凯撒密码无法正常使用C

Question

I've been working on a Caesar cipher problem for a problem set but I ran into a minor problem. Whenever the cipher value gets more the the ascii of 'z', I want it to bounce back to 'a' but I'm not able to figure out how to do so. Here is the code:

#include <cs50.h>
#include <ctype.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main(int argc, string argv[])
{
    if(argc !=2 && !isdigit(argv[1]))
    {
        return 1;
    }

    //convert input to int and get the string
    int k = atoi(argv[1]);
    k=k%26;
    //printf("%d" ,k);
    //get he text

    char *s;
    s=GetString();
    int i, n=strlen(s);

    //checking each character
    for(i=0;i<n;i++)
    {
        if(s[i]==' ')
        {
            s[i]=' ';
        }
        else
        {
            s[i]=s[i]+k;
        }
        printf("%c" ,s[i]);
    }

}

This is quite a basic code. Any help will be appreciated.

PS here is an example with key 4.

input-  Vinay Dawani
output- Zmre} He{erm

Answer 1

Assuming that the code set in use is not EBCDIC (which has non-alphabetic characters in between alphabetic characters), then letters of the Latin alphabet (unaccented az) are coded as contiguous code points in all the common code sets. The rotation of the alphabet is simple when you know how.

Fundamentally, you convert each letter into an offset 0..25 from the letter a or A (depending on case), add the encoding key, take the result modulo 26, and add the starting letter ( a or A ) back. Note that the arithmetic takes place as int , but the result is assigned back to a (possibly signed) char .

#include <cs50.h>
#include <ctype.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main(int argc, string argv[])
{
    if (argc != 2 || !isdigit((unsigned char)argv[1][0]))
    {
        fprintf(stderr, "Usage: %s shift\n", argv[0]);
        return 1;
    }

    // convert input to int and get the string
    int k = atoi(argv[1]) % 26;
    if (k < 0)
        k += 26;

    char *s = GetString();
    int n = strlen(s);

    printf("Original:  [%s]\n", s);

    // encoding each character
    for (int i = 0; i < n; i++)
    {
        if (isupper((unsigned char)s[i]))
        {
            s[i] = 'A' + (s[i] - 'A' + k) % 26;
        }
        else if (islower((unsigned char)s[i]))
        {
            s[i] = 'a' + (s[i] - 'a' + k) % 26;
        }
    }

    printf("Encrypted: [%s]\n", s);
    return 0;
}

Example run:

$ ./caesar13 3
Caesar's cipher is hardly secure against the lazy dog jumping over the quick brown fox, is it?
Original:  [Caesar's cipher is hardly secure against the lazy dog jumping over the quick brown fox, is it?]
Encrypted: [Fdhvdu'v flskhu lv kdugob vhfxuh djdlqvw wkh odcb grj mxpslqj ryhu wkh txlfn eurzq ira, lv lw?]
$ ./caesar50 23
Fdhvdu'v flskhu lv kdugob vhfxuh djdlqvw wkh odcb grj mxpslqj ryhu wkh txlfn eurzq ira, lv lw?
Original:  [Fdhvdu'v flskhu lv kdugob vhfxuh djdlqvw wkh odcb grj mxpslqj ryhu wkh txlfn eurzq ira, lv lw?]
Encrypted: [Caesar's cipher is hardly secure against the lazy dog jumping over the quick brown fox, is it?]
$

The CS50 library is readily available online.

Answer 2

After you add k to the letter, check if it's higher than z . If it is, just subtract the number of letters in the alphabet to wrap it around.

s[i] = s[i]+k;
if (s[i] > 'z') {
    s[i] = s[i] - 26;
}

to deal with upper and lower case, you need separate tests in the main loop.

for(i=0;i<n;i++)
{
    if(islower(s[i]))
    {
        s[i] += k;
        if (s[i] > 'z') {
            s[i] -= 26;
        }
    }
    else if (isupper(s[i]))
    {
        s[i] += k;
        if (s[i] > 'Z') {
            s[i] -= 26;
        }
    }
    printf("%c" ,s[i]);
}

Notice that I removed the test for if (s[i] == ' ') . That's not necessary -- anything that isn't a letter is simply left alone.