编码/解码程序的逻辑错误 (C)

Question

I am a beginner in a C programming class, and our assignment for the third chapter of our textbook was to create an encode/decode program that takes in four digits from user input and rearranges them depending on whether or not the user wants them to be either encoded or decoded. I am also having trouble with getting the program to do both decoding and encoding in one file . However, when I fire up the program, whenever I select encode and type in my four digits, the program not only gives me a massive number that should only be four digits as the encoded result. It also gives me the "press any key to continue" prompt, signifying the end of the program, instead of starting over completely and going back to where it asks the user to either decode or encode.

The output is also attached.

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I've tried looking around and changing some of the variables from int to double, as well as checking my math... I'm not 100% sure why the encryption and decryption aren't limited to anything besides four digits.

As for getting the program to start over, I've tried using a do-while loop, but it still didn't seem to work, so I deleted it.. I also tried using another if statement and then using break, but it was marked as an error.

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The assignment says that, "each digit should be encrypted by adding 7 and taking the remainder after division by 10; after encrypting each digit, swap the first and third digits, and then swap the second and fourth digits. Decryption should reverse the process. Program must do encode and decode in one file.

Here is the example that my instructor posted for this, this is what the output should look like.

Enter a four digit number: 1234
Encoded Digits: 0189
Continue (1) Exit (0): 1
Encode (1) Decode (2): 2
Enter a four digit number: 0189
Decoded Digits: 1234
Continue (1) Exit (0): 0

Here is my code:

///Compiler used: Microsoft Visual Studio
///Language: C
#include <string>
#pragma warning(disable: 4996) 
#include <math.h>
#include<stdio.h>
#include<stdlib.h>

int main()
{

    int Encodechoice = 1;
    int inputdigit1 = 0;
    int Decodechoice = 2;
    int UserChoice = 0;
    int ContinueChoice = 0;
    int UserDigit1 = 0;
    int UserDigit2 = 0;
    int UserDigit3 = 0;
    int UserDigit4 = 0;

    {
        printf("(1) Encode, (2) Decode\n");
        printf("Make your selection.\n");//Asks user to make selection
        scanf_s("%d", &UserChoice);

        if (UserChoice == 1)//begin encryption

        {
            UserDigit1 = 0;
            UserDigit2 = 0;
            UserDigit3 = 0;
            UserDigit4 = 0;
            printf("Encode: Enter FOUR integers.\n");
            scanf_s("%d", &UserDigit1, &UserDigit2, &UserDigit3, &UserDigit4);
            int EncodedIntegers1 = (UserDigit1 + 7) % 10;
            int EncodedIntegers2 = (UserDigit2 + 7) % 10;
            int EncodedIntegers3 = (UserDigit3 + 7) % 10;
            int EncodedIntegers4 = (UserDigit4 + 7) % 10;
            printf("Encoded result:\n");
            printf("%d", "%d", "%d", "%d\n", EncodedIntegers3, &EncodedIntegers4, &EncodedIntegers1, &EncodedIntegers2); /// swap order of integers
        }///end if

        if (UserChoice == 2)///begin decryption
        {
            UserDigit1 = 0;
            UserDigit2 = 0;
            UserDigit3 = 0;
            UserDigit4 = 0;
            printf("Decode: Enter FOUR integers.\n");
            scanf_s("%d", &UserDigit1, &UserDigit2, &UserDigit3, &UserDigit4);
            int DecodedIntegers1 = (UserDigit1 - 7) * 10;
            int DecodedIntegers2 = (UserDigit2 - 7) * 10;
            int DecodedIntegers3 = (UserDigit3 - 7) * 10;
            int DecodedIntegers4 = (UserDigit4 - 7) * 10;
            printf("Decoded result:\n");
            printf("%d", "%d", "%d", "%d\n", DecodedIntegers1, &DecodedIntegers2, &DecodedIntegers3, &DecodedIntegers4); /// keep order the same to decrypt
        }///end if
        system("pause");
        return 0;
    }
}

Output:

Make your selection.
1
Encode: Enter FOUR integers.
1234
Encoded result:
11893608 Continue? (1) Yes, (0) No
Make your selection.
1
Press any key to continue . . .```

Answer 1

You scanf is wrong. I would rather do this

char UserInputNumber[5];
printf("Encode: Enter FOUR integers.\n");
scanf("%s", UserInputNumber);
// optional error checking the input: length is 4? All char is a number?
UserDigit1 = (UserInputNumber[0] - '0');
UserDigit2 = (UserInputNumber[1] - '0');
UserDigit3 = (UserInputNumber[2] - '0');
UserDigit4 = (UserInputNumber[3] - '0');

Also your printf should look like this:

printf("%d%d%d%d\n", EncodedIntegers3, EncodedIntegers4, EncodedIntegers1, EncodedIntegers2);

With the & you tried to print their memory address not their value, but also your format string was wrong too.

Answer 2

The easiest way is probably work directly on a string consisting of the four digits:

char d[5], e[5] = { 0 };
scanf("%4s", d)

This is for encoding:

for (int i = 0; i < 4; i++) e[(i+2)%4]=(d[i]-41)%10+'0';

For decoding, you have to change the magic 41 to 45 . The result is in e .