Single line for loops

[Aange10]

In some of my classes, the teacher is using a very odd syntax. I'm not quite sure what is happening, so I'm starting to get lost in my class. Could somebody explain how these for loops are iterating? It'd probably help a lot to explain it in a nested format. I think I'd get it if I saw it like I'm used to seeing it.

Python:

t2 for t1 in my_list for t2 in my_list2

Thanks

[Aange10]

To make it easier, also, lets say

Python:

my_list = [0,1,2] my_list2 = [0,1,2,3]

[chrisbrown]

Are you missing some brackets? I don't recognize that syntax but

Python:

[ [t2 for t1 in my_list] for t2 in my_list2 ]

is a nested list comprehension.


How much do you know about list comprehensions? Do you know what this does?

Python:

[t for t in my_list]

[Aange10]

There may be those brackets there.

And somewhat. I'm not sure what happens to 't' in

Python:

t for t in my_list

but i know that the 'for t in my_list' will just iterate through the list and store the elements value to t.

[chrisbrown]

OK good stuff. You're right about t being the variable of iteration. But where a for loop simply evaluates the expression(s) within it, a list comprehension builds a new list by applying those expressions to each element as it iterates through the original list. You could encapsulate the same behaviour in a function:

Python:

def list_comp(a_list, f):     newlist = []     for t in a_list:         newlist.append( f(t) )     return newlist # Test [ f(t) for t in a_list ] == list_comp(a_list, f)     # True

where f is any function that can be applied to every element of a_list.

So for the same my_list you provided,

Python:

[ f(t) for t in my_list ]

evaluates to the list:

Python:

[ f(0), f(1), f(2) ]

If f(t) = t like in my example, then the new list will be an element-wise copy of the original.

If f(t) = 0, you can produce a list of zeroes equal in length to the original.

If t(f) = [ t**0, t**1, t**2, t**3, (etc) ] you can produce a list of lists, where each element of the produced list is a list of powers for each element in the original list. (Wordy, I know...)

If f(t) = [ t for t2 in my_list2 ] ?

[Aange10]

Thankyou for explaining that! I understand. It now xD. Very cool tool.

Thanks! + karma