How to insert a column/row of ones into a matrix?

One way to do it is:

function [result] = prependOnes(matrix)
  result = [ones(rows(matrix),1) matrix];
end

prependOnes(b)

In GNU Octave:

Concatenate two matrices together horizontally: separate the matrices by a Space,
and enclose the result in square brackets:

X = [ones(rows(X), 1) X];

Simpler examples:
Glue on one number (add a horizontal "Column" element with a Comma):

octave:1> [[3,4,5],8]
ans =
   3   4   5   8

Glue on another matrix horizontally (as additional Columns) by using a Comma:

octave:2> [[3,4,5],[7,8,9]]
ans = 
   3   4   5   7   8   9

Glue on another matrix vertically (as additional Rows) by using a Semicolon:

octave:3> [[3,4,5];[7,8,9]]
ans =
   3   4   5
   7   8   9

An easy inline way to do this is:

b = [ones(size(b, 1), 1) b];

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