pass user input as lambda function

Question

I'm learning python and I'm not sure how to pass a string to lambda.

This is a basic example of Riemann sum:

#!/usr/bin/env python
# -*- coding: utf-8 -*-

import numpy as np
#f0 = str(input("ingrese funcion:"))
f = lambda x: x**2
sum = 0.0
a = float(input("ingrese límite inferior:"))
b = float(input("ingrese límite superior:"))
n = int(input("ingrese cantidad de subintervalos:"))
i = b-a #la longitud del intervalo
dx = i/n #que diferencia habrá cada vez que evalúes
for xi in np.arange(a,b,dx):
    sum += dx*f(xi)
print "Valor de la suma de Riemann de f en [a,b] = " + str(sum)

My idea was to let f0 as a user defined string (e.g. x**2 + 10) and then pass f0 to f. Of course doing that should return an "undefined" error.

Do you want the user to specify the function?

cs95
– cs95

2017-09-04 23:10:14 +00:00
Commented Sep 4, 2017 at 23:10 — cs95
– cs95, Commented Sep 4, 2017 at 23:10
yes, in the comment below I did put what worked

pachadotdev
– pachadotdev

2017-09-04 23:53:10 +00:00
Commented Sep 4, 2017 at 23:53 — pachadotdev
– pachadotdev, Commented Sep 4, 2017 at 23:53

wrvb · Accepted Answer · 2017-09-04 23:20:13Z

1

The easiest way to do this is using eval:

f0 = "x**2"
f = eval("lambda x:" + f0)
print f(2) # 4

Note that this will accept and execute any python expression. This means that error checking can be difficult (because there are almost no constraints on the function the user can provide) and opens major security holes (because the user can cause your program to execute arbitrary code). This isn't a problem for quick demonstrations, but should be avoided in anything like a production system.

answered Sep 4, 2017 at 23:20

wrvb

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