Thread: Help with cipher key problem

Ok, Hello all, new to this forum and C for that matter, but if anyone could help at all it would be greatly appreciated.

I have been tasked to create a program to read a simple substitution cipher key from a file, then a plaintext file, and write the converted text to the output file - no problem. Now I have to create a program to create the inverse key from the original. I can generate the key myself on paper but I can't see for the life of me how I would generate the inverse within the program. I'm not asking for it to be written out for me, just some insights onto the approach I should take would be really useful, as I've been staring at this thing for hours and it's driving me up the wall!!

Many Thanks
-Chris

So have a struct containing two characters

Code:

struct foo { char clear; char encrypted; }

Then have say

Code:

struct foo table[26];

And initialise however you please, say

Code:

table[0].clear = 'a'; table[0].encrypted = 'q';

Encryption searches for a matching .clear and outputs the .encrypted
Decryption searchs for a matching .encrypted and outputs .clear

Apologies, I should have described the situation a bit better. I'm only supposed to use arrays and loops to acheive this. I have a key array within my cipher program and a loop which reads the ascii code of the character in each part of the array one by one, it then subtracts 65 and uses that value to determine what position of the cipher array to take the output character from. this allows there to be an inverse key...ie

plaintext ABCDEFGHIJKLMNOPQRSTUVWXYZ
key LKASDFJHGMZNXCVBQWPOERTIUY
inverse CPNE....etc

The inverse is such because A got turned into L, but C was turned into A, so in the inverse key, A becomes C and so on and so forth.

So basically i need to create a program to generate this inverse key using only loops and arrays.

Many thanks in advance
-Chris

So use two arrays then and forget the structure.

Have solved the problem, the solution came to me whilst I was trying to get to sleep =)

Thanks for all your help guys

-Chris