Ways to shift keyboard's key to the right by 2 digits using C

Question

I am trying to shift keyboard's key 2 digits to the right, for example if user wants to type "a" they have to press the "d" key on the keyboard, "p" to "]".

Which mean if the user input is: "p m[ojku d, d]]'t/",

then the output will be: "i bought an apple,".

Excluding the uppercase key and last the keys in its row on the keyboard.

The way I am doing it is to check every char of the string and compare its ASCII to each case, it works perfectly fine. But I feel so dumb by doing so, wanna know is there any algorithm or smarter way to accomplish this.

while (fgets(inputString, 500, stdin)) {
    stringLength = strlen(inputString);

    for (int i = 0; i < stringLength; i++) {
        switch (inputString[i]) {
            case 100:
                outputString[i] = 'a';
                break;
            case 109:
                outputString[i] = 'b';
                break;
            case 98:
                outputString[i] = 'c';
            case 47:
                outputString[i] = ',';
                break;
            case 50:
                outputString[i] = '`';
                break;
            case 92:
                outputString[i] = '[';
                break;
            default:
                outputString[i] = inputString[i];
        }
    }
    printf("%s", outputString);
}

Answer 1

Here is a generic replace function using a lookup table. It should be able to do the job. I have filled in conversions for 'd' and 's'. You can fill in the rest. A NUL character in the table indicates no replacement.

const char table[256] = {
             0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, // 0
             0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, // 16
             0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, // 32
             0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, // 48
             0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, // 64
             0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, // 80
             0, 0, 0, 0, 'a', 0, 's', 0, 0, 0, 0, 0, 0, 0, 0, 0, // 96
             0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, // 112
             0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, // 128
             0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, // 144
             0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, // 160
             0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, // 176
             0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, // 192
             0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, // 208
             0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, // 224
             0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0  // 240
};

void replace(char *str, const char table[256])
{
    while(*str) {
        if(table[*str])
            *str = table[(unsigned char)*str];
        str++;
    }
}

int main(void)
{
    char str[] = "dfdffd";
    replace(str, table);
    printf("%s\n", str);
}

If you prefer writing to a new string instead, you can use this. It will work even if input and output is the same.

void replace(char *dest, const char *src, const char table[256])
{
    while(*src) {
        if(table[*src])
            *dest = table[(unsigned char)*src];
        src++;
        dest++;
    }
}