Effect of semicolon after 'for' loop

Question

Say I want to print a message in C five times using a for loop. Why is it that if I add a semicolon after for loop like this:

for (i=0;i<5;i+         


        

Answer 1

Semicolon is a legitimate statement called null statement ^* that means "do nothing". Since the for loop executes a single operation (which could be a block enclosed in {}) semicolon is treated as the body of the loop, resulting in the behavior that you observed.

The following code

 for (i=0;i<5;i++);
 {
     printf("hello\n");
 }

is interpreted as follows:

• Repeat five times for (i=0;i<5;i++)
• ... do nothing (semicolon)
• Open a new scope for local variables {
• ... Print "hello"
• Close the scope }

As you can see, the operation that gets repeated is ;, not the printf.

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^* See K&R, section 1.5.2