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So I want to integrate a double integral with constants in it, like a, b, etc where the user can asign the value of this constants:

The limits of the integral are x[0,1] and y[-1,2]

import numpy as np
import scipy.integrate as integrate

def g(y,x,a):
    return a*x*y

a = int(input('Insert a value --> '))
result = integrate.dblquad(g, 0, 1, lambda x: -1, lambda x: 2, args=(a))[0]
print(result)

But I get this error and I don't understant why:

TypeError: integrate() argument after * must be an iterable, not int

I don't understand it. Because when I do the same but with quad() Python does it correctly:

import numpy as np
import scipy.integrate as integrate

def g(x,a):
    return a*x

a = int(input('Insert a value --> '))
result = integrate.quad(g, 0, 1, args=(a))[0]
print(result)

With the result:

0.5
Vadim Kotov
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Álvaro Romero Mato
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1 Answers1

3

The problem here is that the value you provide in the optional argument args is a tuple. In the case of quad, it is what the function expects, but for dblquad, a sequence is required. Even though tuples are sequences (immutable ones), it seems that scipy makes a difference here and therefore it is why an error is raised. However it is misleading as a tuple is definitely an iterable. Anyway, this should work:

result = integrate.dblquad(g, 0, 1, lambda x: -1, lambda x: 2, args=[a])[0]
Patol75
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    *"The problem here is that the value you provide in the optional argument args is a tuple. "* Actually, `(a)` is not a tuple; it is exactly the same as `a`. To make a tuple of length 1 containing `a`, use `(a,)`. (Your solution works too.) – Warren Weckesser Nov 12 '18 at 14:09