Multiplying boolean with float?

Question

I don't understand the line q.append(p[i] * (hit * pHit + (1-hit) * pMiss)), because the variable hit is a boolean value. That boolean value comes from hit = (Z == world[i])

What's going on there? I only have a basic understanding of Python...

p = [0.2, 0.2, 0.2, 0.2, 0.2]

world = ['green', 'red', 'red', 'green', 'green']
Z = 'red'
pHit = 0.6
pMiss = 0.2

def sense(p, Z):
    q=[]
    for i in range(len(p)):
        hit = (Z == world[i])
        q.append(p[i] * (hit * pHit + (1-hit) * pMiss))
        s = sum(q)
        for i in range(len(p)):
            q[i]=q[i]/s      
    return q

print sense(p,Z)

score 17 · Answer 1 · answered Nov 11 '12 at 14:45

17

In arithmetic, booleans are treated as integers. True is treated as 1 and False is treated as 0.

>>> True + 1
    2
>>> False * 20
    0
>>> True * 20
    20

answered Nov 11 '12 at 14:45

Tim

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score 14 · Answer 2 · edited May 23 '17 at 10:30

14

In python, booleans are a subclass of int:

>>> isinstance(True, int)
True

They are basically 1 and 0:

>>> True * 1
1
>>> False * 1
0

See Why is bool a subclass of int?

edited May 23 '17 at 10:30

Community

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1

answered Nov 11 '12 at 14:45

Martijn Pieters

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score 6 · Accepted Answer · answered Nov 11 '12 at 14:48

6

True is 1 and False is 0, as others have answered. So basically, what it does (and what should've been written) is:

p[i] * (pHit if hit else pMiss)

answered Nov 11 '12 at 14:48

Pavel Anossov

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Multiplying boolean with float?

3 Answers3

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