Delete matrix columns not containing 2 or more instances of numbers between a specific range - MATLAB

Question

If i've got a matrix which is something like:

400     450     450     300
450     300     450     340
450     200     300     210
300     100     300     200
200     200     100     100
475     300     225     300

What is the best way to delete all columns which do not contain numbers between a specific range - for example 400-500 - at least twice or more. In this example, columns B and D will be removed as B contains a number in this range only once and D contains nothing within this range.

The desired output would be:

400     450 
450     450
450     300
300     300
200     100
475     225

I've tried to use an IF statement to scan each column and move matches into a new matrix but this seems inefficient.

Answer 1

Assuming A to be the input matrix, you can use this matrix indexing approach -

A(:,sum(A>=400 & A<=450,1)>=2)

This must be a pretty efficient approach to your problem.

sum(A>=400 & A<=450,1)>=2 gets us a logical array of columns that satisfy our needs and then we index into the columns of A with it.

Note: If the number of columns to be removed is a very small number as compared to the total number of columns, you can directly remove them instead of indexing into the matched columns for a more efficient approach in that special situation. So, in that case you can do this -

A(:,sum(A>=400 & A<=450,1)<2)=[];