How do I write and read files on one program? - c

Question

I'd like to write two multiplication tables in a txt file on one program, read the written file, and print it out on the screen.
C2374 error occurs in my code.
#define _CRT_SECURE_NO_WARNINGS
#include<stdio.h>
int main(void)
{
//File Write
FILE* fp = fopen("99.txt", "wt");
if (fp == NULL)
{
puts("file open fail.");
return 1;
}
int i = 2;
int j;
for (j = 1; j <= 9; j++)
{
fprintf(fp, "\t %d X %d = %2d \n", i, j, i * j);
}
//File Read
char str[100];
FILE* fp = fopen("99.txt", "rt");
while (fgets(str, sizeof(str), fp) != NULL)
printf(str);
fclose(fp);
return 0;
}
How can I get the 'desired output'?
desired output
I'd really appreciate your help.

Answer 1

You cannot declare FILE *fp twice. Minimized scope of variables. Changed loop condition from j <= 9 to j < 10 which how it's usually done. Use freopen() to change the mode of the file handle:
#include <stdio.h>
int main(void) {
FILE* fp = fopen("99.txt", "wt");
if(!fp) {
puts("file open fail.");
return 1;
}
for (int i = 2, j = 1; j < 10; j++)
fprintf(fp, "\t %d X %d = %2d \n", i, j, i * j);
fp = freopen(NULL, "r", fp);
if(!fp) {
puts("file reopen fail.");
return 1;
}
char str[100];
while (fgets(str, sizeof(str), fp) != NULL)
printf(str);
fclose(fp);
}
and expected output:
2 X 1 = 2
2 X 2 = 4
2 X 3 = 6
2 X 4 = 8
2 X 5 = 10
2 X 6 = 12
2 X 7 = 14
2 X 8 = 16
2 X 9 = 18