Why my Caesar cipher code can not run?

Question

#include<stdio.h>
int main()
{
    int amount,i;
    char message[81],encry[81];
    printf("Enter message to be encrypted : ");
    gets(message);
    printf("Enter shift amount (1-25) : ");
    scanf("%d");
    for(i=0;message!='\0';i++)
    {
        if(message[i] >='A' && message[i] <='Z')
        encry[i]=((message[i]-'A')+amount)%26+'A';
    }
    printf("Encrypted message : ");
    int j;
   for(j=0;message[j]!='\0';j++)
    printf("%c",encry[j]);

 return 0;}

I try to write a caesar cipher code but it can not run. Please help me. Thank you.

Answer 1

1 .

 printf("Enter shift amount (1-25) : ");
 scanf("%d");
           ^ missing variable here (you need to pass an argument of type int *)

Where will you take input key then ?

You need to take input in amount variable like this -

 printf("Enter shift amount (1-25) : ");
 scanf("%d",&amount);

So , you don't store anything in amount and it is uninitialized .And you use it , therefore it causes undefined behaviour.

2. In this loop -

 for(i=0;message!='\0';i++)      // you compare here char * with char

condition should be -

 for(i=0;message[i]!='\0';i++)

3. Don't use gets to take input, it won't prevent overflow . Use fgets -

fgets(message,81,stdin);

Answer 2

1. Error in scanf("%d") syntax it has to be like scanf("<format specfier>",&variable);

   So the fix is scanf("%d", &amount)

2. int j; is not a good practice to add the variable before for loop.

Answer 3

Problems:

1. Here:

    scanf("%d"); 

   The %d requires a second argument(of type int* ) for storing the scanned number. You probably wanted:

    scanf("%d", &amount); 

2. You are comparing a char and a char* here:

    for(i=0;message!='\\0';i++) 

   You probably wanted

    for(i = 0; message[i] != '\\0'; i++) 

---

Don't use gets() as it is deprecated. Use fgets instead:

fgets(message, sizeof(message), stdin);

Note that fgets consumes the newline character and stores it in message (if there is space).