CODEWITHSUNDEEP
arraysmatlabmatrix
Adam West
Adam West

Reputation: 105

How to reference indexes of one array with all values from another array in Matlab

New to Matlab and am trying to convert a binary message to characters in Matlab. I was able to translate the binary message to a single column in decimal (without the bin2dec function, as I was instructed not to), with each number corresponding to a character in a string array.

I am having trouble getting the character string array to show its values at the values for the indexes of another array. It works when I reference individual indexes of an array (e.g. message = char_array(decimal(1)) returns a value of k), but not when I try to reference the whole column or the whole array of decimal (e.g. message = char_array(decimal(:,:)) and message = char_array(decimal(:,1)) do not work and give the same error). I know this can accomplished with for loops, but we have been instructed not to use those yet.

Is there some other trick to getting the entire message to display at once? This needs to work in such a way that it could be used for thousands of letters at once with one simple line of code.

The code so far:

 clear;clc

 % Array of characters (first value is a space, last is ! which denotes the end of the message)

 char_array =  ' abcdefghijklmnopqrstuvwxyz!'; 

 % The secret message in binary

 secret =   [0 1 1 0 0;
             0 0 0 0 1; 
             1 0 0 0 0; 
             0 1 0 0 0;
             1 0 0 1 0;
             0 1 1 1 1;
             0 0 0 0 1;
             0 1 0 0 1;
             0 0 1 1 1;
             0 0 0 0 0;
             1 0 0 1 0;
             0 1 1 1 1;
             0 0 0 1 1;
             0 1 0 1 1;
             1 0 0 1 1;
             1 1 0 1 1];


 % Assigns a decimal value to each column to help convert binary values to decimal

 conv = [16 8 4 2 1]; 

 % Converts binary numbers in each column of secret message to a decimal value

 convert = [secret(:,1)*conv(1), secret(:,2)*conv(2), secret(:,3)*conv(3), secret(:,4)*conv(4), secret(:,5)*conv(5)]; 

 % Converts unadded decimal values into columns to prepare for addition

 binary = convert'; 

 % Adds numbers in each column, then transposes it to a single column of decimal values representing values of each row of original message

 decimal = sum(binary)'; 

 % Should display character from character array for each index value of "decimal", but returns "Subscript indices must either be real positive integers or logicals." error each time. Works for individual entries such as decimal(1), though.

 message = char_array(decimal(:,:))

Upvotes: 1

Views: 179

Answers (2)

Divakar
Divakar

Reputation: 221514

Fast matrix multiplication based technique for a generic compact one-liner solution -

message = char_array(secret*(2.^(size(secret,2)-1:-1:0)')+1)

Upvotes: 2

Adam West
Adam West

Reputation: 105

My code was fine. The problem was that one of the values from decimal was zero, and you cannot reference an array value of zero! :)

If you add 1 to each value of decimal to compensate for the space at the beginning of the character array, it yields the message correctly.

Towards the bottom of the code:

% Adds numbers in each column, then transposes it to a single column of decimal values representing values of each row of original message

decimal = sum(binary)'; 

% Adds 1 to each value of "decimal"

decimal2 = decimal+1; 

message = char_array(decimal2(:,:))

Which decodes the message properly. (And yes, Laphroaig does indeed rock!)

Upvotes: 2

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