Help me to understand the termination parameter of this C++ for loop

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! =(

Answer 2

if sequence not valid, or is the member called "total" is reached by i.

The ? says if sequence is not valid the loop ends when i = 0. (so it wont run)

Answer 3

You're seeing two different operators.

1. The conditional operator

   condition ? truePart : falsePart
   

   This operator check whether condition is true, and evaluates to either truePart or falsePart.

2. The deferenced member operator

   pointer->member
   

   This is equivalent to (*pointer).member.

Your expression will evaluate to 0 if sequence is null, and the total property of the structure that sequence points to if it isn't null.

Answer 4

there are two things at work here:

the ? and : syntax are the syntax for a ternary operation

http://en.wikipedia.org/wiki/Ternary_operation

other answers here explain ternary operators concisely

the -> syntax is to access a member from a pointer as opposed to a reference or value

this code is equivalent to:

int total = 0;
if(sequence){
    total = sequence->total;
}

for( i=0; i < total; ++i){
    ...
}

Answer 5

? : is the ternary operator. It evaluates the expression before the ?, and the result is the expression after the ? if true, or the expression after the : if false.

-> is what I'd call "operator arrow", but what it essentially does is dereference the pointer sequence, and then access a member called total.

Two things here:

1. The purpose of this is to ensure sequence is not a NULL pointer before attempting to dereference it with ->,
2. The loop should be rewritten like this:
   for( i = 0; sequence && i < sequence->total; i++ )
   That removes the confusion regarding what is going on.