How to solve undeclared variable in the case below

Question

I am a beginner at c++ and I want to ask about this specific error. I don't know why it always appears that I didn't declare the variable wherein in fact, I did (see my code below).

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palme · Answer 1 · 2020-02-05T07:22:29.737

0

You initiated the decoder array in both cases in an if statement. So the compiler does not know it outside of the if. Write the variable before your first if-else statement and than change it, if needed, within the if statement.

edited Feb 05 '20 at 07:22

answered Feb 05 '20 at 07:06

palme

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score 0 · Answer 2 · answered Feb 05 '20 at 07:25

The problem is in the array declaration, it is declared inside of a scope:

#include <iostream>

using namespace std;

int main()
{
    int a = 2; // local variable

    if (true)
    {
        int a = 4; // local if statement variable
        cout << a << endl; // here the output will be 4
    }
    cout << a << endl; // here the output will be 2, also here 'a' from the if statement, does not longer exist, when the if statement is ended all the variables declared inside will be deleted

    return 0;
}

You can also create your own scope with out 'if' statement:

int a = 2; // local variable

{
    int a = 4; // local scope statement variable
    cout << a << endl; // here the output will be 4
}
cout << a << endl; // here the output will be 2, also 'a' from the previous scope does not exist

score 0 · Answer 3 · answered Feb 05 '20 at 07:51

Here is a useful link to learn about local variables and scope.

Your particular problem is that your variable decoder is local to the block scope (you declared it within a if statement), and you try to use it outside that scope. However, once exited the scope, all variables local to that scope are destroyed.

if (number = 987)
{
    int decoder[3] = {0, 1, 2};
} // and here decoder is destroyed

You could declare decoder outside the block and define it inside the block:

int decoder[3];
if (number = 987)
{
    // populate decoder
}
// decoder still exists

How to solve undeclared variable in the case below

3 Answers3