How to check the amount of times a number is present?

You need to look into what methods std::string has available. One of them is length(). It will tell you how long the string is. Another is the [] operator which will give you the character at a given location. With these things, and assuming you have a bunch of numbers in the string a, you can write logic like this:

int alength = a.length();
for( int i = 0; i < alength; ++i )
{
    char cvalue = a[i];
    if( cvalue == ' ' )
        continue;      // skip spaces

    // the character is not a space
    // do something with it here
}

Exactly what you do with the characters there, I can't tell you because the description of your problem is lacking. For example, what does "correct" for your problem mean?

Store all input in a vector and than use a comparison to count the instances. At first make a search to find unique data and than count these.

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