CODEWITHSUNDEEP
pythonfloating-point
Ma0
Ma0

Reputation: 15204

Why does this print -0.0?

Can anybody please explain why the following happens?

print(-1 * (605 % 11))        #-> 0
print(-1 * (0.5*1210 % 11))   #-> -0.0
print(-1 * (0.5*1210) % 11)   #-> 0.0

Especially the -0.0 is baffling..

Upvotes: 1

Views: 342

Answers (1)

user2357112
user2357112

Reputation: 280564

print(-1 * (605 % 11))        #-> 0

Integer arithmetic. No surprises here.

print(-1 * (0.5*1210 % 11))   #-> -0.0

Here's where you might get surprised. 0.5*1210 % 11 evaluates to floating-point zero, and then -1 * 0.0 results in negative zero, which is a thing in floating-point. It exists because it makes edge cases of some numeric algorithms easier to implement.

print(-1 * (0.5*1210) % 11)   #-> 0.0

Here's where someone more familiar with floating-point than with Python might get surprised. -1 * (0.5*1210) evaluates to -605.0, but then in -605.0 % 11, Python defines the % operation as returning a result of the same sign as the denominator, rather than the numerator, so this returns 0.0 instead of -0.0.

Upvotes: 6

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