How to use 2 forloop in matlab?

I have a Mat-file. My Mat-file have 204 length. How can i just get from length 1 to 51 from my Mat-file? How can i use another forloop in my code to get i = 1 till i = 51? This is my code.

a = 0;

m = matfile('elm_output.mat');
output = m.output;

for i = 1:length(output)
        if output(i) == 1
            a = a+1;
        end
end

Answer 1

[Bhaskar R]

required  = output(1:51);

[Bhaskar R]

Do you mean?

required  = output(1:51);
one_two_values = required(required == 1 | required == 2)

[Muhammad Andi Yusran]

Yes Mr. Bhaskar, but i can figure it out now. Thanks for your attention sir. Thank you so much.