For Loop questions

Question

I'm used to seeing For Loops in this format:

for number in l: sum = sum + number

I was browsing some forums and came across this piece of code:

count_chars = ".arPZ" string = "Phillip S. is doing a really good job." counts = tuple(string.count(d) for(d) in count_chars) print counts

I'm not sure if that is really a For loop, so I decided to rewrite it in a way that I understood:

tuple( for(d) in count_chars: string.count(d))

Needless to say, it failed lol. So can someone explain what is going on, and explain the folly of my logic? Thanks!!

This is called a generator expression. I am sure someone will post a more expert answer than I will write. — anon582847382
– anon582847382, Commented Apr 6, 2014 at 17:07
Thanks for getting me in the right direction. I had no idea what a generator expression was! — dyao
– dyao, Commented Apr 6, 2014 at 17:10
Here is a short video about list comprehensions and it's siblings, including generator expressions. — Gareth Latty
– Gareth Latty, Commented Apr 6, 2014 at 17:13
@user3386440 Nobody posted anything (correct) so I wrote something up, see below. — anon582847382
– anon582847382, Commented Apr 6, 2014 at 17:14
possible duplicate of Python `for` syntax: block code vs single line generator expressions — Ffisegydd
– Ffisegydd, Commented Apr 6, 2014 at 17:23

anon582847382 · Accepted Answer · 2014-04-06 17:19:46Z

4

It's not quite a for loop as such, but a generator expression. What it basically does is return an iterator where each element is the amount of time every character in count_chars occurs in d. It then adds all of these elements into a tuple.

It is (roughly) equivalent to:

counts = [] for d in count_chars: counts.append(string.count(d)) counts = tuple(counts)

edited Apr 6, 2014 at 17:19

answered Apr 6, 2014 at 17:13

anon582847382

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2 Comments

anon582847382 Over a year ago

@user3386440 declare an empty list counts. For every element in count_chars, count how many times it is in string and add that number to the list counts. Convert counts to a tuple.

dyao Over a year ago

Thanks again. I'll look up append on my own but I think I got the rest of it. Assume I didn't want it to appear on a list ie: (2,2,1,1,0). Instead I want it to be be simply print out. I will show you want I mean in the OP.

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