How do I repeat a 'for' loop using a 'do while' loop

Question

This is my Vigenere Cipher code. I'm trying to use the do while loop to repeat the iteration over the key entered by the user. The key is applied to the plaintext word entered by the user, one letter at a time. I need to be able to iterate over the key until the end of the plaintext word, in case the key is shorter than the word. I've tried to add the repeat using a do-while loop with the first for loop being the key iteration.

The last line of the 'do while' loop is throwing the error of undeclared identifier i. This is the only error I'm getting. The inside 'for' loop is from the Caesar Cipher and passed all checks. I think my do while loop is wrong. If I add a definition of char* word[i] or word[i] I get an overshadowing error regardless of where I put it. I would like to use this code rather than completely change it so I understand if it's possible to do it this way. Any suggestions, however, would be welcome.

int main(int argc, char* argv[])
{
    if (argc<2) //key
    {
        printf("Please enter your word key"); //prompts if no key is entered
        return 1;
    }

    char* key = (argv[1]);

    if(argc>=2)
    {
        printf("plaintext:");
        char* word = GetString();

        printf("ciphertext:");

        do
        {  //starts loop to repeat following for loop

            for(int l=0; l<strlen(key); l++) //iterate over letters in key
            {
                int num=l;
                for(int i=0; i<strlen(word); i++)  //iterates through word entered by user as plaintext
                {
                    if(isupper(word[i]))  //if original characters are uppercase
                    {
                        int cipher = (word[i] + num -65) % 26 + 65;
                        printf("%c", cipher);
                    }
                    else if(islower(word[i]))  //if original characters are lowercase
                    {
                        int cipher = (word[i] + num - 97) % 26 + 97;
                        printf("%c", (cipher));
                    }
                    else    //all other types of characters
                    {
                        printf("%c", word[i]);
                    }
                }
            }
            printf("\n");
        }while((word[i])<strlen(word));  // loop to recommence iterating over letters in the key   (i throwing undeclared identifier error)
    }
}

Answer 1

I think you have too many loops [levels].

If the key length runs out before the word is finished, you restart encrypting the word from the beginning of the word (i.e. a bug).

The main thrust is to loop through all word chars. A single loop that increments i works, providing it also increments l [modulo key length].

Here's a cleaned up version [please pardon the gratuitous style cleanup]:

#include <stdio.h>
#include <string.h>
#include <ctype.h>

// NOTE: I don't have GetString on my system
char *FakeGetString(void);

int
main(int argc, char *argv[])
{

    // key
    // prompts if no key is entered
    if (argc < 2) {
        printf("Please enter your word key");
        return 1;
    }

    char *key = (argv[1]);
    int klen = strlen(key);

    if (argc >= 2) {
        printf("plaintext:");

#if 0
        char *word = GetString();
#else
        char *word = FakeGetString();
#endif

        int wlen = strlen(word);

        printf("ciphertext:");

        // current key index
        int l = 0;

        // starts loop to repeat following for loop
        // iterates through word entered by user as plaintext
        // advance to next key char [with wrap to beginning if we're short]
        for (int i = 0;  i < wlen; ++i, l = (l + 1) % klen) {
            int num = key[l];
            int cipher;

            // if original characters are uppercase
            if (isupper(word[i])) {
                cipher = (word[i] + num - 65) % 26 + 65;
            }

            // if original characters are lowercase
            else if (islower(word[i])) {
                cipher = (word[i] + num - 97) % 26 + 97;
            }

            // all other types of characters
            else {
                cipher = word[i];
            }

            printf("%c", cipher);
        }

        printf("\n");
    }

    return 0;
}

// NOTE: I don't have GetString on my system
char *
FakeGetString(void)
{
    static char buf[1000];
    char *cp;

    fgets(buf,sizeof(buf),stdin);
    cp = strchr(buf,'\n');
    if (cp != NULL)
        *cp = 0;

    return buf;
}

---

UPDATE:

The above code I wrote was pretty close but wasn't Vigenere because your original equation was off. The key value has to be an offset/row number, so it needs to have 'A' subtracted from it (i.e. keywords can only be uppercase).

So, here is the corrected version [with some additional cleanups]:

#include <stdio.h>
#include <string.h>
#include <ctype.h>

// NOTE: I don't have GetString on my system
char *FakeGetString(void);

int
baseof(int chr)
{
    int base;

    // if original character is uppercase
    if (isupper(chr)) {
        base = 'A';
    }

    // if original character is lowercase
    else if (islower(chr)) {
        base = 'a';
    }

    // anything else
    else
        base = 0;

    return base;
}

int
main(int argc, char *argv[])
{
    int kval;
    int base;
    int i;

    // key
    // prompts if no key is entered
    if (argc < 2) {
        printf("Please enter your word key\n");
        return 1;
    }

    char *key = argv[1];
    int klen = strlen(key);

    // key must be uppercase and we only want row numbers
    for (i = 0;  i < klen;  ++i) {
        kval = key[i];
        base = baseof(kval);

        if (base) {
            key[i] = kval - base;
            continue;
        }

        printf("Key value must be only A-Z\n");
        return 1;
    }

    if (argc >= 2) {
        printf("plaintext:");

#if 0
        char *word = GetString();
#else
        char *word = FakeGetString();
#endif
        int wlen = strlen(word);

        printf("ciphertext:");

        // starts loop to repeat following for loop
        // iterates through word entered by user as plaintext
        // advance to next key char [with wrap to beginning if we're short]
        for (i = 0;  i < wlen;  ++i) {
            int wval = word[i];
            int cipher;

            base = baseof(wval);

            // uppercase or lowercase
            if (base) {
                kval = key[i % klen];
                cipher = ((wval - base) + kval) % 26 + base;
            }

            // all other types of characters
            else {
                cipher = wval;
            }

            printf("%c",cipher);
        }

        printf("\n");
    }

    return 0;
}

// NOTE: I don't have GetString on my system
char *
FakeGetString(void)
{
    static char buf[1000];
    char *cp;

    fgets(buf,sizeof(buf),stdin);
    cp = strchr(buf,'\n');
    if (cp != NULL)
        *cp = 0;

    return buf;
}

---

UPDATE #2:

Your code passed all check50 checks except this one: :( encrypts "world, say hello!" as "xoqmd, rby gflkp!" using "baz" as keyword \ expected output, but not "ciphertext:xoqmd, szz gflkp!\n". It didn't encipher just the word 'say' properly, which is weird.

I tested it using the test data/example from the Wikipedia page on Vigenere, but it only had one ready test example [without spaces or punctuation].

This was the only check which contained a space (before the word 'say'). The spaces have to be directly copied. Perhaps that's why.

The spaces were directly copied, so that was okay. But ...

The correct way is that when a non-alpha char is copied, the key index must not be incremented.

My version used i to index the phrase and i % klen to index the key, so the key index would always be incremented [effectively]. That is the bug.

Ironically, I had wondered about this, but didn't have the expanded test data at the time.

So, the solution is to separate the index variables [again :-)].

Here's the corrected version. When I was fixing it I renamed the i variable to something more descriptive (e.g. widx) and (re)created the index variable for the key (e.g. kidx).

Notice that, now, kidx is only incremented when actually encrypting a character. The "pass through" case does not

#include <stdio.h>
#include <string.h>
#include <ctype.h>

// NOTE: I don't have GetString on my system
char *FakeGetString(void);

int
baseof(int chr)
{
    int base;

    // if original character is uppercase
    if (isupper(chr)) {
        base = 'A';
    }

    // if original character is lowercase
    else if (islower(chr)) {
        base = 'a';
    }

    // anything else
    else
        base = 0;

    return base;
}

int
main(int argc, char *argv[])
{
    int kval;
    int base;
    int widx;
    int kidx;

    // key
    // prompts if no key is entered
    if (argc < 2) {
        printf("Please enter your word key\n");
        return 1;
    }

    char *key = argv[1];
    int klen = strlen(key);

    // key must be uppercase and we only want row numbers
    for (kidx = 0;  kidx < klen;  ++kidx) {
        kval = key[kidx];
        base = baseof(kval);

        if (base) {
            key[kidx] = kval - base;
            continue;
        }

        printf("Key value must be only A-Z\n");
        return 1;
    }

    if (argc < 2)
        return 1;

    printf("plaintext:");

#if 0
    char *word = GetString();
#else
    char *word = FakeGetString();
#endif
    int wlen = strlen(word);

    printf("ciphertext:");

    kidx = 0;

    // starts loop to repeat following for loop
    // iterates through word entered by user as plaintext
    // advance to next key char [with wrap to beginning if we're short]
    for (widx = 0;  widx < wlen;  ++widx) {
        int wval = word[widx];
        int cipher;

        base = baseof(wval);

        // uppercase or lowercase
        if (base) {
            kval = key[kidx];
            cipher = ((wval - base) + kval) % 26 + base;
            kidx = (kidx + 1) % klen;
        }

        // all other types of characters
        else {
            cipher = wval;
        }

        printf("%c",cipher);
    }

    printf("\n");

    return 0;
}

// NOTE: I don't have GetString on my system
char *
FakeGetString(void)
{
    static char buf[1000];
    char *cp;

    fgets(buf,sizeof(buf),stdin);
    cp = strchr(buf,'\n');
    if (cp != NULL)
        *cp = 0;

    return buf;
}