How can I extract information from an input file and put them in an array in C?

Question

I have an input.txt file:

• First line: m and on the following m lines I have pairs of 2 numbers a and n

I have to calculate a^n and the result can have max 10^8 digits. I made the program using arrays and giving values to a and n. It returned the correct answer. But what I have to do is to read the information from the file, calculate a^n and write the numbers in an output.txt file.

Example:

In input.txt I have on the first line

2
2 10
1 100000

output.txt

1024
1

As I said, the program works with any numbers wrote by me in the main program. Now I need help to extract the numbers from input.txt, put them in array and after I calculate them to return them in output.txt.

---

Non-compiling code copied from codepad.org

#include <stdio.h>
#include <stdlib.h>

const int MAX_NR_DIGITS = 10000;

void longNumClone(char *from, char *to)
{
    int i;
    for (i = 0; i < MAX_NR_DIGITS; i++)
        to[i] = from[i];
}

void longNumPrint(char *num, int sizeNum) {
    int i;
    for (i = sizeNum-1; i >= 0; i--) {
        printf("%i", num[i]);
    }
}

int longNumDiv2(char *rez, char *num, int sizeNum) {
    int i;
    num[sizeNum] = 0;
    rez[sizeNum] = 0;

    for (i = sizeNum - 1; i >= 0; i--) {
        rez[i] = ((num[i+1] - rez[i+1]*2) * 10 + num[i]) / 2;
    }

    if (rez[sizeNum - 1] == 0) {
        return sizeNum - 1;
    }
    return sizeNum;
}

int multiply(char *rez, char *a, int sizeA, char *b, int sizeB) {
    int sizeRez = 0, i;
    for (i = 0; i < MAX_NR_DIGITS; i++) {
        rez[i] = 0;
    }

    int temp = 0, j;
    for (i = 0; i < sizeB; i++) {
        for (j = 0; j < sizeA; j++) {
            temp = b[i] * a[j];

            int inc = 0;
            do {
                temp = temp + rez[i + j + inc];
                rez[i + j + inc] = temp % 10;
                if (i + j + inc + 1 > sizeRez) {
                    sizeRez = i + j + inc + 1;
                }
                temp = temp / 10;
                inc++;
            }while (temp != 0);
        }
    }

    return sizeRez;
}

int superpow(char *rez, char *base, int sizeBase, char *exp, int sizeExp) {
    int i;
    char temp[MAX_NR_DIGITS];
    char temp1[MAX_NR_DIGITS];
    char temp2[MAX_NR_DIGITS];

    for (i = 0; i < MAX_NR_DIGITS; i++) {
        rez[i] = 0;
    }

    int sizeRez = 1;
    rez[0] = 1;

    while (sizeExp > 0) {
        if (exp[0] & 1) {
            longNumClone(rez, temp);
            sizeRez = multiply(rez, temp, sizeRez, base, sizeBase);
        }

        longNumClone(exp, temp);
        sizeExp = longNumDiv2(exp, temp, sizeExp);

        longNumClone(base, temp1);
        longNumClone(base, temp2);
        sizeBase = multiply(base, temp1, sizeBase, temp2, sizeBase);

          }

    return sizeRez;
}

int main() {

    // 4437053125^625
    char a[MAX_NR_DIGITS] = {5, 2, 1, 3, 5, 0, 7, 3, 4, 4};
    int sizeA = 10;
    char n[MAX_NR_DIGITS] = {5, 2, 6};
    int sizeN = 3;

      char rez[MAX_NR_DIGITS];
    int sizeRez = 0;


    sizeRez = superpow(rez, a, sizeA, n, sizeN);
    longNumPrint(rez, sizeRez);

    return 0;
}

The arrays in main() are deemed variable-length arrays (VLAs) because MAX_NR_DIGITS is not deemed to be an integer constant expression, and you can't initialize VLAs. It would be OK in C++, but not in C.

Answer 1

Look into fscanf for reading a certain format from a file, and fprintf for writing a certain format to a file.

An example given some initialised FILE *read_file and FILE *write_file could be

int count, int a, n;
fscanf(read_file, "%d ", &count);

for (i = 0; i < count; i++) {
    fscanf(read_file, "%d %d ", &a, &n);
    ... 
    /* calculate the output result */
    ...
    fprintf(write_file, "%d\n", result);
}