How to replace a character in text file using C? - c

Question

I need to read a text file (E3-5.txt), and search for character c1 to be replaced by c2.
This is my incomplete code:
#include<stdio.h>
#include<stdlib.h>
#include<string.h>
int main()
{
char c;
char c1 = 'm';
char c2 = 'a';
int i;
FILE* fp;
fp = fopen("C:\\E3-5.txt", "r+");
if (fp == NULL)
{
printf("File not found!");
return 0;
}
for(c = getc(fp); c != EOF; c = getc(fp))
{
if(c == 'm')
{
i = ftell(fp);
printf("\nPosition %d", i);
}
}
}
I am having trouble how to locate the position of c1 in the text and how to rewrite it.
Edit:
I used the code from the answer, but it didn't change the text.
This is the new code:
#include<stdio.h>
#include<stdlib.h>
#include<string.h>
int main()
{
char c;
char c1 = 'm';
char c2 = 'a';
int i;
FILE* fp;
fp = fopen("C:\\E3-5.txt", "rb+");
if (fp == NULL)
{
printf("File not found!");
return 0;
}
else
{
for(c = getc(fp); c != EOF; c = fgetc(fp))
{
if(c == c1)
{
fseek(fp, -1, SEEK_CUR);
fputc(c2, fp);
}
else
{
return 0;
}
}
}
return 0;
}
The program returned 0 without writing anything in the text

Answer 1

Here you have a very naive one:
int freplace(FILE *f, char needle, char repl)
{
int result = 1;
int c;
if(f)
{
while((c = fgetc(f)) != EOF)
{
if(c == needle)
{
fseek(f, -1, SEEK_CUR);
fputc(repl, f);
//all I/O functions require error handling
}
}
}
return result;
}

Answer 2

getc() returns an int so you need to declare int c not char c to check for the EOF.
ftell() gets the location. Use fwrite() or fputc() to write to file at that location by setting with fseek().
Go to https://en.cppreference.com/w/c for reference. Lots of beginners fail to read all of the standard library functions, and some even reinvent the wheel.

Answer 3

You really don't want to directly manipulate a file. Ever. Doing so is just asking for data corruption. Instead, create a new file and move it when you're done. Also, it's a lot easier to write the code. You can do so with something like:
#include <stdio.h>
#include <unistd.h>
int
main(int argc, char **argv)
{
int c1 = argc > 1 ? argv[1][0] : 'm';
int c2 = argc > 2 ? argv[2][0] : 'a';
const char *path = argc > 3 ? argv[3] : "stdin";
FILE *in = argc > 3 ? fopen(path, "r") : stdin;
if( in == NULL ){
perror(path);
return 1;
}
FILE *out = stdout;
char tmp[1024] = ".tmpXXXXX";
char *outpath = "stdout";
if( argc > 3 ){
outpath = tmp;
int fd = mkstemp(tmp);
if( fd == -1 ){
perror("mkstemp");
return 1;
}
if( (out = fdopen(fd, "w")) == NULL ){
perror(tmp);
return 1;
}
}
int c;
while( (c = fgetc(in)) != EOF ){
if( c == c1 ){
c = c2;
}
if( fputc(c, out) == EOF ){
perror(outpath);
return 1;
}
}
if( argc > 3 ){
if( fclose(out) ){
perror(outpath);
return 1;
}
if( rename(outpath, path) ){
perror(path);
return 1;
}
}
return 0;
}

Answer 4

String replace
Just for completeness, here is a bit of code to replace a word in a file! This will replace a single character, so of course it answers the question and shows some useful examples.
This is also my first and only non-trivial golden program, written in May 1994! Although you can certainly find fault with it, it worked as intended and my co-workers and I used it many different ways for sysadmin-related tasks. Compiled on MS C/C++
#include <stdio.h>
#include <string.h>
#define err(e) {_fcloseall(); fprintf(stderr, \
"USAGE: chg source dest oldstr newstr\n%s\n",e); exit(1);}
main (int argc,char *argv[])
{
FILE *in,*out;
char buffer[200];
char *old,*new;
int i,j,k;
if (argc!=5)
err("invalid # of parameters");
if ((in=fopen(argv[1],"r"))==NULL)
err("Can't open source");
if ((out=fopen(argv[2],"w"))==NULL)
err("Can't open dest");
old=argv[3];
new=argv[4];
if (*old=='"')
old++;
if (*new=='"')
new++;
if (i=strlen(old) && old[i-1]=='"')
old[i-1]=0;
if (i=strlen(new) && new[i-1]=='"')
new[i-1]=0;
if (!*old)
err("Can't search for nothing!");
if (!*new)
err("Can't replace nothing!");
j=0;
while (!feof(in))
{
if ((buffer[j]=fgetc(in))==EOF)
break;
buffer[j+1]=0;
j++;
if (!old[j-1])
{
fprintf(out,new);
fputc(buffer[j-1],out);
j=0;
}
else if (_strnicmp(buffer,old,j))
{
fprintf(out,buffer);
j=0;
}
else if (j>195)
err("Internal error, buffer filled past 195");
}
}