How to read integers from file into dynamic array

Question

I am trying to read integers from a text file and put them into a dynamic array that will be represented as vectors and matrices for an assigment.

An example of a few lines from the input file:

3#456
33#123456789

The numbers before the pound sign represent the elements of the vector or matrix, so 3# would mean a three element vector and 33# would mean a matrix with 3 rows and 3 columns.

Reading those isn't really a problem as we were told we can assume we know which lines are matrices and which are vectors, however, I have never working with C++ file I/O so I don't know how to iterate through the numbers 4,5,6 and put them into a 3, 9, 12, etc, element dynamically created array. Here's somewhat of a sample of what I'm working with.

int *a;
int size_a;
char x;

ifstream infile("input.txt");

if (infile.is_open())
{
    infile >> size_a;

    // The x is basically a junk variable used to go past the '#'
    // when reading the file
    infile >> x;

    a = new int[size_a];
}

After that, I have no real idea of how to loop until the end of the line and put the remaining elements in the array. For example in this line, the numbers 4, 5, and 6 would need to be put into the a array, then break from adding elements and go to the next line to work on the next array, which I don't know how to do either. Any ideas?

No need for vectors; new gives the op the ability to create arrays whose size is known not in run-time but compile time. You are right these are not dynamically resizable arrays, but they do meet OP's requirements, which does not include resizing-- just not knowing the size a priori. — EyasSH
– EyasSH, Commented Mar 2, 2015 at 5:15
You'll need to stream each digit into a char variable (say "c"), then you can convert it from ASCII to a number using the expression c - '0'. Use those numbers to get the right size_a etc value(s), as well as the element values to write into the arrays/matrices. — Tony Delroy
– Tony Delroy, Commented Mar 2, 2015 at 5:23

brettwhiteman · Accepted Answer · 2015-03-02 06:19:05Z

The below code will do this for you. Note that you do not need to use new here - you should just use a std::vector. In that case, the number before the '#' is unneeded as you do not need to specify the size of the array when you create it.

For that reason I have used new here anyway to show you how to read both parts of the file.

#include <iostream>
#include <fstream>
#include <string>

int main()
{
    std::ifstream file("input.txt");

    if(file.good()) {
        std::string line;
        std::getline(file, line);
        std::string::size_type pos = line.find('#');
        std::string strSize = line.substr(0, pos);
        std::string strValues = line.substr(pos + 1);
        int size = 0;

        for(char c : strSize) {
            if(size == 0)
                size = static_cast<int>(c - '0');
            else
                size *= static_cast<int>(c - '0');
        }

        int* values = new int[size];

        for(int i = 0; i < size; ++i) {
            values[i] = static_cast<int>(strValues[i] - '0');
        }

        std::cout << "Array of size " << size << " has the following values:" << std::endl;

        for(int i = 0; i < size; ++i) {
            std::cout << values[i] << std::endl;
        }

        delete[] values;
    }
}

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