Question

I do not understand the termination parameter of this for loop. What does it mean? Specifically, what do the ?, ->, and : 0 represent?

for( i = 0; i < (sequence ? sequence->total : 0); i++ )

Answer 1

This: (sequence ? sequence->total : 0) (it’s called a “ternary if”, since it takes three inputs) is like saying:

if (sequence)
    replaceEntireExpressionWith(sequence->total);
else
    replaceEntireExpressionWith(0);

-> is a dereferencer, just like *, but it makes user data-types like structs easy to use.

sequence->total means sequence is a pointer to a one of those data types, and you are accessing the total property of what it is pointing to. It’s exactly the same as:

(*sequence).total;

So the loop:

for( i = 0; i < (sequence ? sequence->total : 0); i++ )

exits when sequence evaluates to false, since 0 == false.

The ternary if construction is used to make sure they aren’t dereferencing (->) a null pointer, because if they just put sequence->total as the condition, they would be dereferencing it every time. Unhappy! =(