Caesar Cipher in C Help

[Ashley Demers]

I am taking a second semester programming course in C and have to write this program that I have to write a program that encrypts a message using a Caesar cipher. The user will enter the message to be encrypted and the value of the key. We are to assume that the message will not exceed 80 characters. We are supposed to include in the code something for the alphabet to wrap around like if the key is 2 then y would be replaced by a. The hint given for this is: to handle the wrap-around problem for upper-case characters, use the expression
((ch-‘A’) + k) % 26 + ‘A’
to calculate the encrypted version of an upper-case letter, where k is the key (i.e., the shift amount). You will need a similar expression for lower-case letters.
I am having a lot of trouble trying to figure this out, and we are supposed to be using arrays
when i try to compile i get error and cannot figure out what I am doing wrong#include <stdio.h>
#include <string.h>

int main()
{
   int shift;
   char message[80];
   printf("Enter message to be encrypted: ");
   scanf("%s", &message);
   printf("Enter shift amount (1-25): ");
   scanf("%d", &shift);
 
 
   for(int i=0;i<strlen(message);i++)
   {
      if (message[i]=!' ')
     if('A'>= message[i] <= 'Z')
         {
        message[i] = ((message[i]-'A') + shift) % 26 + 'A';
     }
     else if('a'>= message[i] <='z')
     {
            message[i] =((message[i]-'a') + shift) % 26 + 'a';
     }
   }
   printf("Encrypted message: %s", message);
   return 0;
}

[jjdot]

I think you should enable all warning messages of your compiler. gcc -Wall used on your program gives:

t.c:16:1: warning: suggest parentheses around assignment used as truth value
t.c:17:1: warning: comparisons like ‘X<=Y<=Z’ do not have their mathematical meaning
t.c:21:1: warning: comparisons like ‘X<=Y<=Z’ do not have their mathematical meaning
t.c:16:4: warning: suggest explicit braces to avoid ambiguous ‘else’

I think you have problems with C syntax:

disequality is != and not =!
x <= y <= z is written x <= y && y <= z


if (message[i]=!' ')