Replacing specific text of a file in C - c

Question

Alright, so basically what I have to do is change all the numbers of a text file to dollar sign, I know how to scan for the specific character but I am stuck on how to replace that specific character with dollar sign. I don't want to use fseek or any library commands, how do I proceed and why isn't my code working?
#include<stdio.h>
main()
{
FILE* fptr;
char filename[50];
char string[100];
int i;
printf("Enter the name of the file to be opened: ");
scanf("%s",filename);
fptr=fopen(filename,"w");
if(fptr==NULL)
{
printf("Error occurred, try again.");
return 0;
}
fgets(string,"%s",fptr);
do
{
if(string[i]>='1' && string[i]<='9')
{
string[i]='$';
}
}
while(i!=100);
fclose(fptr);
}

Answer 1

There are basically two approaches at first glance, the first is to use fseek() and the second to read the file in its entirety and replace the characters to your criteria and finally write that in one shot. You can choose either of the approaches depending on your need. For large file you should prefer the former and for small file you can prefer the latter.
Here's an example code of the former:
#include <stdio.h>
int main() {
// Open the file
FILE *fptr = fopen("input.txt", "r+");
if (!fptr) {
printf("Error occurred, try again.");
return -1;
}
int c;
// Iterate through all characters in a file
while ((c = getc(fptr)) != EOF) {
// Check if this current character is a digit?
if (c >= '0' && c <= '9') {
// Go one character back
if (fseek(fptr, -1, SEEK_CUR) != 0) {
fprintf(stderr, "Error while going one char back\n");
return -1;
}
// Replace the character with a '$'
if (fputc('$', fptr) == EOF) {
fprintf(stderr, "Error while trying to replace\n");
return -1;
}
}
}
// Flush the changes to the disk
if (fflush(fptr) != 0) {
fprintf(stderr, "Error while flushing to disk\n");
return -1;
}
// Close the file
fclose(fptr);
return 0;
}