not declared in this scope (arrays in C++)

Question

I'm a beginner in C++ programming I have a text file which has 1 million prime numbers separated by spaces. I want to put them in an int array primes[]. Following is the code that I have written:

int main()
{
    ifstream infile;
    infile.open("C:/Users/DELL/Desktop/primes1.txt");

    //check for error
    if(infile.fail()){cerr<<"Error Opening File"<<endl;
    exit(1);}
    int i=0;
    primes = new int[1000001];
    while(i != infile.eof()){
        infile>>primes[i];
        i++;
    }

    cout<<  primes[4]  <<endl;

    return 0;
}

When I build and run, it gives the following error:

"Error: 'primes' was not declared in this scope"

What is the solution to this?

`primes = new int[1000001];` --> `int *primes = new int[1000001];` — P.P, Jun 05 '14 at 10:50
To echo what @FredOverflow said, your loop is very wrong, and it's still wrong after applying advice from the answers below. — Lightness Races in Orbit, Jun 05 '14 at 10:59
If you've already been question-banned in the past, then you should take more care _this time_, not promising you'll get around to it in the future and begging people not to downvote you in the meantime. — Lightness Races in Orbit, Jun 05 '14 at 11:18

score 5 · Answer 1 · answered Jun 05 '14 at 11:02

5

I highly recommend to use a vector instead of an array on the heap to prevent resource leaks:

std::vector<int> primes;
int p;
while (infile >> p)
{
    primes.push_back(p);
}

answered Jun 05 '14 at 11:02

fredoverflow

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score 5 · Answer 2 · answered Jun 05 '14 at 11:11

5

C++ ~~is beautiful~~ has nice stuff in its library that allows you to do this in a high-level, concise, declarative manner:

std::vector<int> primes(std::istream_iterator<int>{infile},
                        std::istream_iterator<int>{});

answered Jun 05 '14 at 11:11

+1 I'd love to see the C++98 solution as well! – fredoverflow Jun 05 '14 at 11:11
@FredOverflow substitute the braces for parentheses, and parenthesise either argument to the constructor of `std::vector`. – Jun 05 '14 at 11:13
Or use std::copy with back_inserter. – Cat Plus Plus Jun 05 '14 at 11:14
3

By the way you can replace the second parameter with just {} so `std::vector primes(std::istream_iterator{infile},{});` works. – Veritas Jun 05 '14 at 11:15

score 1 · Answer 3 · answered Jun 05 '14 at 10:52

1

What is the solution to this?

Declare primes. The identifier must be declared first in order to assign something to it.

For example:

int *primes = new int[1000001];

answered Jun 05 '14 at 10:52

Karadur

1,226
9
16

not declared in this scope (arrays in C++)

3 Answers3