Multiplying 1D Array in Python

Question

If I have a 1D array in Python for example:

a = (10,20,30,40,50)

How can I multiply this by an integer for example 2 to produce:

b = (20,40,60,80,100)

I have tried:

b = a*2

But it doesn't seem to do anything.

`a * 2` does do _something:_ `(10, 20, 30, 40, 50, 10, 20, 30, 40, 50)` — Matt Ball, Sep 01 '12 at 14:54

Matthew Adams · Answer 1 · 2012-09-01T15:13:07.487

3

Tuples are immutable; use lists ([] instead of ()) if you're going to want to change the contents of the actual array.

To make a new list that has elements twice those of the tuple, loop over the tuple and multiply each element:

b = []
for num in a:
    b.append(2*num)

This can be shortened to

b = [2*num for num in a]

Note that If you really want the final result to still be a tuple, you can use use

b = tuple([2*num for num in a])

I believe the closest thing you can get to your original syntax without using third party libraries would be

>>> map(lambda n: n*2, [1,2,3])
[2, 4, 6]

which is basically just a fancy way of saying, "take the function f(n) = 2n and apply if to the list [1,2,3]".

edited Sep 01 '12 at 15:13

answered Sep 01 '12 at 14:52

Matthew Adams

Tuples are not the problem here. The end result of using a list instead of a tuple is the same in terms of the elements: `[10, 20, 30, 40, 50, 10, 20, 30, 40, 50]`. -1 – Matt Ball Sep 01 '12 at 14:55
@MattBall Totally right. (I clicked submit before enough of my answer was down.) – Matthew Adams Sep 01 '12 at 15:03
1

map function doesn't modify the list, it just returns a new one. – MatthieuW Sep 01 '12 at 15:19
@MatthieuW True. I wrote that, thought about it for a second, and then realized I was being kinda stupid. Also, everyone involved in this question now has some form of the name Matthew... – Matthew Adams Sep 01 '12 at 15:34

score 3 · Accepted Answer · answered Sep 01 '12 at 14:53

3

Use the following:

>>> b = [2 * i for i in a]
>>> b
[20, 40, 60, 80, 100]

a * 2 will duplicate your set:

>>> a = (10,20,30,40,50)
>>> a * 2
(10, 20, 30, 40, 50, 10, 20, 30, 40, 50)

answered Sep 01 '12 at 14:53

João Silva

@cia09mn Welcome to SO. Since this answer worked for you, please mark it as accepted by clicking the green check mark. – Matthew Adams Sep 01 '12 at 15:16

score 3 · Answer 3 · edited Mar 30 '23 at 18:00

3

For a more natural way of working with numbers, you may want to consider numpy. Using numpy, your code would like like this:

import numpy as np
a = np.array([10,20,30,40,50])
b = a*2

edited Mar 30 '23 at 18:00

Glorfindel

answered Sep 01 '12 at 14:56

Guy Adini

3 Answers3