CODEWITHSUNDEEP
c++for-loopiteratorinitializationunordered-set
Asta Lee
Asta Lee

Reputation: 35

"iterator cannot be defined in the current scope" error

I am a novice C++ programmer working through a simple problem to print out name-and-score pairs together. Here I have used a std::unordered_set for the names and a vector for the scores (to accept duplicate scores, but not names) and that works fine.

But one thing puzzles me about the result, and that's that if I try to initialize the iterator in the for loop, the compiler gives me an error that says

the iterator "cannot be defined in the current scope."

This gives the error:

for (int i = 0, std::unordered_set<std::string>::iterator it = names.begin();
                                                i < names.size(); i++, it++)
{
    std::cout << *it << ", " << scores[i] << '\n';
}

But moved outside the loop, it works fine:

std::unordered_set<std::string>::iterator it = names.begin();
for (int i = 0; i < names.size(); i++, it++)
{
    std::cout << *it << ", " << scores[i] << '\n';
}

Why must the iterator be initialized outside the loop here? Sorry for the simple question, I've searched elsewhere and have not found a clear answer for this.

Upvotes: 2

Views: 389

Answers (1)

JeJo
JeJo

Reputation: 32722

In C++ for-loop

for ( declaration-or-expression(optional) ; declaration-or-expression(optional) ; expression(optional) )
      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

init-statement

  • either an expression statement (which may be a null statement ";")
  • a simple declaration, typically a declaration of a loop counter variable with initializer, but it may declare arbitrary many variables Note that any init-statement must end with a semicolon ;, which is why it is often described informally as an expression or a declaration followed by a semicolon.

Therefore, you can either declare the variable of the same type; ex:

for (int i = 0, j = 1; ... ; i++, j++) {...}
     ^^^^^^^^^^^^^^^^

or initialize the variables of any type that you have declared previously. Ex

std::unordered_set<std::string>::iterator it;
int i;
for (it = names.begin(),  i = 0; i < names.size(); i++, it++) { ... }
     ^^^^^^^^^^^^^^^^^^^^^^^^^^^

Hence, your attempt is failed.

Upvotes: 3

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