it is reached in the order of execution before the first evaluation of the controlling expression. If clause-1 is an expression, it is evaluated as a void expression before the first evaluation of the controlling expression.¹³³⁾

Both clause-1 and expression-3 can be omitted. An omitted expression-2 is replaced by a nonzero constant.

¹³³⁾ Thus, clause-1 specifies initialization for the loop, possibly declaring one or more variables for use in the loop; the controlling expression, expression-2, specifies an evaluation made before each iteration, such that execution of the loop continues until the expression compares equal to 0; and expression-3 specifies an operation (such as incrementing) that is performed after each iteration.

Solution 2:

The comma simply separates two expressions and is valid anywhere in C where a normal expression is allowed. These are executed in order from left to right. The value of the rightmost expression is the value of the overall expression.

for loops consist of three parts, any of which may also be empty; one (the first) is executed at the beginning, and one (the third) at the end of each iteration. These parts usually initialize and increment a counter, respectively; but they may do anything.

The second part is a test that is executed at the beginning of each execution. If the test yields false, the loop is aborted. That's all there is to it.

Solution 3:

The C style for loop consists of three expressions:

for (initializer; condition; counter) statement_or_statement_block;

• The initializer runs once, when the loop starts.
• The condition is checked before each iteration. The loop runs as long it evaluates to true.
• The counter runs once after each iteration.

Each of these parts can be an expression valid in the language you write the loop in. That means they can be used more creatively. Anything you want to do beforehand can go into the initializer, anything you want to do in between can go into the condition or the counter, up to the point where the loop has no body anymore.

To achieve that, the comma operator comes in very handy. It allows you to chain expressions together to form a single new expression. Most of the time it is used that way in a for loop, the other implications of the comma operator (e.g. value assignment considerations) play a minor role.

Even though you can do clever things by using syntax creatively - I would stay clear of it until I find a really good reason to do so. Playing code golf with for loops makes code harder to read and understand (and maintain).

The wikipedia has a nice article on the for loop as well.

Solution 4:

Everything is optional in a for loop. We can initialize more than one variable, we can check for more than one condition, we can iterate more than one variable using the comma operator.

The following for loop will take you into an infinite loop. Be careful by checking the condition.

for(;;)