#100DaysOfCode in Python Transcripts
Chapter: Appendix: Python language concepts
Lecture: Concept: for-in

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0:01 In Python we have a fantastically simple way
0:04 to work with collections and sequences.
0:06 It's called the "for...in" loop and it looks like this.
0:08 You just say for some variable name in some collection,
0:13 so here we have "for item in items" and that creates the variable called item
0:16 and we are looping over the collection items,
0:18 it just goes through them one at a time, so here it will go through this loop three times,
0:22 first time it will print the item is "cat", the second time it will print the item is "hat"
0:26 and then finally the item is "mat".
0:29 And then it will just keep going, it will break out the loop and continue on.
0:31 Some languages have numerical "for" loops, or things like that,
0:34 in Python there is no numerical "for" loop,
0:37 there is only these for in loops working with iterables and sequences.
0:41 Because you don't have to worry about indexes and checking links
0:44 and possible off-by-one errors, you know,
0:46 is it less than or less than or equal to, it goes in the test in the normal "for" loop.
0:50 This is a very safe and natural way to process a collection.
0:54 Now, there may be times when you actually need the number,
0:57 if you want to say the first item is "cat", the second item is "hat",
0:59 the third item is "mat", this makes it a little bit challenging.
1:04 Technically, you could do it by creating an outside variable, and incrementing,
1:06 but that would not be the proper Pythonic way.
1:09 The Pythonic way is to use this enumerate function, which takes a collection
1:13 and converts it into a sequence of tuples
1:17 where the first element in the tuple is this idx value, that's the index, the number.
1:21 And the second item is the same value that you had above.
1:24 So first time through its index is zero, item is cat;
1:27 second time through, index is one, item is hat, and so on.
1:30 So these are the two primary ways to loop over collections in Python.
1:35 Remember, if you need to get the index back,
1:38 don't sneak some variable in there, just use enumerate.