Rounding: round, trunc, floor, ceil

The sensor reports 294.2318 K. On the display you want to see “21.1”, not “21.0818”. And when you work out how many gas canisters you need, the answer must not be half a canister. There are four instructions for that.

The four rounding instructions#

Negative numbers#

For positive numbers trunc and floor are the same. For negative ones they are not: trunc only cuts and therefore moves towards zero, floor always moves down. For −2.7 that means: trunc gives −2, floor gives −3. On Mars at −40 °C that is not an edge case, it is everyday life.

Rounding to digits#

There is no instruction “round to one decimal”. You shift the decimal point, round and shift it back:

l r0 d0 Temperature
sub r0 r0 273.15     # e.g. 21.0818
mul r1 r0 10         # 210.818
round r1 r1          # 211
div r1 r1 10         # 21.1
s d1 Setting r1

For two digits use 100 instead of 10.

Whole pieces#

When something only exists in whole pieces, the question is which direction is safe:

div r2 r3 50         # mol per canister: 50
ceil r2 r2           # rather one canister too many

ceil gives you enough canisters, floor tells you how many full canisters you can already fill.