这是关于实现凯撒密码来加密用户给定的输入

Question

The code takes input(the string) to be encoded and a key from the user such that when the key is added to the input the input is increased by the amount of the given key . For ex. if the key is 2, so the input A changes to C, b changes to d, and so on.

I have written a code for the same but cannot get the output.

int main()
{
  int x,i,y,c;
  char text[20];

  printf("enter the plaintext:");
  gets(text);
  printf("enter the key: ");
  scanf("%d",&x);
  for (y=0;y<strlen(text);y++)
  {
    if (text[i]>='a'&&text[i]<='z'&&text[i]>='A'&&text[i]<='Z' )
    {
      int c=(int)text[i]+x;
      printf("%c\n",text[i]);
    }
  }
}

The result that i am getting is blank. kindly help me.

Answer 1

There are a lot of problems in your proposal, it is needed to check the inputs success, you iterate on y rather than on i , you compute the new char code but you do not print it

Here a corrected proposal :

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

int main()
{
  char text[100];

  printf("enter the plaintext:");

  if (fgets(text, sizeof(text), stdin) != NULL) {
    int key;

    printf("enter the key: ");
    if (scanf("%d", &key) == 1) {
      int i;

      for (i=0; text[i] != 0; ++i)
      {
       if (isalpha(text[i]))
         putchar(text[i] + key); /* may not be printable */
       else
         putchar(text[i]);
      }
    }
  }
  return 0;
}

Compilation and execution :

pi@raspberrypi:/tmp $ gcc -pedantic -Wextra c.c
pi@raspberrypi:/tmp $ ./a.out
enter the plaintext:the sentence to encode
enter the key: 3
wkh vhqwhqfh wr hqfrgh
pi@raspberrypi:/tmp $ ./a.out
enter the plaintext:Alea jacta est
enter the key: 2
Cngc lcevc guv

For the fun, 32 is not a very good key to encode uppercase characters :

pi@raspberrypi:/tmp $ ./a.out
enter the plaintext:THE FIVE BOXING WIZARDS JUMP QUICKLY.
enter the key: 32
the five boxing wizards jump quickly.

Answer 2

You initialized the y variable instead of i variable
try this :

for (i = 0; i < strlen(text); i++) { ... }

Note :
if you compile your code with the following flags -Wall -Wextra -Werror it will help you so much to know more about errors you might have, like unused variable.
example : gcc -Wall -Werror -Wextra youprogram.c -o output

you have other errors not just this one,
so i suggest to you my solution to your problem : (all output characters will be printable)

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

int     main(void)
{
    char    text[250];
    size_t  i;
    int     key;

    printf("Enter the plaintext : ");
    if (fgets(text, sizeof(text), stdin) != NULL)
    {
            printf("Enter the key : ");
            scanf("%d", &key);
            for(i = 0; i < strlen(text); i++)
            {
                    if (text[i] >= 'a' && text[i] <= 'z')
                            putchar((((text[i] - 'a') + key) % 26) + 'a');
                    else if (text[i] >= 'A' && text[i] <= 'Z')
                            putchar((((text[i] - 'A') + key) % 26) + 'A');
                    else
                            putchar(text[i]);
            }
    }
    return (0);
}