Question

The following question is for C programming.

There is a function that has the header char * decodeSubstitution(char * lcEncodingKey, char * src, char *dest)

What it ultimately is supposed to do is take an encoded array of characters (src), decode it and put it into dest, and return the decoded array of characters. The first step, however, is what I need help with. The first step is to create the decoding key called lcDecodingKey using the encoding key was was passed as an argument. The encoding key passed in this case is an array of characters is called lcEncodingKey and is the array of characters "qwertyuiopasdfghjklzxcvbnm". We ultimately want the decoding key to be "abcdefghijklmnopqrstuvwxyz". The result of this is a 'q' would become an 'a', a 'w' would become a 'b', an 'e' would become a 'c' etc. We had originally encoded the phrase "Ciphers are fun" to "Eohitkl qkt yxf", and now in this function we want to decode "Eohitkl qkt yxf" back to "Ciphers are fun". I just need help with the first part of creating the decoding key using the encoding key.

Answer 1

C CODE:

#include <stdio.h>
#include<string.h>
char * decodeSubstitution(char * lcEncodingKey, char * src, char *dest){
char lcDecodingKey[26];//initialize the decoding key of 26 character as there are 26 alphabets in total
// we would iterate 26 times and each time we take the character from the encoding key and
//calculate it's index in by subtracting from 'a'
//The index which is calculated is used to decode
//e.g index of q will be 17 we will go at 17th index in decoding key and put character 'a' in it as it represents a
//this way we will iterate through lcEncodingKey and calculate index of all and then put the character one by one in it
  
for(int i=0;i<26;++i)
{

char ch=(char)('a'+i);
lcDecodingKey[lcEncodingKey[i]-'a']=ch; //put the character in decoding key
  
}
int l =strlen(src);
//decode function
for(int i=0;i<l;++i)
{
if(src[i]==' ')
dest[i]=' '; //if space then input space in dest
else
{
if(src[i]>='a'&&src[i]<='z')   
dest[i]=lcDecodingKey[src[i]-'a'];
else
dest[i]='A'+lcDecodingKey[src[i]-'A']-'a'; // if the string has capital letter than index will be calculate by subtracting it //from 'A'
//then to put capital letter to its place we need to add 'A' and subtract 'a' as decoding key has characters in lower case //only
}
}
printf("%s ",dest);
return dest;
}
  
int main()
{
char * lcEncodingKey="qwertyuiopasdfghjklzxcvbnm";
char *src="Eohitkl qkt yxf";
char dest[26];
char *d;
d=decodeSubstitution(lcEncodingKey,src,dest);
return 0;
}

DECODING KEY GENERATED IS :  

kxvmcnophqrszyijadlegwbuft