What's allowed in the header of a for loop?

In C#, I've seen some strange and complex logic in the headers of loops.

What is/isn't possible in the h开发者_高级运维eader of a for loop? Is it possible to have more than one incrementor/variable?

Thanks

Is it possible to have more than one incrementor/variable?

Yes, it is possible. That is, this is perfectly legal:

// reverse b into a
for (int i = 0, j = n - 1; i < n; i++, j--) {
    a[i] = b[j]
}

What is/isn't possible in the header of a for loop?

That is exactly what the grammar will tell you. Here is the grammar for a for statement in C#:

for-statement:
    for(for-initializer_opt; for-condition_opt; for-iterator_opt)
        embedded-statement
for-initializer:
    local-variable-declaration
    statement-expression-list
for-condition:
    boolean-expression
for-iterator:
    statement-expression-list
statement-expression-list:
    statement-expression
    statement-expression-list, statement-expression

Note that both the for-initializer and the for-iterator allow compound statements via statement-expression-list. See §8.8.3 of the language specification for addtional details. You'll probably also want to visit §8.5.1 of the specification for exactly what local-variable-declaration entails (hint: int i = 0, j = n - 1, k = 42 is legal but int i = 0, j = n - 1, long k = 42 is not).

The ECMA-334 C# Language Specification

C# Language Specifications