Why does using the same count variable name in nested FOR loops work?

c++ scoping
M Perry picture M Perry · Mar 6, 2010 · Viewed 7.8k times · Source

Why does the following not give an error?

for (int i=0; i<10; ++i) // outer loop
{
    for (int i=0; i<10;++i) // inner loop
    {
    //...do something
    }
//...do something else
}

The way I understand it, variables in braces ({...}) are in scope only within these braces. But the inner loop is inside the braces of the outer loop. So as soon as I declare int i=0 for the inner loop, shouldn't I get an error about multiple definitions?

Answer

fbrereto picture fbrereto · Mar 6, 2010

You are actually making a new variable with the same name as another variable. Since they are in different scopes this is allowed, and the variable in the inner scope "owns" the name. You will not be able to access the outer-scoped i inside the inner scope.

The for loop declaration itself is part of the scope of the for loop, so counts as part of the inner-scope in the case of the second i.

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