Need help converting letters in a string

[Jake Garbelotti]

Hi, I have to write a program when a user enters 26 letters which is a code that corresponds the normal 26 letter (A-Z) in order and then enter their name which can be any where from 1 to 19 letters. Then I have to convert each letter from their name (which is a string) to the letter corresponding to the code they entered. Example:

They enter for their code:
ZYXWVUTSRQPONMLKJIHGFEDCBA

Then enter their name:
JAKE

Then the program spits out their encoded name:
QZPV

I used an array to collect the 26 letters, then a string to collect the name, but how do I change each letter in the string to a new letter based on the array and then reprint it???

[Matticus]

A string is just a special case of a character array, so you can simply swap indices between both array variables. What have you tried so far (i.e. code-wise)?

Here's a link to a recent thread that might give you some clues, though won't give you an answer.

[Jake Garbelotti]

A string is just a special case of a character array, so you can simply swap indices between both array variables. What have you tried so far (i.e. code-wise)?

Here's a link to a recent thread that might give you some clues, though won't give you an answer.

There is some great info in that thread, HOWEVER I don't know how long my user's name is going to be or what their name is going to be. In that thread they are just picking and name and converting it which is simple.

[fnprintf]

I am learning 'c' on my own and did the exercise.

Just use fgets() for user input. C Style strings tutorial.

[smokeyangel]

There is some great info in that thread, HOWEVER I don't know how long my user's name is going to be or what their name is going to be. In that thread they are just picking and name and converting it which is simple.

Looking at that thread, I don't think it makes any difference. They use strlen(name), you can use strlen(name) too. If you have a 20 character array and read "JAKE" into it, strlen will tell you the string has length 4.

"JAKE" is just a 4-character array of chars. So just process each char separately.

Given 'J', you need to work out what letter that corresponds to in the code array. There's more than 1 way to do this -- you could do similarly to the other thread by looping, or you could devise a way of mapping each letter to 0-25 -- A=0,B=1 etc, then index the code array using that mapping. Advantages and disadvantages to either way, up to you to decide.

[Adak]

Jake, if you didn't HAVE a computer, how would you do this?

Write out those small steps you would use. Taken together, they can be the "backbone" of your programs logic. If you don't see the pattern to the logic, do it a few times - or several times. Before long, you will see a pattern you use in solving this, yourself. That's the same pattern your program might use.

Once you get that pattern understood, you can start the program. Post it up when you get that far, and tell us what problems you're having. We can be a lot more helpful after we can look at specific code.

[Jake Garbelotti]

Looking at that thread, I don't think it makes any difference. They use strlen(name), you can use strlen(name) too. If you have a 20 character array and read "JAKE" into it, strlen will tell you the string has length 4.

"JAKE" is just a 4-character array of chars. So just process each char separately.

Given 'J', you need to work out what letter that corresponds to in the code array. There's more than 1 way to do this -- you could do similarly to the other thread by looping, or you could devise a way of mapping each letter to 0-25 -- A=0,B=1 etc, then index the code array using that mapping. Advantages and disadvantages to either way, up to you to decide.

The second way is the way I would like to do it. I just dont know how I can go from like name[0] being any letter to matching it with the proper letter and then assigning the new letter.

[Jake Garbelotti]

This is the start I have so far:

//included libraries#include <stdio.h>
#include <math.h>


//Main Function
int main() {
    //Most of your program should go here.


    //Declare strings and arrays
    int i,j;
    char name[18];
    char cipher[25];
    char alphabet[25] = {'A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z'};


    //Ask for the cipher
    printf("What is your substitution cipher?\n");


    //Scan for the cipher
    for(i=0; i<26; i++){
        scanf("%c", &cipher[i]);
    }


    //Ask for the users name
    printf("What is your name?\n");
    scanf("%s", &name);


















    return 0;
}

[Adak]

This is the start I have so far:

//included libraries
#include <stdio.h>
#include <math.h> //is this needed?


//Main Function
int main() {
    //Most of your program should go here.


    //Declare strings and arrays
    int i,j;
    char name[18];
    char cipher[25];
    char alphabet[25] = {'A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z'};

    /* 
    Note that the alphabet has 26 letters, not 25. A simpler way:
    char alphabet[]={"ABCDEFGHIJKLMNOPQRSTUVWXYZ"};
    Now alphabet[] will have 27 spaces. 26 for the letters, and one    
    more for the end of string char: '\0', which you can't see here, but 
    is required for string functions, in C.
   */


    //Ask for the cipher
    printf("What is your substitution cipher?\n");


    //Scan for the cipher
    for(i=0; i<26; i++){
        scanf("%c", &cipher[i]);
    }

    /* why not the simpler
    scanf("%s", cipher); ?
    */


    //Ask for the users name
    printf("What is your name?\n");
    scanf("%s", &name);
    /* correct is:
    scanf("%s", name);
    the name of the array, IS the address to the array's first element
   */

    return 0;
}

The above is all syntax stuff. Now, how would you solve this puzzle? No code needed for this part,yet. Pure puzzle solving! Of course, you can do it, but can you see the WAY that you solve it - the pattern of logic you use to do that?

You can use that same pattern of logic, in your program.

[smokeyangel]

The second way is the way I would like to do it. I just dont know how I can go from like name[0] being any letter to matching it with the proper letter and then assigning the new letter.

Just think about how you'd do it yourself (as a person not a computer). Let's say you want to substitute 'J'. You figure out that J is the 10th letter in the alphabet, so you pick the 10th cipher letter.

Characters in C have an underlying representation as a number. For example, ASCII: Ascii Table - ASCII character codes and html, octal, hex and decimal chart conversion
(Of course ASCII isn't the only character encoding in use, but for a program like this I think it's fair to assume everything will be in ASCII. Just remember that that's not something you can universally assume with all programs. )

int i = 'C';     // i is 67
char c = 86; // c is 60, can be written 'V'

If the value of 'A' was 0, 'B' 1 and so on, it'd be really easy, right? You could just use the letter itself to index into the cipher array

   coded_char = cipher[name[i]];

It's not quite that easy - if you look at the ASCII values you can see that A=65,B=66 and so on. So you'd need to map these to 0,1 before indexing the array, by subtracting 65. A starting point would probably be to try to map the name onto your alphabet array-- then once you get back the same name you input, you can just swap in the cipher array.

That's how I'd do it anyway. You need to be careful to check that the input letter is valid, or you'll access outside the array and crash your program (probably). Likewise if you put in lowercase characters -- these have different encodings to uppercase, so you need to handle that.

[Jake Garbelotti]

Thanks for the help, and I understand how it works, like J should change to the 10th letter of the cipher string, I just have no idea how to assign J a number and change it to the corresponding value before printing the name again. Do I have to use a new string for the new name???

[smokeyangel]

Whatever suits your needs. You can create a new string, or write over the old one, or just print the characters as you encode them. Examples below:

for (i = 0; i < strlen(name); i++)
{
     char c = name[i]; // get the letter
     // use the value of 'c' to work out an index num between 0-25
     putc(cipher[index]); // print character 
     printf("%c", cipher[index]); // alternative way of printing character
     newname[i] = cipher[index]; // write the coded name into another buffer
     name[i] = cipher[index]; // write over the old string

}

For getting the index into the array you just subtract the value of 'A' from the letter. Because 'A' is 65 and you want to access element 0 of cipher for 'A': 65-65=0. Then B is 66... 66-65=1

int index = name[i] - 'A'; // or 
int index = name[i] - 65;  //if you find it clearer

Breaking it down a bit using a couple of temporary variables:

char plain_char = name[i];
int index = plain_char - 65;
char coded_char = cipher[index]

Before I forget:

    char cipher[25];
    char alphabet[25] = {'A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z'};

Your arrays are too small, only 25 elements and you need 26. If you decide to use scanf to get the cipher string then you'll need 27 -- 26 characters and the null terminator at the end of the string.

Ah - actually I see Adak already said that.


I suggest you start by just writing a loop that prints out the original name without trying to encode it. Get that compiled and working, then start trying to work on the cipher part. I think from the code you posted that you haven't actually tried to compile it yet (it wouldn't compiler) -- it's best to be compiling and testing as you go -- the more untested code you have to debug at the end the harder it'll be for you to spot mistakes.

[Jake Garbelotti]

Thanks you so much for your help! I now have to code working perfectly and your help has greatly helped me understand what is going on with strings. Thanks once again!