Multiple User-inputs, Default input, and String-to-Array Conversion

Question

Hello all I would like to make a program that takes in two user inputs. The first is a scalar and the second is a provided list or array. The number would be a multiplier for which to scale the array and the output would be the multiplied array.

import numpy as np

a = [1, 3, 5, 7, 9]
b = [2, 4, 6, 8, 10]

Also, for added convenience to the user, I'd like there to be a default scalar of 1 if no number is entered.

I am sorry for the lack of code but everything I've tried has not worked anyway.

I know that using

x, y = input().split()

will enable the user to enter two inputs such as

3 a

or

5.5 b

which is how I'd like the inputs to be entered. However, while the number can be converted to a float, I do not know how to interpret the letter a or b from a string to the their array names.

float(x)*np.array(y)

The following error I believe to have occurred due to the mismatch of data types.

 float(x)*np.array(y)
TypeError: ufunc 'multiply' did not contain a loop with signature matching types dtype('<U32') dtype('<U32') dtype('<U32')

If anyone can offer a solution or if there is a better way to do this than your input would be greatly appreciated. Thank you.

Do you wish to take any list as an input from the user or some pre-defined lists like a and b? — Som Shekhar Mukherjee
– Som Shekhar Mukherjee, Commented May 11, 2020 at 18:37
The lists are predefined. The user would just enter the scalar and the name of the list. — Plums
– Plums, Commented May 11, 2020 at 18:39
You should note that you need numpy arrays instead of lists, otherwise multiplication won't work. — Som Shekhar Mukherjee
– Som Shekhar Mukherjee, Commented May 11, 2020 at 18:48

Som Shekhar Mukherjee · Accepted Answer · 2020-05-11 19:01:21Z

2

I guess this helps! This also handles the case where the user doesn't specify a multiplier, then it automatically defaults to 1

import numpy as np

a = np.array([1, 3, 5, 7, 9])
b = np.array([2, 4, 6, 8, 10])

y,*x = input().split()
if not x:
    x=[1]

print(float(x[0])*eval(y))

edited May 11, 2020 at 19:01

answered May 11, 2020 at 18:46

Som Shekhar Mukherjee

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3 Comments

Plums Over a year ago

Thank you for your input. However, do you happen to know if you can set a default value of 1 for the coefficient if no number is entered?

Som Shekhar Mukherjee Over a year ago

@Plums I have updated my answer, that solves this issue as well, have a look!

Plums Over a year ago

Thank you! This concludes my question!

Reupiey · Accepted Answer · 2020-05-11 18:45:55Z

1

There are two solutions for your issue, one is cleaner.

The first one (dirty) consists in using eval, because the a or b your user will pass as input will be evaluated as a string, not as your variable named a. So you have to return float(x) * eval(y).

But the use of eval is heavily discouraged.

I think a cleaner way is to declare your pre-defined arrays in a dict and to look for the key the user provided like that:

import numpy as np

arrays = {
    "a": np.array([1, 3, 5, 7, 9]),
    "b": np.array([2, 4, 6, 8, 10])
}

coefficient, array_name = input().split()

print(float(coefficient) * arrays[array_name])

NB: In your original code, a = [1, 3, 5, 7, 9] defines a list. And you cannot multiply a float and a list. You could eventually multiply and int and a list but that is not what you want, it will duplicate your list :)

edited May 11, 2020 at 18:45

answered May 11, 2020 at 18:40

Reupiey

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1 Comment

Plums Over a year ago

Thank you this answer has greatly helped. However, do you happen to know if you can set a default value of 1 for the coefficient if no number is entered?

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