C - Replacing char with string - c

Question

I am writing a program that encodes text such that it can be put into a URL. I have the user inputting a string and if it contains special characters (#, %, &, ?, etc.) to replace them with their corresponding character codes (%23, %25, %26, %3F, etc.). The problem is that the special characters are only of length 1 and the codes are of length 3. The codes end up replacing characters after the special one. This is the code I am using to do the replacement.
char *p = enteredCharStr;
while ((p = strstr(p, specialCharArr[x])) != NULL )
{
char *substr;
substr = strstr(enteredCharStr, specialChar[x]);
strncpy(substr, charCodesArr[x], 3);
p++;
}
Example output from using my program with input: "this=this&that"
this%3Dis%26at
I would like the output to be:
this%3Dthis%26that
Any idea on how to implement what I am trying to do in C (no libraries)?

Answer 1

One way to approach this problem would be to allocate a second string that is three times as large as enteredCharStr and copy the characters over one by one and when you see special character write the replaement instead. You want it to be three times as large since in the worst case you need to replace nearly all the characters.

Answer 2

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int isspecial(int c){
static char table[] = "#%&?=<>"; //add etc..
return strchr(table, c) ? 1 : 0;
}
char *encode(const char *s){
size_t capa = 1024;
char *buff=malloc(capa*sizeof(char));
size_t size = 0;
for(;*s;++s){
if(size + 3 > capa){
capa += 32;
buff = realloc(buff, capa*sizeof(char));
}
if(isspecial(*s)){
size+=sprintf(buff+size, "%%%02x", *s);
} else {
size+=sprintf(buff+size, "%c", *s);
}
}
if(size == capa){
buff=realloc(buff, (size+1)*sizeof(char));
}
buff[size++]='\0';
return realloc(buff, size*sizeof(char));
}
int main(void){
char *enteredCharStr = "this=this&that";
char *p = encode(enteredCharStr);
printf("%s\n", p);
free(p);
return 0;
}

Answer 3

You need to make a new string. Here's an example:
char *str = "abc$ddd";
char *p = str;
char *buf = malloc(strlen(str)+1);
char *pbuf = buf;
while(*p) {
if(*p != '$') *pbuf ++ = *p;
p++;
}
It will copy from str to buf all non-$,byte per byte.
Note that in your case,you need to perform the right computation of size of new string.

Answer 4

A C 'string' is a fixed-size array of characters, and therefore there is no built-in notion of insertion. You're effectively asking how to insert n characters into the middle of an array.
One strategy come to mind:
To insert a string of length x at position i of an array of length n:
Resize the array to size n+x (using something like realloc).
Shuffle every character beyond position i to position i+x.
Write your string into the x positions now freed by this shuffle operation.
Alternatively, allocate a new array that is big enough to hold your target string (i.e., with all the substitutions applied), and then write your result into that by copying from the target array until you encounter a character you'd like to replace, then copy from the replacement string, then continue reading from the original source array.

Answer 5

I'm copying characters over one by one, and if I see a special character, (In this code only "#")
I copy in 3 characters, incrementing the index into the output buffer by 3.
You can also do something smarter to guess the buffer size, and perhaps loop over the entire operation, doubling the size of the buffer each time it overruns.
#include<stdio.h>
#include<stdlib.h>
int main(int argc, char* argv[]){
if (argc != 2) {
exit(1);
}
char* input = argv[1];
int bufferSize = 128;
char* output = malloc(bufferSize);
int outIndex = 0;
int inIndex = 0;
while(input[inIndex] != '\0'){
switch (input[inIndex])
{
case '#':·
if(outIndex + 4 > bufferSize){
// Overflow, retry or something.
exit(2);
}
output[outIndex] = '%';
output[outIndex+1] = '2';
output[outIndex+2] = '3';
outIndex = outIndex + 3;
inIndex = inIndex + 1;
break;
// Other cases
default:
if(outIndex + 2 > bufferSize){
exit(2);
}
output[outIndex] = input[inIndex];
outIndex = outIndex + 1;
inIndex = inIndex + 1;
break;
}
}
output[outIndex] = '\0';
printf("%s\n", output);
return 0;
}