c++ C++ How to assign an number (character type) in a char array to an actual integer value?

[MikeHoward]

My goal is to read the first line of my colors.txt file and assign that integer value to `blue`. However, in my assignment I am instructed to use a char array `red` in order to assist in getting the first line of code. I have succeeded in doing so, but am having trouble with assigning it to `blue`.

    int main() {
        ifstream col; 
        char red[50]; 
        int blue = 0; 
        
        col.open("colors.txt");
        col.getline(red, 2, '\n');
        cout << "Blue:  " << red << endl; 
    }

**colors.txt file below**

 

9
8
7
6

So far I have thought about using the atoi function, but I'm not exactly sure if it will work in this case. Would it work if I copied the character array into the heap and then used a pointer to get the value?

[Mr_KoKa]

After you read a number to red array you can use atoi on it, you don't need to allocate another array for it.

 

[Questargon]

The easy way out:

 

#include <iostream>
#include <fstream>

int main() {
  ifstream col; 
  char red[50]; 
  int blue = 0; 

  col.open("colors.txt");
  col.getline(red, 2, '\n');
  // Use atoi to convert C-Style-String to an int value. 
  // This is NOT recommended though, because it doesn't handle errors well,
  // see here under "Exceptions": http://www.cplusplus.com/reference/cstdlib/atoi/
  blue = atoi(red); 
  cout << "Blue:  " << blue << endl; 
}

And a bit more complex, but correct:

#include <iostream>
#include <fstream>
#include <stdlib.h>     /* strtol */
#include <ctype.h>      /* isspace */

int main() {
    ifstream col; 
    char red[50]; 
    int blue = 0; 

    col.open("colors.txt");
    col.getline(red, 2, '\n');

    // "endp" points to the end where conversion stopped
    // see http://www.cplusplus.com/reference/cstdlib/strtol/         
    char *endp;
    blue = strtol(red, &endp, 10);
    // Check, whether the whole string was converted into a number
    // If that's the case, endp points towards the character directly after the number part.
    // That should be a whitespace or a 0 if the number has been converted completely.
    if (*endp == 0 || isspace(*endp))
      cout << "Blue:  " << blue << endl; 
    else
      cerr << "Invalid number: Unconvertable characters detected!" << endl;
}

HTH ;-)

Edited September 22, 2016 by Questargon
Handling eventual whitespaces after the number part of the string.