A is less than a threshold

m=256;
e=0.5;
    for x=1:m
        for y=1:m
            if A(x,y)<0.5
                s_dark(x,y)=A(x,y)^e;
            end
        end
    end

Hello everyone.I have some problem in this code.The values less than 0.5 shold be stored in a variable s_dark.But i didn't get it.It store values less than 0.5 and also greater than 0.5.I've attached the s_dark variable below.please see through it.your help is really appreciated.

Answer 1

[Mehmed Saad]

Because in your code

 s_dark(x,y)=A(x,y)^e;

suppose A(x,y) = 0.47;

where e = 0.5;

 0.47^0.5
ans =

    0.6856

That is why you are getting values greater than 0.5

i think what you are trying to do is

m=256;
e=0.5;
A = A.^e;
    for x=1:m
        for y=1:m
            if A(x,y)<0.5
                s_dark(x,y)=A(x,y);
            end
        end
    end

i am assuming that s_dark is initialized in the code earlier

[Rocky]

Thank you for your quick response.I have one doubt,when i ran the code the last value of s_dark has 1.1866..can you tell me why it is coming like this.I'm attaching the variable value for your reference..

[Mehmed Saad]

two possibilities

1. Wrong initialization
2. Negative values i.e. -1.4080 maybe as it is still less than 0.5

[Rocky]

Okay.Thank you for your help

[Image Analyst]

What value do you think it should be? What is A(256, 256)?

Why are you using this for loop instead of my vectorized code like most MATLABers would use?

[Rocky]

I need to check each and every values is less than 0.5 or not.for that reason I'm using for loop..A(x,y) where x and y are rows and columns of a matrix..Here I've taken image size as 256×256..so I put 1:256.

[Image Analyst]

I guess you haven't yet encountered the part of MATLAB where you learned how to use vectorized array operations. Too bad, though I thought they covered that really early on. So that whole set of nested for loops and if statement (7 lines of code) is done simply by these two lines of code:

map = A < 0.5; % Logical matrix of where A is less than 0.5.
s_dark(map) = A(map) .^ e;

You'll need to learn this easy concept if you want to use MATLAB much.

Anyway, you didn't answer the question of what you expected the bottom right element of A to be. What did you expect for A(256, 256) instead of what you showed?

[Stephen23]

Matrix and array operations are introduced in the second part of the basic tutorials:

https://www.mathworks.com/help/matlab/getting-started-with-matlab.html

[Rocky]

Thank you for providing the link I will definitely go through it @stephen codeldick

[Rocky]

@Image Analyst..In my above code I simply comparing the A matrix with a threshold..if it is less than that threshold the values are stored in dark variable.And I raising e to the power of dark one..This is what I am doing.while using for loop instead of 0:255 I'm taking 1:256..I think I have clarified your question.If I'm wrong with my assumption you can point out my mistakes..

Answer 2

[Image Analyst]

Not exactly sure what you want where A > 0.5, a value of 0 or of the original A? So here are both options:

% Initialization steps.
m = 256;
A = 0.7 * rand(m, m)
e = 0.5;

% OPTION 1
% s_dark equals A except where A is less than 0.5 where
% it will replace values of A less than 0.5 with
% the value raised to the e power
s_dark = A; % Initialize;
map = A < 0.5; % Logical matrix of where A is less than 0.5.
s_dark(map) = A(map) .^ e;

% OPTION 2
% s_dark equals 0 except where A is less than 0.5 where
% it will be the values of A raised to the e power
s_dark = zeros(size(A)); % Initialize;
map = A < 0.5; % Logical matrix of where A is less than 0.5.
s_dark(map) = A(map) .^ e;

[Rocky]

Thanks a lot for your explanation..