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 <= currentSize; hole = child)
What does this mean?
Thanks.
views:
259answers:
8
+1
A:
It means that the initial value of hole was set before we got to the loop
John Knoeller
2009-12-28 20:14:36
+15
A:
It just means that the user chose not to set a variable to their own starting value.
for(int i = 0; i < x; i++)
is equivalent to...
int i = 0;
for( ; i < x; i++)
EDIT (in response to comments): These aren't exactly equivalent. the scope of the variable i is different.
Sometimes the latter is used to break up the code. You can also drop out the third statement if your indexing variable is modified within the for loop itself...
int i = 0;
for(; i < x;)
{
...
i++
...
}
And if you drop out the second statement then you have an infinite loop.
for(;;)
{
runs indefinitely
}
Pace
2009-12-28 20:15:03
Declaring i inside the for loop pre-initialization isn't *quite* equivalent to declaring it just before the loop. In the former case, its scope is the body of the for statement -- i.e., you can't access it after the for statement's body closes. In the latter case, the variable i can still be accessed after the for loop body.
Edward Loper
2009-12-28 21:01:27
While you *can* use `for(;i<x;)`, it's stylistically better to go with the smipler and easier-to-read `while(i<x)`.
Edward Loper
2009-12-28 21:02:36
Actually those aren't equivalent. "i" is declared only for that scope when declared in the for keyword.
2009-12-28 23:05:01
I also like to go with smipler code. Also, it should be as Redditable as possible. :)
dar7yl
2009-12-29 00:09:31
Actually, you don't need to set any variable in the first expression of a `for` statement. Try this: `for (std::cout<<"first\n"; false;);`. As Edward said in his comment, it is a pre-initialization step.
Thomas Matthews
2009-12-29 06:30:22
+1
A:
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++) { }
Ashish
2009-12-28 20:15:47
C99 allows you to use the "C++" form, so it is no longer necessary to declare the variable outside the for statement. However, in both C and C++, a variable declared inside the for statement goes out of scope after that statement. If you want to use the variable after the for statement is finished, the for(;i<10;i++) syntax is necessary.
Chinmay Kanchi
2009-12-28 21:05:01
+1
A:
You could omit any of the parameters of a for loop. ie: for(;;) {} is about the same as while(true) {}
Alan Alvarez
2009-12-28 20:16:12
+7
A:
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
}
cyborg
2009-12-28 20:16:24
+1
A:
It means that the initial value of hole was set before we got to the loop.
Looks like a list traversal of some kind.
Michal Sznajder
2009-12-28 20:16:33
+1
A:
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)
codaddict
2009-12-28 20:17:35
+1
A:
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;
(Can someone fix the code tags? I can't get them to work.)