What is the scope of the counter variable in a for loop?

Question

I get the following error in Visual Studio 2008:

Error 1 A local variable named 'i' cannot be declared in this scope because it would give a different meaning to 'i', which is already used in a 'child' scope to denote something else

This is my code:

for (int i = 0; i < 3; i++)
{
  string str = "";
}

int i = 0; // scope error
string str = ""; // no scope error

I understand that str ceases to exist once the loop terminates, but I also thought that the scope of i was confined to the for loop as well.

So i has the same scope as a variable declared just outside of the for loop?

Edit:

Just to be clear, I am using C#. I am debating removing the "C" tag. However, since the correct answer explains the difference between the two, I figure it makes sense to leave both tags.

I had an error in my code comment above:

for (int i = 0; i < 3; i++)
{
  string str = "";
}

int i = 0; // scope error
string str = ""; // also scope error,
                 // because it's equivalent to declaring
                 // string str =""; before the for loop (see below)

Answer 1

I think you're all confusing C++ and C#.

In C++, it used to be that the scope of a variable declared in a for expression was external to the block that followed it. This was changed, some time ago, so that the scope of a variable declared in a for expression was internal to the block that followed it. C# follows this later approach. But neither has anything to do with this.

What's going on here is that C# doesn't allow one scope to hide a variable with the same name in an outer scope.

So, in C++, this used to be illegal. Now it's legal.

for (int i; ; )
{
}
for (int i; ; )
{
}

And the same thing is legal in C#. There are three scopes, the outer in which 'i' is not defined, and two child scopes each of which declares its own 'i'.

But what you are doing is this:

int i;
for (int i; ; )
{
}

Here, there are two scopes. An outer which declares an 'i', and an inner which also declares an 'i'. This is legal in C++ - the outer 'i' is hidden - but it's illegal in C#, regardless of whether the inner scope is a for loop, a while loop, or whatever.

Try this:

int i;
while (true)
{
    int i;
}

It's the same problem. C# does not allow variables with the same name in nested scopes.

Answer 2

The incrementor does not exist after the for loop.

for (int i = 0; i < 10; i++) { }
int b = i; // this complains i doesn't exist
int i = 0; // this complains i would change a child scope version because the for's {} is a child scope of current scope

The reason you can't redeclare i after the for loop is because in the IL it would actually declare it before the for loop, because declarations occur at the top of the scope.

Answer 3

Yea. Syntactically: The new scope is inside the block defined by the curly strings. Functionally: There are cases in which you may want to check the final value of the loop variable (for example, if you break).

Answer 4

Just some background information: The sequence doesn't come into it. There's just the idea of scopes - the method scope and then the for loop's scope. As such 'once the loop terminates' isn't accurate.

You're posting therefore reads the same as this:

int i = 0; // scope error
string str = ""; // no scope error

for (int i = 0; i < 3; i++)
{
  string str = "";
}

I find that thinking about it like this makes the answers 'fit' in my mental model better..