🔒 understanding the syntax of list comprehensions

[crisp herald]

I need help understanding list comprehensions in python.
per w3 the syntax for a list comprehension is:

newlist = [expression for item in iterable if condition == True]

after watching youtube videos I see people inputting what they want for the expression part of the syntax.

and than we use the for loop to feed the numbers into the expression.

So basically we create an expression or a formula that will apply the calculation for the for loop first, and than write the actual for loop. is this correct?

is there a simpler way to explain the syntax?

[rose gazelle]

@crisp herald

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[crude pendant]

hang on i have an amazing graphic for this

[crisp herald]

reviewing

color coded!

okay so the blue is the variable holding the data

red seems to be the action that is taken on the data

green is the for loop itself

[crude pendant]

not necessarily

[crisp herald]

yello is the operation in the for loop?

[crude pendant]

red is just what the list is constructed out of

in for item in cart, item represents any given element of cart

in a list comprehension, [item for item in cart] means "a list with every element in cart"

[crisp herald]

thinking

[crude pendant]

the first term, before the "for" syntax. is added to the list "for every thing in an iterable"

so if a list has 4 things, and i did [ 10 for i in list_with_4_things ]

[crisp herald]

okay so add this to the list when the condition is true?

[crude pendant]

i would have a list which looks like [10, 10, 10, 10]

[crisp herald]

reviewing

10 for i in list with 4 things -> so it iterates 4 times and there are 4 things so it looks at the code before the for loop and will output that code up to the number of times the condition is met

[mental owl]

comprehensions actually combine the functionality of map and filter
if you know what those are then it might be clearer now
if you don't then just don't worry about this for now

[crude pendant]

condition is an optional part of the syntax, but yes. if you have [ x for x in y if x % 2 == 0 ]
then you make a new list, where any element (which we call x for a "placeholder" essentially) in y is added to the new list, if it satisfied x % 2 == 0 (aka x is even)

[crude pendant]

theres no "code" before the for loop, you cant do something like [def f(x): return 1 for i in list_with_4_things]

its just an expression. to put it simply, anything can go in place of the "red" color coding in the original image, if it can go on the right side of an equal-sign

x = "anything you can put here"

["you can also put here" for i in range(3)]

[crisp herald]

does the for loop feed values into the expression?

[crude pendant]

it can

[crisp herald]

can i see a simple example

[mental owl]

!e

l = [i+1 for i in range(5)]
print(l)

[crisp herald]

err you gave me one

[rose gazelle]

@mental owl :white_check_mark: Your 3.12 eval job has completed with return code 0.

[1, 2, 3, 4, 5]

[crisp herald]

okay i see the list for 5 numbers using the range function

so basically

i gets added by one

and uh

the for loop creates the range of numbers

1,2,3,4,5

so would those numbers

be added

and it be

2,3,4,5,6

[crude pendant]

lst = [1, 2, 3, 4]
new_lst = [ i for i in lst if i % 2 == 0 ]

# "i" is created by the "for i in lst", its the iterator variable for the for loop and represents the elements of "lst"

# the list is then constructed of all "i" in "lst" such that i % 2 == 0

[crude pendant]

range(5) is 0, 1, 2, 3, 4

[crisp herald]

oh

okay so in the prior example

it would have been 0,1,2,3,4

and than the plus one would happen

so the output is

1,2,3,4,5

i see

[crude pendant]

this

l = [i+1 for i in range(5)]
print(l)

is analogous to this:

l = []
for i in range(5):
  l.append(i+1)
print(l)

if you cant understand the second one, i would learn the basics before shortcuts like this

[crisp herald]

but the expression seems to be the operation or equation that runs when the for loop will use after it has run its own calculations and the results of those calculations will generate the final result or new list

[crisp herald]

so in this example

list of numbers 1-4
and the expression is that i will beplaced in the list or final result if the for loops condition is met. in this case the for loop checks if i does not have a reminder after being divded by 2.

so the output would be 2,4

[crude pendant]

yes

[crisp herald]

the point is have fewer lines of code and make it more readable

@mental owl @crude pendant thank you, you are both effective teachers

[crude pendant]

fewer lines of code is not always better, readability is always the main concern

sometimes more efficient algorithms are actually more lines

[crisp herald]

!solved

[rose gazelle]

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