凯撒密码问题（C）

Question

The purpose of this code is to create a Caesar cipher that only encodes alphanumeric characters with 3 keys, where the 1st letter is incremented by the 1st key, 2nd character by the 2nd key, 3rd character by the 3rd key, 4th character by the 1st key etc... with wrap around (Z + 1 --> a), (a - 1 --> Z).

I've completed the assignment completely, the only issue I have is that my negative wrap around doesn't work (a - 1 --> Z). It asks for the keys, takes input, and then returns nothing (but still allows the user to type and hit enter without any result). Here is my code:

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

char sentence[101] = { '\0' };
char alphabet[52]={'a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z','A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z'};

int locateletter(char tolocate){
    int i=0;
    for(i=0; i<52; i++){
            if(alphabet[i] == tolocate)
                    return i;
    }
}

int main(void){
    int key1 = 0;
    int key2 = 0;
    int key3 = 0;
    printf("Sentence: ");
    scanf("%101[^\n]", sentence);
    if( sentence[100] != '\0' ){
        printf("You entered more than 100 characters. Block Caesar Cipher is exiting. Goodbye.\n");
        exit;
    }else{
        printf("Keys: ");

        scanf("%d %d %d", &key1, &key2, &key3);
        int i=0;
        for(i=0; i<100; i=i+3){
                if(isalpha(sentence[i])){
                        int position = locateletter(sentence[i]);
                        while((position+key1)>51){
                                key1 = position+key1-52;
                                position=0;
                        }
                        while((position+key1)<0){
                                key1 = key1+position+52;
                                position=0;
                        }
                        sentence[i] = alphabet[position+key1];
                }
        }
        int k=1;
        for(k=1; k<100; k=k+3){
                if(isalpha(sentence[k])){
                        int position = locateletter(sentence[k]);
                        while((position+key2)>51){
                                key2 = position+key2-52;
                                position=0;
                        }
                        while((position+key2)<0){
                                key1 = key2+position+52;
                                position=0;
                        }
                        sentence[k] = alphabet[position+key2];
                }
        }
        int t=2;
        for(t=2; t<100; t=t+3){
                if(isalpha(sentence[t])){
                        int position = locateletter(sentence[t]);
                        while((position+key3)>51){
                                key3 = position+key3-52;
                                position=0;
                        }
                        while((position+key3)<0){
                                key1 = key3+position+52;
                                position=0;
                        }
                        sentence[t] = alphabet[position+key3];
                }
        }
        printf("Cipher: %s\nDone.\n", sentence);

    }
    return 0;
}

Answer 1

I found the error. My while loop for negative wrap around assigns the values to key1 instead of key2/3.