How is Google Calculator implemented?

When you search in Google "100F to C" how does it know to convert from Fahrenheit to Celsius? Similarly, conversion from d开发者_开发百科ifferent currencies and simple calculation.

What is the data structure used, or is it simple pattern matching the strings?

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It's not exactly simple pattern matching. Evaluating the mathematical expressions you can enter is not trivial. For example, here's an algorithm that evaluates a math expression. That's just the evaluation, there's probably a lot of code to detect if it's even valid.

For the currencies conversion and other units, that's simple pattern matching.

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it's simple pattern matching

try
100 kmh in mph = no calculation
100 kph in mph = 62.1371192 mph