Preventing dividing by zero in list comprehensions

Question

I have the following code:

scores = [matrix[i][i] / sum(matrix[i]) for (i, scores) in enumerate(matrix)]

My problem is that sum(matrix[i]) could be 0 in some cases, resulting in a ZeroDivisionError. But because matrix[i][i] is also 0 in that case, I solved this as follows:

scores = [divide(matrix[i][i], sum(matrix[i])) for (i, scores) in enumerate(matrix)]

The function divide(x, y) returns 1 if y == 0 and (x / y) if y > 0. But I wonder if there is an easier way. Maybe I could use some ternary operator, but does that exist in Python?

score 6 · Accepted Answer · edited Oct 07 '19 at 14:49

6

Yes, in Python it's called the conditional expression:

[matrix[i][i] / sum(matrix[i]) if sum(matrix[i]) != 0 else 0 
 for (i, scores) in enumerate(matrix)]

edited Oct 07 '19 at 14:49

Georgy

12,464
7
65
73

answered Jun 21 '13 at 12:44

TerryA

58,805
11
114
143

But this will make `sum` calculated twice. – ovgolovin Jun 21 '13 at 12:45
@ovgolovin Sadly yes, but it's a solution nonetheless :3 – TerryA Jun 21 '13 at 12:47
Yeah, no doubt this is a solution. +1 for that. I just wonder if it is possible to reuse the result of `sum` inside list comprehension. – ovgolovin Jun 21 '13 at 12:48
So, the proposed solution is faster? – Lewistrick Jun 21 '13 at 12:49
@Lewistrick Depends on how you implement `divide`. If it calculates `sum` twice, it is no faster. – ovgolovin Jun 21 '13 at 12:50
@ovgolovin No, that doesn't depend on the implementation of `divide`, because it calculates `sum` before `divide`. – Lewistrick Jun 21 '13 at 12:51
@Lewistrick Then your solution is presumably faster as `sum` is calculated once, and here it is calculated twice. – ovgolovin Jun 21 '13 at 12:52

score 2 · Answer 2 · edited Oct 07 '19 at 14:50

2

[(lambda x, y: 0 if y == 0 else x/y)(row[i], sum(row))
 for i, row in enumerate(matrix)]

edited Oct 07 '19 at 14:50

Georgy

12,464
7
65
73

answered Jun 21 '13 at 12:57

falsetru

357,413
63
732
636

score 1 · Answer 3 · edited Oct 07 '19 at 14:50

1

Ternary conditionals do exist:

'blah' if True else 'wee'

edited Oct 07 '19 at 14:50

Georgy

12,464
7
65
73

answered Jun 21 '13 at 12:45

William Cummings

41
3

Preventing dividing by zero in list comprehensions

3 Answers3

Linked

Related