Struggling with wraparound counting in C - c

Question

I'm a newbie, so apologies if I don't explain myself well. If it helps, I'm doing this for the Caesar problem set as part of the Harvard CS50x OpenCourseWare.
I'm trying to convert user generated plain text to cipher text using a simple key. To accomplish this I'm attempting to use a wraparound counting formula in my last function. However, sometimes I get blanks that print out instead of the new characters... Help!
EDIT: I'm using a key of 5 and the plaintext "Helloz!" to test. Expect to see Mjqqte!
instead am seeing blank spaces.
#include <stdio.h>
#include <cs50.h>
#include <string.h>
#include <math.h>
#include <ctype.h>
#include <stdlib.h>
int convert(string n);
string k, text;
char text;
int r, c, t,x;
bool validate(string n);
//int encrypted(string n);
int main(int argc, string argv[])
{
//accept single command-line argument, non negative integer, k with appropriate error
k = argv[1];
if (argc > 1 && argc <= 2)
{
//printf("Success\n%s\n", argv[1]);
// print individual characters of argv[i]
validate(k);
}
else //if wrong input then print error message and main should return 1
{
printf("Usage: ./caesar key\n");
return 1;
}
text = get_string("plaintext:");
t = atoi(k);
printf("%i\n", t);
convert (text);
printf("\n");
}
//output "ciphertext:" without a newline, with the characters roated by k positions
//after output, print a newline and exit by returning 0 from main
bool validate(string n)
{
for (int i = 0; k[i] != '\0'; i++)
{
if (48 <= k[i] && k[i] <= 57)
{
//printf("%c\n", k[i]);
}
else
{
printf("./caesar key\n");
return 1;
// save for later: printf("%s \n", k);
}
}
return r;
}
int convert(string n)
{
//if fits within a range, Reads individual characters
for (int i = 0; i < text[i]; i++)
{
if (isalpha(text[i]))
{
x = text[i];
//printf("%i\n", x);
c = (x+t) % 26;
// printf("%i\n",c);
printf("%c", c);
}
else
{
printf("%i", text[i]);
}
}
return 0;
}

Answer 1

Here's an implementation that could work for you:
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <ctype.h>
void convert(char *text, unsigned char k) {
for (unsigned int i = 0; i < strlen(text); i++) {
if (isalpha(text[i])) {
// Contains the 3 leftmost bits, containing the uppercase/lowercase part.
char c = (text[i] / 32) * 32;
// Perform the shifting with modulo on the alphabetic index of the letter.
text[i] = c + ((text[i] % 32) + k) % 26;
}
}
}
int main(int argc, char *argv[]) {
unsigned char k = strtol(argv[1], NULL, 10);
char text[64];
printf("Using key %d.\n", (int) k);
printf("Plaintext: ");
fgets(text, 64, stdin);
// Remove newline.
text[strlen(text) - 1] = 0;
convert(text, k);
printf("Ciphertext: %s.\n", text);
return 0;
}
Test run:
>>> cipher 4
Using key 4.
Plaintext: Test mE Right Away!!1
Ciphertext: Xiwx qI Vmklx Eaec!!1.