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Flavours of rounding
How often do you round various numbers in your day-to-day job? Do you know what type of rounding do you actually use?
Let's compare several programming languages and their default (meaning without additional parameters if the rounding function allows it) rounding techniques.
I'll use several notions of rounding: banker's rounding, away from zero and round half up. More info about different techniques can be found on Wikipedia.
In dotnet (Framework, Core, 5+) banker's rounding is used by default.
Math.Round(0.5); // 0
Math.Round(1.5); // 2
Math.Round(2.5); // 2
Math.Round(3.5); // 4
Math.Round(-23.5); // -24If you need away from zero, use the following
Math.Round(0.5, MidpointRounding.AwayFromZero); // 1
Math.Round(1.5, MidpointRounding.AwayFromZero); // 2
Math.Round(2.5, MidpointRounding.AwayFromZero); // 3
Math.Round(3.5, MidpointRounding.AwayFromZero); // 4
Math.Round(-23.5, MidpointRounding.AwayFromZero); // -24If you need round half up, use the following
Math.Round(0.5, MidpointRounding.ToPositiveInfinity); // 1
Math.Round(1.5, MidpointRounding.ToPositiveInfinity); // 2
Math.Round(2.5, MidpointRounding.ToPositiveInfinity); // 3
Math.Round(3.5, MidpointRounding.ToPositiveInfinity); // 4
Math.Round(-23.5, MidpointRounding.ToPositiveInfinity); // -23In JavaScript round half up is used be default.
Math.round(0.5); // 1
Math.round(1.5); // 2
Math.round(2.5); // 3
Math.round(3.5); // 4
Math.round(-23.5); // -23In Python 2.7 away from zero is used by default.
round(0.5) # 1
round(1.5) # 2
round(2.5) # 3
round(3.5) # 4
round(-23.5) # -24But in Python 3+ banker's rounding is used by default.
round(0.5) # 0
round(1.5) # 2
round(2.5) # 2
round(3.5) # 4
round(-23.5) # -24This was quite surprising, to be honest.
In Java (JDK 1.8.0, 9, 10, 11) round half up is used by default.
Math.round(0.5); // 1
Math.round(1.5); // 2
Math.round(2.5); // 3
Math.round(3.5); // 4
Math.round(-23.5); // -23In Go away from zero is used by default.
math.Round(0.5) // 1
math.Round(1.5) // 2
math.Round(2.5) // 3
math.Round(3.5) // 4
math.Round(-23.5) // -24But if you want banker's rounding there is a default function for this too.
math.RoundToEven(0.5) // 0
math.RoundToEven(1.5) // 2
math.RoundToEven(2.5) // 2
math.RoundToEven(3.5) // 4
math.RoundToEven(-23.5) // -24In PHP away from zero is used by default.
round(0.5); # 1
round(1.5); # 2
round(2.5); # 3
round(3.5); # 4
round(-23.5); # -2426