Caesar - unexpected results - weird character added to cypher

Question

My code seems to be working but when I enter only one or few characters in my plaintext, my cyphertext returns weird characters... I had a look at my code and try couple of solutions and I can see that the issue is fixed when I remove the usage of strlen. Based on reading on internet, I understood that strlen could have brought rubbish data in my array but I can't find a solution to clean my arrays or to make strlen work better.

for example:

plaintext: a
ciphertext: b���

Thank you for any advice!

#include <cs50.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>

int main(int argc, string argv[])
{
    string key = argv[1];

    //check if there is only one argument
    if (argc != 2)
    {
        printf("Usage: ./caesar key\n");
        return 1;
    } 

    //check if the string has only digit
    for (int i = 0 ; i < strlen(key) ; i++)
    {
        if (key[i] < '0' || key[i] > '9')
        {
            printf("Usage: ./caesar key\n");
            return 1;  
        } 
    }
    int k = atoi(argv[1]);
    printf("%i\n", k);

    //Prompt for Plain text
    string p = get_string("plaintext: ");
    int lenp = strlen(p);

    //Create ciphertext
    char c[lenp];

    for (int i = 0; i < lenp; i++)
    {        
        int pn = (int) p[i];

        //no change if character is not a letter
        if (pn < 65 || pn > 122 || (pn > 90 && pn < 97))
        {
            c[i] = p[i];
        }
        // to keep uppercase
        else if (pn >= 65 && pn <= 90)
        {
            pn = pn - 65;
            int cn = ((pn + k) % 26) + 65 ;
            c[i] = (char) cn;
        }

        //to keep lowercase
        else 
        {
            pn = pn - 97;
            int cn = ((pn + k) % 26) + 97 ;
            c[i] = (char) cn;
        }
    }
    printf("ciphertext: %s\n", c);
    return 0;
}

Answer 1

Hmmm.....

When you created the char array c, did you leave room for, and add an end of string marker?

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Answer 2

i had the same issue originally.

in your line char c[lenp];

you need to leave room for the \0 in your array since lenp is just reading the legnth of p as read by a human without factoring in the \0. so by replacing lenp with lenp+1 your outputs shouldn't show those odd looking characters hope it helps and good luck!