Concatenation only works when it feels like it?

Question

I am trying to get the following code to work:

#include <iostream>
#include <string>
#include <vector>
#include <fstream>
#include <sstream>
#include <cmath>

using namespace std;

bool prime_test(int num);
void stringRotation(string& str);

int main()
{
    vector<string> primes;
    ifstream infile("PRIMES1T.txt");

    // checks to see if there was any problems opening the .txt
    if (infile.is_open()) {
        string line = "";
        while(getline(infile,line)) {
            primes.push_back(line);
        }

        // rotates our string and tests if the number is still prime
        vector<string> primes2;
        for (int i = 0; i < primes.size(); i++) {
            string str = primes[i];
            for (int j = 0; j < str.length(); j++) {
                stringRotation(str);
                int value = atoi(str.c_str());
                if (prime_test(value) == false) {
                    break;
                }
                if (j == str.length()-1) {
                    if (prime_test(value) == true) {
                        primes2.push_back(primes[i]);
                    }
                }
            }
        }

        cout << "There are " << primes2.size() << " primes that work.";
        cout << endl;
    }
    else {
        cout << "File failed to open." << endl;
    }

    return 0;
}

// tests to see if num is a prime number
bool prime_test(int num) {
    if (num == 1) {
        return false;
    }
    // Finds first integer value larger than the sqrt of num
    // since that is all we really need.
    double dnum = num;
    double sqrt_dnum = sqrt(dnum);
    int counter = ceil(sqrt_dnum);

    for (int i = 2; i < counter; i++) {
        if (num == 2) {
            break;
        }
        if (num%i == 0) {
            return false;
        }
    }
    return true;
}

// rotates a string
void stringRotation(string& str) {
    int len = str.length();

    // converts a char variable into a string variable
    stringstream ss;
    string ch;
    char c = str.at(0);
    ss << c;
    ss >> ch;

    str = str.substr(1,str.length());
    str = str.append(ch);
    cout << str << endl;
}

What it does is it takes a prime number say 999983, cuts off the first digit 9, and then adds it to the end of the rest of the number so that it spits out the new number 999839. It then tests whether or not this new number is prime or not and repeats the process until the original number is returned. If the number is prime every time we do this process, then we add that number to the vector primes2.

The problem I have is that the stringRotation function does not work properly for some reason. I have tested it by trying to outputting the string before adding the digit that was removed and outputting the string after adding the digit. It does not concatenate properly. It will cut off the first digit in 999983 so that we have str = '99983' and ch = '9' but then when I do str.append(ch), it still gives me 99983. I have also tried variations like str = str.append(ch) and str = str + ch.

I have tried copying just the function over to a different .cpp file to compile only adding a declaration for str by setting str to "999983" and it works fine.

EDIT I changed stringRotation to:

void stringRotation(string& str) {
    int len = str.length();
    char ch = str.at(0);

    cout << ch << endl;

    str = str.substr(1,str.length());
    str.append(1,ch);
    cout << str << endl;
}

but the problem still persists. I have also tried string.push_back(ch) with no luck.

Answer 1

In your programmer career, you will need to always make sure that your input is handled well. If you are loading data from a file which is not guaranteed to have a specific content scheme, you will always need to make sure that you prepare your data before parsing. In this particular case you need to make sure that your "numbers" are indeed numbers and execute your stringRotation on values which are guaranteed to be numbers.