C: undeclared identifier error in 'do-while' loop - Vigenere Cipher

Question

I'm getting an undeclared identifier error for the i in word[i] in the last of the 'do-while' loop. (word[i]) was accepted previously in the for loop as it is defined in the get-string at the top of the code(not shown here). For some reason it is not recognised in the do-while loop outside of the 'for' loops. I have tried defining it again in various parts of the code even immediately outside and inside the 'do' but then error says the definition is overshadowed by the previous one in the 'get_string'. The 'do-while' loop needs to iterate at the same time as the two internal 'for' loops so both internal 'for' loops and the 'do while' loop need to access the same (word[i]). I would appreciate advice. I can add more original code if the question is unclear. I have edited to avoid spoilers.

I would like to use my existing code, if possible, rather than use someone else's, as I know the internal 'for' loop works perfectly as it was the one I used for Caesar.

   do   //loop to iterate through the letters in the key   
    {     

    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 key word entered by user
            {   

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

            (I REMOVED CODE HERE DUE SPOILERS)

            else    //all other types of characters
                {
                printf("%c", word[i]); 
                } 
            }
        }        
        printf("\n");

    }    

    while((word[i])<strlen(word));  

    }  

}

Answer 1

Variables in C are declared in block-scope. You can access all variables in current scope, and in scopes that contain the current scope (unless shadowed by variables of the same name declared in more inner scopes).

You would have to declare i in a scope that's above (in a hierarchical sense) the do..while loop, though some of your code makes little sense to me (innermost loop looks almost ok).

You have 1 do..while and 2 for loops, while you need only one loop over the text. Also, in (word[i])<strlen(word), you try to compare a character to a string length. Characters might be very short integers, but here there's no relation to the string length.

In Vigenère ciphers, you'd keep two indices, one for the plaintext, and one for the key. The one for the key is advanced only when you use the current key character for encryption, if you just copy a non-letter, it stays the same.

Example structure:

int keylen = strlen(key);
int textlen = strlen(plaintext);
for (int textpos = 0, keypos = 0; textpos < textlen; textpos++)
{
    if (isalpha(plaintext[textpos])) {
        // encrypt plaintext[textpos] using key[keypos]
        // and print encrypted letter

        // keypos increment version 1:
        keypos++;
        if (keypos >= keylen)
        {
            keypos = 0;
        }
        // version 2:
        keypos = (keypos + 1) % keylen;
        // (use only one of the two)
    }
    else
    {
        // print plaintext[textpos]
    }
}

Only textpos is incremented automatically, keypos instead is incremented on encrypting a letter. Another version would be keypos++; without a wrap-around in incrementing, and encrypt using key[keypos % keylen] instead of key[keypos].