Help with this code - logical indexing and num2cell -

Hi everyone ! Would you mind taking a look at this code and explaining it to me-specifically the lines marked.

But first here is what the code does:

The function replace_me is defined like this:

function w = replace_me(v,a,b,c).

The first input argument v is a vector, while a, b, and c are all scalars. The function replaces every element of v that is equal to a with b and c. For example, the command

>> x = replace_me([1 2 3],2,4,5);

makes x equal to [1 4 5 3]. If c is omitted, it replaces occurrences of a with two copies of b. If b is also omitted, it replaces each a with two zeros.

    function [result] = replace_me(v,a,b,c)
    if nargin == 3
        c = b;
    end
    if nargin == 2
        c = 0;
        b = 0;
    end
    result = v;
    equ = result == a;          %here
    result = num2cell(result);  %here
    result(equ) = {[b c]};      %here
    result = [result{:}];       %here
    end

[Stephen23]

You can also use strrep to do this:

>> strrep([1,2,3],2,[4,5]) 
ans =
     1     4     5     3

Answer 1

[Cedric]

The line that you mention defines a vector equ of logical elements which are true (1) where the condition is true (relational operator ==) and false (0) otherwise. Arrays of logicals can be used for indexing arrays (numeric or cell). This is what is done then in the following:

 result(equ) = {[b c]};

There are several things here that you can study for understanding the code better:

• The logical type/class.
• Relational operators.
• Logical indexing.

These three topics are well developed in MATLAB help/doc. To give you an example, let's define a numeric array:

 >> x = [1, 8, 3, 5, 7, 2] ;

Now we apply a relational operator > (greater than) between x and the value 3, and we save the outcome of the test in a variable named isGt3 :

 >> isGt3 = x > 3
 isGt3 =
     0     1     0     1     1     0

You can see that elements of isGt3 are 0 where the relation is false, and 1 where it is true. But these 0 and 1 are not numbers:

 >> class( isGt3 )
 ans =
     logical

They are booleans, symbolized/represented by symbols 0 and 1. Now it is interesting to note that you can use this vector of logicals for performing logical indexing, which is extracting all elements associated with a logical index that is true:

 >> x(isGt3)
 ans =
     8     5     7

and here you see elements at indices 2, 4, 5 of isGt3 which are true, indexed/addressed/extracted elements of x with same indices.