Thread: definition inside block

Off the top of my head I can't think of any advantages.

As for disadvantages:

The scope of variable is only within the whlie loop. so if another function needed to use it, it wouldn't have access to it.

>The scope of variable is only within the whlie loop. so if another
>function needed to use it, it wouldn't have access to it
That's also one of the advantages. Another disadvantage is that the memory for that variable is allocated and freed with every iteration of the loop, often causing a performance drop. Generally, you only need to create variables at the top of a function if the function was written properly.

-Prelude

Originally posted by Prelude
>The scope of variable is only within the whlie loop. so if another
>function needed to use it, it wouldn't have access to it
That's also one of the advantages. Another disadvantage is that the memory for that variable is allocated and freed with every iteration of the loop, often causing a performance drop. Generally, you only need to create variables at the top of a function if the function was written properly.

-Prelude

You know, Prelude,

I really wished you were one of my instructors at school becuase what I posted was the def. we were taught for scope.

I'll pay you $.05 a day for tutoring!

This is really C++ but it kinda falls withing the same scope*smile*

Code:

void f()
{
    for(int i = 0; i < 5; i++)
    {
        something();
    }

    for(int i = 0; i < 5; i++)
    {
        something_else();
    }
}

This will generate an error because in this case i is not local to the for loop.

Originally posted by Barjor
This is really C++ but it kinda falls withing the same scope*smile*

Code:

void f()
{
    for(int i = 0; i < 5; i++)
    {
        something();
    }

    for(int i = 0; i < 5; i++)
    {
        something_else();
    }
}

This will generate an error because in this case i is not local to the for loop.

What? That is legal C++ code. The reason it doesn't work in C is that C doesn't allow variable declarations in the middle of the scope. For example:

Code:

/*C*/
int main( void )
{
    printf("Hello.\n");
    int x; /* invalid this has to go before the printf */

    {
        int y; /* valid; this happens first in the scope */
        printf("Wheee...");
    }
    return x;
}


//C++
int main( void )
{
    cout << "Hello" << endl;
    int x; // Valid. C++ Allows variable declaration almost anywhere.

    {
        int y; // valid; this happens first in the scope
        printf("Wheee...");
    }

    return x; //return some random undefined value to the OS...
}

That is the difference between C++ and C regarding variable declarations in a given scope.

Quzah.

No it's not valid C++. In my compiler it says redefenition of i. My point is that i is not local to the for loop

Originally posted by Salem
> This will generate an error because in this case i is not local to the for loop
This is true of the old C++ standard, but not the new C++ standard. The new standard specifically makes such declarations exist just for the loop itself, not the rest of the block.

Now when you say it. I kinda remeber reading something that VC++ didn't follow the standard about for loops. Anyone know if this is fixed in VC.NET? How about this?

Code:

for(int i = 0; i < 5; i++)
{
    for(int i = 0; i < 5; i++)
    {
          i++;//will this change the second i but not the first?
    }
}

Or will that not even compile?

This code compiled in VC 6.0 sp5

Code:

#include <iostream.h>

int main()
{
	for(int i = 0; i < 3; i++)
	{
			for(int i = 0; i < 3; i++)
			{
				cout<<"Inner"<<i<<"\n";
			}

		cout<<"Outer"<<i<<"\n";
	}

	return 0;
}

gives me this output

Code:

Inner0
Inner1
Inner2
Outer3
Inner0
Inner1
Inner2
Outer3
Inner0
Inner1
Inner2
Outer3

So how come the outer loop runs three times when the outer i is 3 and really should break the outer loop?
If I do it this way

Code:

#include <iostream.h>

int main()
{
	for(int i = 0; i < 3; i++)
	{
		cout<<"Outer"<<i<<"\n";

		for(int i = 0; i < 3; i++)
		{
			cout<<"Inner"<<i<<"\n";
		}
	}

	return 0;
}

I get this output

Code:

Outer0
Inner0
Inner1
Inner2
Outer1
Inner0
Inner1
Inner2
Outer2
Inner0
Inner1
Inner2

This is more in line with what I would expect. It looks like the inner i's scope is "leaking" on my first piece of code

If I compile my first code in K-Develop it gives me these varnings

Code:

name lookup of 'i' changed
matches this 'i' under ISO standard rules
matches this 'i' under old rules

and my output is

Code:

Inner0
Inner1
Inner2
Outer0
Inner0
Inner1
Inner2
Outer1
Inner0
Inner1
Inner2
Outer2

As it should
So I guess you where right Quazah and Salem and I and MS where wrong

>Doesn't seem to be true all the time - tried it with GCC, and all
>the variable space is allocated at the start of the function.
>Scope in this case is enforced at compile time, not run time.
Agreed, I ran a test on MSVC in debug mode and the space was reallocated with each iteration. So either the result will be different with each implementation or it would only show up when the compiler makes no optimizations.

-Prelude