Why does my Matlab code not work correctly?

Question

My code

    b(abs(b(1:3:length(b))) > 0.75) = 0.75

What it's supposed to do:

    b1 = b(1:3:end);
    b1(abs(b1)>0.75) = 0.75;
    b(1:3:end) = b1;

How are these two not the same?

Answer 1

The indexing part b(1:3:end) returns a short vector of zeros and ones, so will only change the i -th entry of b (for i in the first third-ish of b ) to 0.75 if the absolute value of the 3*i + 1 -th entry is greater than 0.75 .

For example:

b = [-0.684; 0.941; 0.914; -0.029; 0.6; -0.716; -0.156; 0.831; 0.584; 0.919];
b_index = abs(b(1:3:length(b)))>0.75

would return

b_index =

     0
     0
     0
     1

and b(b_index) = 0.75 would change the 4th entry of b to 0.75 .

One way of doing this as a one-liner is

b(1:3:end) = b(1:3:end).*(abs(b(1:3:end))<=0.75)) + 0.75*((b(1:3:end)>0.75));

but I think the three-liner is a bit clearer.