Move from Excel to Python with Pandas Transcripts
Chapter: Appendix: Python language concepts
Lecture: Concept: lambdas

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0:01 In Python, functions are first class citizens,
0:03 and what that means is they are represented by a class instances of them,
0:07 particular functions are objects they can be passed around
0:11 just like other custom types you create just like built-in types, like strings and numbers.
0:16 So we are going to leverage that fact in a simple little bit of code I have here
0:19 called find significant numbers.
0:21 Now, maybe we want to look for all even numbers,
0:24 all odd numbers, all prime numbers, any of those sorts of things.
0:27 But this function is written to allow you to specify what it means
0:32 for a number to be significant, so you can reuse this finding functionality
0:36 but what determines significance is variable,
0:40 it could be specified by multiple functions being passed in
0:43 and that's what we are calling predicate
0:45 because this ability to pass functions around and create and use them in different
0:50 ways especially as parameters or parts of expressions,
0:53 Python has this concept of lambdas.
0:56 So let's explore this by starting with some numbers,
0:58 here we have the Fibonacci numbers
1:00 and maybe we want to find just the odd Fibonacci numbers.
1:03 So we can start with the sequence and we can use this "find significant numbers" thing
1:07 along with the special test method we can write the checks for odd numbers.
1:11 So, in Python we can write this like so,
1:14 and we can say the significant numbers we are looking for is... call the function,
1:17 pass the number set we want to filter on
1:19 and then we can write this lambda expression instead of creating the whole new function.
1:23 So instead of above having the def and a separate block
1:26 and all that kind of stuff, we can just inline a little bit of code,
1:30 so we indicate this by saying lambda and then we say the parameters,
1:34 there can be zero, one or many parameters,
1:37 here we just have one called x, and we say colon to define the block that we want to run,
1:42 and we set the expression that we are going to return when this function is called,
1:46 we don't use the return keyword we just say when you call this function
1:49 here is the thing that it does in return, so we are doing a little test, True or False,
1:53 and we ask "if x % 2 == 1" that's all the odd numbers, not the even ones,
1:58 so when we run this code it loops over all the Fibonacci numbers
2:01 runs a test for oddness and it pulls out as you can see below
2:05 just the odd ones, for example 8 is not in there.