How to prepend a column to a matrix?

Question

Ok, imagine I have this Matrix: {{1,2},{2,3}}, and I'd rather have {{4,1,2},{5,2,3}}. That is, I prepended a column to the matrix. Is there an easy way to do it?

My best proposal is this:

PrependColumn[vector_List, matrix_List] := 
 Outer[Prepend[#1, #2] &, matrix, vector, 1]

But it obfuscates the code and constantly requires loading more and more code. Isn't this built in somehow?

Answer 1

Since ArrayFlatten was introduced in Mathematica 6 the least obfuscated solution must be

matrix = {{1, 2}, {2, 3}}
vector = {{4}, {5}}

ArrayFlatten@{{vector, matrix}}

A nice trick is that replacing any matrix block with 0 gives you a zero block of the right size.

Answer 2

I believe the most common way is to transpose, prepend, and transpose again:

PrependColumn[vector_List, matrix_List] := 
  Transpose[Prepend[Transpose[matrix], vector]]