Abs, absolute value

Sometimes a number is negative: like -100. But we want a distance—an absolute value. With abs we compute absolute values.

With an if-statement, we can test for negative numbers and make them positive. But imagine doing this many times in a program. Abs is clearer.

Core example

A common math method is abs. This computes absolute values. It removes the negative sign (if there is one) from the number and returns that.

# Negative number.
n = -100.5

# Absolute value.
print(abs(n))

# Positive numbers are not changed.
print(abs(100.5))

# Zero is left alone.
print(abs(0))
100.5
100.5
0

Benchmark, abs

Are math methods fast? Could we rewrite the abs() method with an if-else statement to compute absolute values faster?

import time

print(time.time())

# Version 1: compute absolute value with abs.
a = -1
i = 0
while i < 10000000:
    b = abs(a)
    i += 1

print(time.time())

# Version 2: compute absolute value with if-statement.
a = -1
i = 0
while i < 10000000:
    if a < 0:
        b = -a
    else:
        b = a
    i += 1

print(time.time())
1346355970.511
1346355973.081 (Abs = 2.57  s)
1346355975.509 (If  = 2.428 s)

Hash codes

When computing a hash code (a number based on data and used for lookup) we sometimes end up with a negative value. Abs fixes that.

With absolute values, we convert negative numbers to positive ones. This helps when computing "distances" from positions. With abs, a built-in, we do this with no extra code.