Thread: Please i need someone to explain this code step by step

  1. #1
    Registered User
    Join Date
    Oct 2020
    Posts
    1

    Please i need someone to explain this code step by step

    Hello all I'm new to c language programming can someone explain to me the below code,
    I know that it is a playfair encryption in c language. But I need step by step explanation of the code please


    ----------------------------------------------------------------

    Code:
    include <stdio.h>
    #include <conio.h>
    #include <string.h>
    #include <ctype.h>
    #define Matrix 5
    int ch;
    
    
    void pfemulator(char c1, char c2, char key[Matrix][Matrix]) {
    
    
       
        int i, j, w, x, y, z;
        for (i = 0; i < Matrix; i++) {
            for (j = 0; j < Matrix; j++) {
                if (c1 == key[i][j]) {
                    w = i;
                    x = j;
                } else if (c2 == key[i][j]) {
                    y = i;
                    z = j;
                }
            }
        }
    
    
        if (w == y) {
            if(ch==1){
                x = (x + 1) % 5;
                z = (z + 1) % 5;
            }
            else{
                x = ((x - 1) % 5+5)%5;
                z = ((z - 1) % 5+5)%5;
            }
            printf("%c%c", key[w][x], key[y][z]);
        } else if (x == z) {
            if(ch==1){
                w = (w + 1) % 5;
                y = (y + 1) % 5;
            }
            else{
                w = ((w - 1) % 5+5)%5;
                y = ((y - 1) % 5+5)%5;
            }
            printf("%c%c", key[w][x], key[y][z]);
        }
        else {
            printf("%c%c", key[w][z], key[y][x]);
        }
    
    
    }
    
    
    void removeDuplicates(char str[]){
        int hash[256]        =  {0};
        int currentIndex     = 0;
        int lastUniqueIndex  = 0;
        while(*(str+currentIndex)){
            char temp = *(str+currentIndex);
            if(0 == hash[temp]){
                hash[temp] = 1;
                *(str+lastUniqueIndex) = temp;
                lastUniqueIndex++;
            }
    
    
            currentIndex++;
        }
        *(str+lastUniqueIndex) = '\0';
    
    
    }
    
    
    
    
    void main() {
        int i, j, k = 0, l, m = 0, n;
        char key[Matrix][Matrix], keyminus[25], keystr[10], str[25] = {
            0
        };
    
    
        char alpa[] = {
            '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'
        };
     
        printf("\n ======= Playfair Cipher emulator =======\n");
        printf("\n1.Encryption\n2.Decryption\n\nSelect an option (1 or 2): ");
        scanf("%d",&ch);
        if(ch!=1 && ch!=2){ printf("Invalid Selection"); return;}
        // if the user enters wrong choice(1-2) the program displays an error message
        fflush(stdin);
        printf("\nEnter the key: ");
        gets(keystr);
        printf("Enter the plain/encrypted text: ");
        gets(str);
        removeDuplicates(keystr); 
        n = strlen(keystr);
    
    
        
        for (i = 0; i < n; i++) {
            if (keystr[i] == 'j') keystr[i] = 'i';
            else if (keystr[i] == 'J') keystr[i] = 'I';
            keystr[i] = toupper(keystr[i]);
        }
       
        for (i = 0; i < strlen(str); i++) {
            if (str[i] == 'j') str[i] = 'i';
            else if (str[i] == 'J') str[i] = 'I';
            str[i] = toupper(str[i]);
        }
    
    
       
        j = 0;
        for (i = 0; i < 26; i++) {
            for (k = 0; k < n; k++) {
                if (keystr[k] == alpa[i]) break;
                else if (alpa[i] == 'J') break;
            }
            if (k == n) {
                keyminus[j] = alpa[i];
                j++;
            }
        }
    
    
    
    
      
        k = 0;
        for (i = 0; i < Matrix; i++) {
            for (j = 0; j < Matrix; j++) {
                if (k < n) {
                    key[i][j] = keystr[k];
                    k++;
                } else {
                    key[i][j] = keyminus[m];
                    m++;
                }
                printf("%c ", key[i][j]);
            }
            printf("\n");
        }
    
    
      
        printf("\nEntered text :%s\nOutput Text :", str);
        for (i = 0; i < strlen(str); i++) {
            if (str[i] == 'J') str[i] = 'I';
            if (str[i + 1] == '\0') pfemulator(str[i], 'X', key);
            else {
                if (str[i + 1] == 'J') str[i + 1] = 'I';
                if (str[i] == str[i + 1]) pfemulator(str[i], 'X', key);
                else {
                    pfemulator(str[i], str[i + 1], key);
                    i++;
                }
            }
        }
    
    
        if(ch==2) printf(" \n\n(Remove unnecessary X if found)\n");
    }

  2. #2
    and the hat of int overfl Salem's Avatar
    Join Date
    Aug 2001
    Location
    The edge of the known universe
    Posts
    38,293
    > void main()
    ...
    > fflush(stdin);
    ...
    > gets(keystr);
    My first thought would be to find someone not stuck with a 1980's DOS mentality to copy/steal code from.

    > I know that it is a playfair encryption in c language
    Gee - yah think!?
    printf("\n ======= Playfair Cipher emulator =======\n");


    > But I need step by step explanation of the code please
    Sure, do you know about the goldilocks problem?

    That it's a waste of our time busting a gut to give you what you ask, when there's a 66% chance of you responding with "too easy" or "too hard".

    You show us what you know / have doubts about / don't know a thing and then we'll fill in the blanks.

    > Hello all I'm new to c language programming can someone explain to me the below code
    Yeah, that code isn't really newbie code material.
    What else is going on?
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.

Popular pages Recent additions subscribe to a feed

Similar Threads

  1. Replies: 6
    Last Post: 08-09-2015, 12:30 PM
  2. Replies: 1
    Last Post: 03-08-2013, 03:16 AM
  3. Step by step read data from exe file to memory.
    By JoBlack in forum C++ Programming
    Replies: 1
    Last Post: 06-23-2012, 02:02 PM
  4. Replies: 4
    Last Post: 05-26-2011, 06:51 AM
  5. why does the memory increase step by step?
    By zcrself in forum C Programming
    Replies: 9
    Last Post: 07-14-2010, 12:04 AM

Tags for this Thread