Built-in Knowledge: Entities, Units, and Dates

Prerequisites: Associations

The Wolfram Language includes curated data about the real world, and types for units and dates that make calculations with them safe.

Quantities and units

UnitConvert[Quantity[26.2, "Miles"], "Kilometers"]
Quantity[100, "Kilometers"]/Quantity[2, "Hours"]
UnitConvert[Quantity[100., "DegreesFahrenheit"], "DegreesCelsius"]
QuantityMagnitude[Quantity[5, "Meters"]]

42.1648 km
50 km/h
37.7778 °C
5

Quantity[magnitude, unit] carries its unit through arithmetic; dividing kilometers by hours gives a speed.
Adding incompatible units (meters plus seconds) is an error instead of a silent bug.
Unit names are spelled out ("Meters", "Pounds"); in a notebook, Ctrl + = lets you type natural language like 3 feet and converts it to the right Quantity (this needs an internet connection).

Dates

DateDifference[DateObject[{2024, 1, 1}, "Day"], DateObject[{2024, 3, 15}, "Day"]]
DayName[DateObject[{2024, 3, 15}]]

74 days
Friday

DateObject, DatePlus, DateRange, DateString, and Now cover date arithmetic and formatting, including time zones.

Entities

An entity is a real-world thing — a country, a city, a chemical, a planet — with properties you can query. Entity data comes from Wolfram's servers, so these need an internet connection, and values such as populations change over time:

(* illustrative: needs an internet connection *)
france = Entity["Country", "France"];
france["Capital"]
france["Population"]
EntityValue[EntityClass["Country", "EuropeanUnion"], "Population"]
Interpreter["Country"]["japan"]

france["Capital"] returns another entity — Paris — that you can query further.
france["Population"] returns a Quantity of people.
EntityValue fetches a property for many entities at once.
Interpreter turns free text into entities, dates, numbers, and more.

Similar domain functions exist for chemistry (ChemicalData), finance (FinancialData), astronomy (PlanetData), geography (GeoDistance, GeoGraphics maps), and dozens of other areas.

Free-form input

In a notebook, starting a cell with = sends it to Wolfram|Alpha and returns the answer along with Wolfram Language code you can reuse. WolframAlpha["query"] does the same from code.

Why it matters

Built-in knowledge saves the data-gathering step: a question like "how does GDP per person relate to life expectancy across countries?" is a few lines of code, and units keep the calculations honest. For reproducible work, store the data you fetched (for example with Export or DumpSave), since curated values are updated over time.