How to output a vector/array from a created function

In short, I have an array that has numbers from 0 to 100 using a step of 0.01, I created a function that is supposed to output another array containing 1's and -2's. While looping on the indices of my first array if the number is smaller than 40, I insert 1 in my output array, otherwise I insert a -2. I don't know what's the problem in my code:

function [outputVector] = myOutput(inputVector)

outputVector = [];

for i=1 : size(inputVector)
    if inputVector(i) < 40
        outputVector = [outputVector,1] ;
    else
        outputVector = [outputVector,-2];
    end
end

end

Answer 1

[Shwetank Shrey]

The vector that you would be creating using 0 : 0.01 : 100 would have a size of 1x10001.

>> size(0 : 0.01 : 100)

ans =

           1       10001

Your for loop on the size returns the first dimension whereas you require the second dimension.

Changing

for i=1 : size(inputVector)

to

for i=1 : size(inputVector, 2)

should work for you.

[Stephen23]

size([0:0.01:100])
%    ^          ^ the square brackets are superfluous.

https://www.mathworks.com/matlabcentral/answers/35676-why-not-use-square-brackets

[Shwetank Shrey]

Thanks. I wasn't aware of this and will edit the answer accordingly.

Answer 2

[James Tursa]

Don't use size(inputVector) for your loop indexing limits since this is a vector. Use numel(inputVector) instead.

Also, you shouldn't be increasing the size of outputVector iteratively inside your loop. Each time you do that, the memory for the variable has to get copied to new memory. This can hurt the performance in a massive way. Instead, pre-allocate and use indexing. E.g., instead of this:

outputVector = [];
        :
        outputVector = [outputVector,1] ;

do this:

outputVector = zeros(size(inputVector));
        :
        outputVector(i) = 1;

There are also ways to calculate your result without using a loop.

[Omar Sinno]

My problem is solved but I'd love to know how to do it without a loop.

[Stephen23]

"I'd love to know how to do it without a loop."

Not only are these simpler than your code, they will also be much more efficient.

Method one: basic arithmetic:

>> V = randi([1,100],1,17)
V =
   93   45   83   48    1   41   14   93   43    8   41   64   46   10   21   48   65
>> 3*(V<40)-2
ans =
   -2   -2   -2   -2    1   -2    1   -2   -2    1   -2   -2   -2    1    1   -2   -2

Method two: basic indexing:

>> X = [-2,1];
>> X(1+(V<40))
ans =
   -2   -2   -2   -2    1   -2    1   -2   -2    1   -2   -2   -2    1    1   -2   -2

[Steven Lord]

A different way that doesn't require logical arithmetic but just logical indexing:

V = randi([1, 100], 1, 17);
result = ones(size(V));
result(~(V < 40)) = -2;

% or
result = repmat(-2, size(V));
result(V < 40) = 1;