My algorithm for replacing characters in string is not working-CodePudding

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];}
}

CodePudding user response：

The problem is the inner for loop

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

For example if text[h] is equal to replace[p] then the character text[h] is changed and the inner loop continues its iterations with the new value of text[h] And the new value of text[h] can be again found in the string replace.

At least you need to insert a break statement

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

You could use the standard function strchr instead of writing manually the inner for loop. For example

string replace_char( string text, string replace, string new )
{
    for ( string p = text; *p != '\0';   p )
    {
        char *target = strchr( replace, *p );
        if ( target != NULL ) *p = new[target - replace];
    }

    return text;
}

Pay attention to that you need to guarantee that lengths of the strings replace and new are equal each other. Otherwise the function will look more complicated.

CodePudding user response：

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]];
    }
}