Reading a matrix from file in C - c

Question

I want to read the matrix from file and store it in an array. But the array is storing only the last value of matrix. Can anyone explain this please?
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main() {
FILE *fp, *fp1;
int n = 0, i, j, a[4][4], b[16];
fp = fopen("Output.txt", "r");
if (fp == NULL) {
printf("\nError; Cannot open file");
exit(1);
}
while (!feof(fp)) {
i = 0;
fscanf(fp, "%d", &n);//reads the file containing matrix
b[i] = n;//this part is not working
printf("%d\n", n);
i++;
}
fclose(fp);
fp1 = fopen("Output2.txt", "w");
for (i = 0; i < 4; i++) {
for (j = 0; j < 4; j++) {
fprintf(fp1, "%d\t", a[i][j] * 2);
}
fprintf(fp1, "\n");//creates file of altered matrix
}
fclose(fp1);
return 0;
}

Answer 1

Your input loop is incorrect:
you reset i to 0 at the beginning of each iteration
you use an incorrect test: while (!feof(fp)). Learn here why: Why is “while ( !feof (file) )” always wrong? . You should instead test the array index against the array length and check if fscanf() succeeds at reading the next value.
Here is a corrected version:
for (i = 0; i < 16; i++) {
if (fscanf(fp,"%d",&n) != 1) { //reads the file containing matrix
fprintf(stderr, "invalid input\n");
exit(1);
}
b[i] = n;
printf("%d\n", n);
}
Note also that you do not read the values into the 2D matrix, so the output loop has undefined behavior because a is uninitialized.
Here is a improved version:
#include <stdio.h>
#include <stdlib.h>
int main() {
FILE *fp;
int i, j, a[4][4];
fp = fopen("Output.txt", "r");
if (fp == NULL) {
fprintf(stderr, "Error: Cannot open file Output.txt for reading\n");
exit(1);
}
for (i = 0; i < 4; i++) {
for (j = 0; j < 4; j++) {
if (fscanf(fp, "%d", &a[i][j]) != 1) {
fprintf(stderr, "invalid input for a[%d][%d]\n", i, j);
fclose(fp);
exit(1);
}
}
}
fclose(fp);
fp1 = fopen("Output2.txt", "w");
if (fp1 == NULL) {
fprintf(stderr, "Error: Cannot open file Output2.txt for writing\n");
exit(1);
}
for (i = 0; i < 4; i++) {
for (j = 0; j < 4; j++) {
fprintf(fp1, "%d\t", a[i][j] * 2);
}
fprintf(fp1, "\n");
}
fclose(fp1);
return 0;
}