#include<stdio.h>
#include<stdlib.h>
#include <ctype.h>
 
 
int encode (int, int);
 
 
int encode(int ch, int key) { 
    if (islower(ch)) {
         ch = (ch-'a' + key) % 26 + 'a';
         ch += (ch < 'a') ? 26 : 0;
    }
    else if (isupper(ch)) {
         ch = (ch-'A' + key) % 26 + 'A';
         ch += (ch < 'A') ? 26 : 0;
    }
    return ch;
}
int main (void) 
{
    FILE *file_in;
    FILE *file_out;
    char ch;
    char text[300];
    int key;
 
 
    printf("Enter the name of the file you wish to see including complete pathway and file\n extension:\n");
    gets(text);
    file_in = fopen(text,"r"); 
 
 
    if( file_in == NULL )
    {
       perror("Error while opening the file.\n");
       exit(EXIT_FAILURE);
    }
       printf("\n The contents of the file are : \n");
     
    while( ( ch = fgetc(file_in) ) != EOF )
    {
       printf("%c",ch);
    }
 
 
    printf("\n \n Enter the name of the file you wish to send the encryption\n to including complete pathway and file\n extension:\n");
    gets(text);
    file_out = fopen(text,"w");
     
    printf("\n Enter the alphabetic offset key you would like to use:");
    scanf("%d", &key);
     
    while( ( ch = fgetc(file_in) ) != EOF )
    { 
        encode(ch, key);
        fprintf(file_out, "%c", ch);
    }
    printf("file has been encoded");
    fclose(file_out);
    fclose(file_in);
     
    return 0;