Solved

C++, how to find a character's matching index within another character array?

Posted on 2009-05-20
4
441 Views
Last Modified: 2012-05-07
Hi all,

I have a character array with base 64 digits "alph" and I have a character array containing a base 64 message, "cTotalImage". My algorithm below works fine, but I was thinking that there MUST be a better way to find a particular character's index within "alph". Does anyone know of a faster general approach to what I'm doing here?

Please let me know if I can clear any of this up! Thanks for your help in advance.

BTW I know there are classes out there that do this, but I'd rather write one of my own so that I can learn and grow my programming knowledge - do not suggest that I go elsewhere for a base 64 decoding class.
char alph[64];

	strcpy(alph, "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/");

	
 

		// Find index of characters in alph

		short iSearch;

		short iDecimals[4] = {0, 0, 0, 0};

		for (iSearch = 0; iSearch < 64; iSearch++)

		{

			if (cTotalImage[iDecodeIndex] == alph[iSearch])

			{

				iDecimals[0] = iSearch;

				break;

			}

		}

		for (iSearch = 0; iSearch < 64; iSearch++)

		{

			if (cTotalImage[iDecodeIndex + 1] == alph[iSearch])

			{

				iDecimals[1] = iSearch;

				break;

			}

		}

		// etc...

Open in new window

0
Comment
Question by:ehensens
  • 2
4 Comments
 
LVL 84

Expert Comment

by:ozo
Comment Utility
// if you are going to do it many times, initialize
int index[CHAR_MAX+1];

for( int i;i<64;i++){
   index[alpha[i] = i;
}

then

for( i=0;i<4; i++ ){
  decimals[i] = index[cTotalImage[iDecodeIndex+i]];
}
0
 

Author Comment

by:ehensens
Comment Utility
Hi ozo,

Thank you for your reponse. I cannot quite understand what you're doing (sorry), could you please add comments and fix the index[alpha[i] = i; part? Thanks!
0
 
LVL 86

Accepted Solution

by:
jkr earned 500 total points
Comment Utility
You can calculate the index if you take into accout that the alphabet is consecutive and the ASCII characters are also, e.g.
#include <ctype.h>
 

// ...
 

if (isupper(cTotalImage[iDecodeIndex]) { // uppercase character
 

  iDecimals[0] = cTotalImage[iDecodeIndex] - 65; // ASCII 'A' == 65

}
 

if (islower(cTotalImage[iDecodeIndex]) { // lowercase character
 

  iDecimals[0] = cTotalImage[iDecodeIndex] - 97 + 25; // ASCII 'a' == 97, add offset until 'Z' in your array

}
 

if (isdigit(cTotalImage[iDecodeIndex]) { // 0 - 9
 

  iDecimals[0] = cTotalImage[iDecodeIndex] - 48 + 50; // ASCII '0' == 97, add offset until 'z' in your array

}
 

// add special treatment for +/

Open in new window

0
 

Author Closing Comment

by:ehensens
Comment Utility
I like this approach, good thinking.
0

Featured Post

How your wiki can always stay up-to-date

Quip doubles as a “living” wiki and a project management tool that evolves with your organization. As you finish projects in Quip, the work remains, easily accessible to all team members, new and old.
- Increase transparency
- Onboard new hires faster
- Access from mobile/offline

Join & Write a Comment

Templates For Beginners Or How To Encourage The Compiler To Work For You Introduction This tutorial is targeted at the reader who is, perhaps, familiar with the basics of C++ but would prefer a little slower introduction to the more ad…
An Outlet in Cocoa is a persistent reference to a GUI control; it connects a property (a variable) to a control.  For example, it is common to create an Outlet for the text field GUI control and change the text that appears in this field via that Ou…
The goal of this video is to provide viewers with basic examples to understand and use conditional statements in the C programming language.
The viewer will be introduced to the technique of using vectors in C++. The video will cover how to define a vector, store values in the vector and retrieve data from the values stored in the vector.

772 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question

Need Help in Real-Time?

Connect with top rated Experts

12 Experts available now in Live!

Get 1:1 Help Now