MATLAB: Does matlab allow array indexing by a string by converting them into ascii codes

indexingMATLABmatrix manipulation

If you (by accident) enter a character array in an array indexing and it returns values not an error. It turns out that MATLAB converts characters to ASCII codes, use them as integer indices, and returns the values. For example:

X = rand(1,100);
X('A') == X(65)

I think this is just confusing, useless, and potentially risky. Does anyone know a good reason why MATLAB even supports this?

Best Answer

In many languages, and certainly in the majority of languages when matlab was created, the character type is just an alias for an integer type, and thus there was absolutely no reason for characters to be treated any different in matlab.

Modern languages do tend to treat the string (not char!) type differently and indeed matlab does too now. As dpb showed, if you use a string then you don't get that problem.

You may find it useless and confusing but for others it's useful. And certainly for somebody with a background in C-type languages, the opposite would be confusing.

In matlab, you could use this feature for caesar-type like cyphers for examples:

cypher('A':'Z') = circshift('A':'Z', 13);  %build caesar cypher
cypher('THE QUICK BROWN FOX')  %use cypher

The feature is used twice above: 1) to build the vector 'ABCD...Z' with the colon operator (if char wasn't treated as a number 'A':'Z' wouldn't work. 2) to index the cyper array.

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MATLAB: Use string or character arrays in code

"Which is preferred and why?"

In my opinion character arrays and string arrays have quite different applications, even though people often seem to treat them as being interchangeable. In a nutshell, I would decide which one to use based on these paradigms:

string array: use when you want to consider each string element as an atomic unit (of course it is possible to dig into the string and mess around with its individual characters, but that is not their raison d'être). For example, strings allow for arrays with identical sizes to corresponding numeric arrays (e.g. simpler assigning of function outputs in a for-loop). The various special methods (e.g. compose, etc.) are sometimes very convenient but not particularly efficient.
character array: use when you need to work with individual characters, e.g. compression, encoding/decoding, or when you need to write very efficient code. With character arrays you can use standard MATLAB indexing and arithmetic operations etc. to work with individual character codes. There is no way to beat this in terms of speed or memory.

MATLAB: Opening different files in for loop with dlmread

You need to call one file at a time in dlmread. Change to:

for k = 1:3
  N = string([1,2,3]);
  caesarN = strcat('caesar',N);
  caesarT = strcat(caesarN, '.txt');
  caesar(k) = dlmread(caesarT{k});
end

You can also use something like this, to generate the names

fnames = dir('filepath\ceasar*');

all files in the current dir beginning with 'ceasar' are stored in fnames.name

Best Answer

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