Alternative for-loop syntax [duplicate]

Answer 1

Unfortunately, this is not easy to read. You are misreading the second case of the for statement. The first semicolon is an integral part of declaration and thus hidden to your eyes. You can easily check such syntax questions by looking into Annex A. There you have:

(6.7) declaration: 
    declaration-specifiers init-declarator-listopt ;
    static_assert-declaration

Answer 2

If you see, the syntax is,

 for ( declaration expression1opt ; expression2opt ) statement

Let's compare it with a general statement

 for (int i = 0; i < 10; i++) printf("%d \t", i);

Here,

• int i = 0; denotes declaration [includes the ;]
• i < 10 denotes expression1opt [optional]
• ; is as per the syntax requirement of ; [must, as described in syntax]
• i++ is the expression2opt [optional]
• printf("%d \t", i); is the statement

Now, in your case,

for (int i = 0, i; i++) { /* ... */ }

• int i = 0, i; denotes declaration
• i++ denotes expression1opt
• ; is missing .....

The last point here produces the error. You need to have the ; to pass the syntax check.