Python Lambda Identity Matrix

Question

I just tried to learn both list comprehensions and Lambda functions. I think I understand the concept but I have been given a task to create a program that when fed in a positive integer creates the identity matrix. Basically if I fed in 2 it would give me: [[1, 0],[0, 1]] and if I gave it 3: [[1, 0, 0],[0, 1, 0], [0, 0, 1] so list within a list.

Now I need to create this all within a lambda function. So that if I type:

FUNCTIONNAME(x) it will retrieve the identity matrix of size x-by-x.

By the way x will always be a positive integer.

This is what I have so far:

FUNCTIONNAME = lambda x: ##insertCodeHere## for i in range(1, x)

I think I am doing it right but I don't know. If anyone has an idea please help!

A `lambda` is **just** a way of defining certain kinds of simple functions in a single expression, without needing to name them. This is mainly useful for using the `lambda` as an argument to another function. If the `lambda` will be given a name immediately, **this defeats the purpose** - just use `def`. — Karl Knechtel, Oct 19 '22 at 21:16

codeape · Accepted Answer · 2013-08-15T08:16:11.647

6

How about:

>>> imatrix = lambda n: [[1 if j == i else 0 for j in range(n)] for i in range(n)]
>>> imatrix(3)
[[1, 0, 0], [0, 1, 0], [0, 0, 1]]

1 if j == i else 0 is an example of Python's conditional expression.

edited Aug 15 '13 at 08:16

answered Aug 15 '13 at 08:08

codeape

97,830
24
159
188

score 4 · Answer 2 · answered Aug 15 '13 at 08:14

4

This would be my favorite way to do it:

identity = lambda x: [[int(i==j) for i in range(x)] for j in range(x)]

It takes advantage of the fact that True maps to 1 and False maps to 0.

answered Aug 15 '13 at 08:14

abeinstein

552
4
11

Henry Gomersall · Answer 3 · 2013-08-15T10:51:53.780

3

Just for completeness (and to highlight how one really should be doing numerical stuff in python):

import numpy
list_eye = lambda n: numpy.eye(n).tolist()

Of course, in practice you'd probably just be using eye(n) by itself and be working with the numpy arrays.

edited Aug 15 '13 at 10:51

answered Aug 15 '13 at 08:32

Henry Gomersall

8,434
3
31
54

Python Lambda Identity Matrix

3 Answers3