convert an array string to intger c++ (beginner) - c++

Question

I have a text file which that contains only numbers inside , and i have successfully pulled the numbers from the file and stored it inside an array:
my problem is that the array is "string" and i cant do mathematical operations on the array like Addition and Subtraction
I have tried to use atoi(array[i][j].c_str()) to convert it to intger
but it gives me only the first digit of a number!
my program looks like this for now , its a mess I know :(
#include <iostream>
#include <fstream>
#include <string>
#include <stdlib.h>
using namespace std;
int main()
{
ifstream iFile("input.txt");
string line;
string array[7][7];
for (int i=0;i<7;i++){
for (int j=0;j<6;j++){
getline(iFile,line);
if (!line.empty()){
array[i][j]=line;
}
else {
break;
}
}
}
cout<<"number of processes is: "<<array[0][0]<<endl;
cout<<"resource types: "<<array[1][0]<<endl<<endl;
cout<<"Allocation Matrix:"<<endl;
cout<<" A B C D"<<endl;
cout<<"0: "<<array[2][0]<<endl;
cout<<"1: "<<array[2][1]<<endl;
cout<<"2: "<<array[2][2]<<endl;
cout<<"3: "<<array[2][3]<<endl;
cout<<"4: "<<array[2][4]<<endl;
cout<<"Max Matrix:"<<endl;
cout<<" A B C D"<<endl;
cout<<"0: "<<array[3][0]<<endl;
cout<<"1: "<<array[3][1]<<endl;
cout<<"2: "<<array[3][2]<<endl;
cout<<"3: "<<array[3][3]<<endl;
cout<<"4: "<<array[3][4]<<endl;
cout<<"Need Matrix:"<<endl;
cout<<" A B C D"<<endl;
//cout<<"0: "<<array[3][1]+array[2][1]<<endl;
//int c= atoi(array[3][1].c_str());
//int c2= atoi(array[3][1].c_str());
//cout<<c+c2<<endl;
return 0;
}
my input.txt file looks like this:
5
4
0 0 1 2
1 0 0 0
1 3 5 4
0 6 3 2
0 0 1 4
0 0 1 2
1 7 5 0
2 3 5 6
0 6 5 2
0 6 5 6
1 5 2 0
1:0 4 2 0
edit:
note:if there is an empty line >> stop!
the program is based on banker algorithm which takes the first number from the input.txt as the numbers of of processes
then takes the second number as the numbers of of resource types
then takes the next numbers that there is no empty line between them as Allocation Matrix
then takes the next numbers that there is no empty line between them as max Matrix
and here is my problem when i want do Subtraction between Allocation Matrix and max Matrix because both are strings !
as for 1:0 4 2 0 it means do some operations with process number 1

Answer 1

You can use atoi but in c++ you have better options.
in c++ you can easily use stringstream to convert these types.
#include <iostream>
#include <string>
#include <sstream>
using namespace std;
int convert_str_to_int(const string& str) {
int val;
stringstream ss;
ss << str;
ss >> val;
return val;
}
int main () {
string str = "1024";
int val = convert_str_to_int(str);
cout << "Val is: " << val << ", val/2 is " << val/2 << endl;
}