strings and numbers for a numeric array name

I am using 6 numeric arrays in my project, their names are: block1, block2 , block3 , block4, block5 , block6

i execute the same function for all of them, i need to make this in a for loop, but i failed to index the elements of the arrays, i tried this:

   for i=1:6

        h = ['block' int2str(i)];
        eval(h)
    end

                for row=1:BRows
                 for col=1:BCols
                if (h(row,col) >= 316 &&  h(row,col) <= 20)
                    h(row,col) = 0;
            elseif (h(row,col) >= 21 &&  h(row,col) <= 40)
                    h(row,col) = 1;
                end
                end

Would you help me please

[Walter Roberson]

No h(row,col) can possibly be simultaneously >= 316 but <= 20.

Meanwhile, consider:

h(h >= 21 & h <= 40) = 1;

[yasmine]

i tried it but this error occurred:

??? Error: File: fg6.m Line: 86 Column: 40

The expression to the left of the equals sign is not a valid target for an assignment.

any other suggestions?

[Jan]

The problem is, that your "h" is *not* the contents of block1, but the string 'block1'. Avoid EVAL.

Answer 1

[the cyclist]

Rather than doing it like that, I strong suggest you used cell arrays, as described in Doug's answer here: http://www.mathworks.com/matlabcentral/answers/143-how-do-i-make-a-series-of-variables-a1-a2-a3-a10

[yasmine]

actually i have a problem with cell arrays in my project, i can't use them

Answer 2

[Jan]

Please explain why you cannot use cell arrays. "eval([block, int2str(i)])" is such an ugly approach, prone to errors and it has remarkable drawbacks for the processing speed in Matlab. I cannot imaging why "h=block{i}" should be worse. Embedding an index in the name of a variable is not smart. Better use an index as index.

Btw. you do not want to do "eval(h)", but:

eval(['h = block', int2str(i)]);

As you see: The EVAL approach is prone to errors!

[yasmine]

am dividing a image into blocks then plot a histogram for every block

so to plot a histogram, i need a numeric array not a cell array and couldn't convert from cell to numeric using cell2mat()

[Jan]

block{1} = rand(2,2);

block{2] = rand{3,4};

disp(block{1}) % <== as you see "block{1}" is a numeric array!

Consider the curly braces. With round parenthesis "block(1)" you get a scalar cell.

Has my Do Not Eval been impressive enough?

[the cyclist]

Here is an example of code that uses numeric array embedded in a cell array, and then makes histograms from them.

block = cell(2,2);

for i = 1:4

block{i} = randn(1000,1);

end

figure

for i=1:4

subplot(2,2,i), hist(block{i},25)

end

Think of a cell array as a container than can hold many other type of MATLAB objects. When you use curly brackets, you are accessing the contents of the container, which in this case is numeric arrays.

[yasmine]

hi

i need to use a database of 100 images each one's size is 256X384. every image is divided into 6 equal blocks(128x128)

and i want to access the pixels in each block.

how can i do this using a for loop or any iterative mean on arrays or cell arrays in matlab

Thank you

[yasmine]

knowing that i want to keep record of all blocks ( not to overwrite on old data)

[Jan]

@yasmine: Please open a new thread for a new question.