Pfthb

I am trying to build a program that will do a simple caesar cipher on a text file with single strings with no spaces on each line. For some reason, my cipher function is not shifting text and I am cutting off the strings at various lengths of characters. Can you see where I am messing up with my function call in the while loop?

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

#define FILE_NAME "./infile.txt"

void caeser (char * ch, int shift)

 int i = 0;
 int len = strlen(ch);

 while (ch[i] < len)
 
 if (islower(ch[i]))
 ch[i] = ((ch[i] - 'a' + shift) % 26 + 'a');
 else
 ch[i] = ((ch[i] - 'A' + shift) % 26 + 'A');
 i++;

 printf("Caesar Cipher = %sn", ch);



int main(void)

 char * c = malloc( sizeof(char) * 1000);


 FILE* fp = fopen (FILE_NAME, "r");
 if (fp == NULL)
 
 printf("Can't open %sn", FILE_NAME);
 exit(EXIT_FAILURE);
 

 while(fgets(c, sizeof(c), fp) != 0)
 
 printf("%sn", c);
 caeser(c, 1);
 

 fclose(fp);
 fp = NULL;
 return 0;

I made a few changes in your code and I marked them in bold.

Array start from 0 and the end is the n-1 character.

you checked if ch[i] < len.
ch[i] is a character in the i place of an array, not a number.

in each iteration, you need to increment i by 1. so you can get the next character.

for better design try to sand a printable format to the main function instead print in the function. you should return a pointer to string and print it in main.

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

#define FILE_NAME "./infile.txt"

void caeser (char * ch, int shift)

 int i = 0;
 int len = strlen(ch);

 while (**i < len-1**)
 
 if (islower(ch[i]))
 
 ch[i] = ((ch[i] - 'a' + shift) % 26 + 'a');
 **i++;**
 
 else
 
 ch[i] = ((ch[i] - 'A' + shift) % 26 + 'A');
 **i++;**
 
 
 printf("Caesar Cipher = %sn", ch);


int main(void)

 char * c = malloc( sizeof(char) * 1000);


 FILE* fp = fopen (FILE_NAME, "r");
 if (fp == NULL)
 
 printf("Can't open %sn", FILE_NAME);
 exit(EXIT_FAILURE);
 

 while(fgets(c, sizeof(c), fp) != 0)
 
 printf("%sn", c);
 caeser(c, 1);
 

 fclose(fp);
 fp = NULL;
 return 0;

Note that fgets may return a "new line character" ('n') at the end of the string. You may want to remove the new line character from the buffer.

If the string has blank spaces, 'n' character, or any characters not in the range A-Z or a-z, then ignore those characters because they don't fit the logic for caeser cipher.

char * c = malloc( sizeof(char) * 1000);
while(fgets(c, sizeof(c), fp) != 0) ... 

As mentioned in comments, c is a pointer and sizeof(c) is usually 4 or 8 in this case. So you are telling fgets to read a maximum of 4 or 8 bytes. But each line in the file could be much longer. If you had declared, lets say char c[1000]; then sizeof(c) would be 1000. Otherwise don't use sizeof operator here.

while(i < len) ...i++;

You want to got to the end of the string, and increment inside the loop, so change the condition to:

 while(i < len) ... i++;

Lastly, c or ch is usually used to denotes a character. This is informal and doesn't really matter, but it's more clear to use buf or str if you are declaring a string.

Example:

void caeser(char *buf, int shift)

 int i = 0;
 int len = strlen(buf);
 while(i < len)
 
 char c = buf[i];
 if(c >= 'a' && c <= 'z')
 buf[i] = (c - 'a' + shift) % 26 + 'a';
 else if(c >= 'A' && c <= 'Z')
 buf[i] = (c - 'A' + shift) % 26 + 'A';
 //else, do nothing if chararacter is not between a-z or A-Z 
 i++;
 


int main(void)

 FILE* fp = fopen(FILE_NAME, "r");
 if(fp == NULL)
 
 printf("Can't open %sn", FILE_NAME);
 exit(EXIT_FAILURE);
 

 int buf_size = 1000;
 char *buf = malloc(sizeof(char) * buf_size);

 while(fgets(buf, buf_size, fp))
 
 //optional: remove the new line character if any
 int len = strlen(buf);
 if(len && buf[len - 1] == 'n')
 buf[len - 1] = 0;

 printf("plain : %sn", buf);
 caeser(buf, 1);
 printf("cipher: %snn", buf);
 

 //free the buffer allocated with malloc
 free(buf);
 fclose(fp);
 return 0;