I can I do logical indexing on a column

Hello everyone,

I have a column which is 840 in length. It contains value s from -12 to 12. I need only values < -8 and >8. I need to keep the length 840. I tried the following. but it says " 0x1 empty double column vector".

v = zeros (size (data)); % Pre-allocate!

v=data(data <-8 & data>8)

thank you for helping.

Chane

Answer 1

[Birdman]

Is it possible for a value to be smaller than -8 and greater than 8 at the same time? Check your logical operator. You need OR operator instead of AND.

 v=data(data <-8 | data>8)

You can not keep the length same. If you want that, fill the other values with a specific value, like zero.

[Guillaume]

In addition to what birdman said, note that in your case, your preallocation line is not a preallocation and will actually slow your code down as you create a matrix that is immediately destroyed and replaced by another one on the next line.

Preallocation is when you create a matrix and then assign values to it elements by elements using indexing. In your case, the v = data... replace the whole matrix.

[Guillaume]

If you want that, fill the other values with a specific value, like zero

Considering that the aim is to keep elements whose absolute value is greater than 8, replacing the other elements by 0 might be counterproductive, they're still smaller than 8. Replacing them by +/-Inf or NaN would make more sense.

[Engdaw Chane]

Birdman and Guillaume, Thank you for your swift replay. the solution works fine.I am working on a time-series data. so I need to keep the values to the month they belong. the remaining months can be zero. but how can fill the zeros?

[Guillaume]

Well, if you want to replace values less than absolute 8 by 0, then:

v(abs(v) < 8) = 0;

You may want <= instead of <

[Engdaw Chane]

Birdman and Guillaume, Thank you!