Question

I have a matrix in MATLAB, lets say:

a = [
  89  79  96  
  72  51  74  
  94  88  87  
  69  47  78
]

I want to subtract from each element the average of its column and divide by the column’s standard deviation. How can I do it in a way which could be implemented to any other matrix without using loops.

thanks

Answer 1

You can use repmat to make your average/std vector the same size as your original matrix, then use direct computation like so:

[rows, cols] = size(a); %#to get the number of rows

avgc= repmat(avg(a),[rows 1]); %# average by column, vertically replicated by number of rows
stdc= repmat(std(a),[rows 1]); %# std by column, vertically replicated by number of rows
%# Here, a, avgc and stdc are the same size
result= (a - avgc) ./ stdc;

Edit:

Judging from a mathworks blog post,bsxfun solution is faster and consumes less memory (see acai answer). For moderate size matrices, I personally prefer repmat that makes code easier to read and debug (for me).