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Move from Excel to Python with Pandas Transcripts

Chapter: Appendix: Python language concepts

Lecture: Concept: lambdas

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In Python, functions are first class citizens,

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and what that means is they are represented by a class instances of them,

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particular functions are objects they can be passed around

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just like other custom types you create just like built-in types, like strings and numbers.

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So we are going to leverage that fact in a simple little bit of code I have here

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called find significant numbers.

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Now, maybe we want to look for all even numbers,

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all odd numbers, all prime numbers, any of those sorts of things.

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But this function is written to allow you to specify what it means

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for a number to be significant, so you can reuse this finding functionality

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but what determines significance is variable,

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it could be specified by multiple functions being passed in

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and that's what we are calling predicate

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because this ability to pass functions around and create and use them in different

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ways especially as parameters or parts of expressions,

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Python has this concept of lambdas.

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So let's explore this by starting with some numbers,

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here we have the Fibonacci numbers

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and maybe we want to find just the odd Fibonacci numbers.

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So we can start with the sequence and we can use this "find significant numbers" thing

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along with the special test method we can write the checks for odd numbers.

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So, in Python we can write this like so,

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and we can say the significant numbers we are looking for is... call the function,

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pass the number set we want to filter on

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and then we can write this lambda expression instead of creating the whole new function.

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So instead of above having the def and a separate block

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and all that kind of stuff, we can just inline a little bit of code,

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so we indicate this by saying lambda and then we say the parameters,

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there can be zero, one or many parameters,

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here we just have one called x, and we say colon to define the block that we want to run,

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and we set the expression that we are going to return when this function is called,

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we don't use the return keyword we just say when you call this function

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here is the thing that it does in return, so we are doing a little test, True or False,

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and we ask "if x % 2 == 1" that's all the odd numbers, not the even ones,

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so when we run this code it loops over all the Fibonacci numbers

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runs a test for oddness and it pulls out as you can see below

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just the odd ones, for example 8 is not in there.