My algorithm for replacing characters in string is not working

For example when I input in the terminal

hello there

it prints

gmllg mgmkm

instead of printing

itssg zitkt

which is not supposed to happen.

Another example is when I input in the terminal

abcdefghijklmnopqrstuvwxyz

it prints

jvmkmnbgghaldfghjklmbcvbnm

the last 5 characters are right but its supposed to print

qwertyuiopasdfghjklzxcvbnm

Anyways I can fix this?

Here is the code below

#include <cs50.h>
#include <stdio.h>
#include <string.h>
void replace_char(string text, string replace, string new);
int main(void){

    string message = get_string("Type message: ");
    string Alpha = "abcdefghijklmnopqrstuvwxyz";
    string Key  =  "qwertyuiopasdfghjklzxcvbnm";
    replace_char(message, Alpha, Key);
    printf("%s\n", message);
}
void replace_char(string text, string replace, string new){

    int strl = strlen(text);
    int h;
    int p;
    for (h = 0; h < strl; h++){
        for (p = 0; p < 26; p++)
        if (text[h] == replace[p])
        text[h] = new[p];}
}

Solution

The problem is that after you've replaced the character you continue searching and may therefore replace the character you just put there. This may happen multiple times. When you find a match, you need to break out of the loop:

void replace_char(string text, string replace, string new){
    int strl = strlen(text);

    for (int h = 0; h < strl; h++){
        for (p = 0; p < 26; p++) {
            if (text[h] == replace[p]) {
                text[h] = new[p];
                break;                   // break out
            }
        }
    }
}

An even simpler version would be to just look up the character in the replacement string directly. This works if the characters in replace are in a contiguous range (and they are 26 of them as you've hardcoded):

void replace_char(string text, string replace, string new){
    int strl = strlen(text);

    for (int h = 0; h < strl; h++){
        if(text[h] >= replace[0] && text[h] <= replace[25])
            text[h] = new[text[h] - replace[0]];
    }
}

Or, make sure that replace is a contiguous range by taking the first and last character in the range instead:

#include <assert.h>

void replace_char(string text, char first, char last, string new) {
    int strl = strlen(text);

    assert(strlen(new) >= last - first);

    for (int h = 0; h < strl; h++) {
        if (text[h] >= first && text[h] <= last)
            text[h] = new[text[h] - first];
    }
}

and call it like so:

replace_char(message, 'a', 'z', Key);