What does it mean when the first “for” parameter is blank?

Question

I have been looking through some code and I have seen several examples where the first element of a for cycle is omitted.

An example:

for ( ; hole*2          


        

Answer 1

That means loop control variable is initialized before the for loop .

For C  code,

int i=0;
for( ; i <10 ; i++) { } //since it does not allow variable declaration in loop 

For C++  code,

for(int i=0 ; i <10 ; i++) { }  

Answer 2

It means that the initial value of hole was set before we got to the loop

Answer 3

You could omit any of the parameters of a for loop. ie: for(;;) {} is about the same as while(true) {}

Answer 4

The for construct is basically ( pre-loop initialisation; loop termination test; end of loop iteration), so this just means there is no initialisation of anything in this for loop.

You could refactor any for loop thusly:

pre-loop initialisation
while (loop termination test) {
...
end of loop iteration
}

Answer 5

Some people have been getting it wrong so I just wanted to clear it up.

int i = 0;
for (; i < 10; i++)

is not the same as

for (int i = 0; i < 10; i++)

Variables declared inside the for keyword are only valid in that scope.

To put it simply.

Valid ("i" was declared outside of the loops scope)

int i = 0;
for (; i < 10; i++)
{
  //Code
}
std::cout << i;

InValid ("i" does not exist outside the loop scope)

for (int i = 0; i < 10; i++)
{
  //Code
}
std::cout << i;

Answer 6

Suppose you wanted to

for (hole=1 ; hole*2 <= currentSize; hole = child)

But the value of hole just before the for loop was already 1, then you can slip this initilization part of the loop:

/* value of hole now is 1.*/
for ( ; hole*2 <= currentSize; hole = child)