For loop code to one line code in python?

Question

Can someone explain the following loop code converted into a one line code?

Code:

total_price = 0
for cost in gift_costs:
    if cost < 25:
        total_price += cost * 1.08

Converted one line code:

total_price = (gift_costs[gift_costs < 25]).sum() * 1.08

"please explain this code" questions are typically off-topic here, unless you specify a very specific aspect you don't understand and there aren't any questions about that aspect already answered elsewhere on the site. — Charles Duffy, Jun 28 '20 at 01:19
Moreover, it's important for code to be a [mcve] -- the shortest possible code someone can copy-and-paste without changes to see the behavior you're asking about. If we just set `gift_costs=[10 20 30 40]` your code [doesn't work](https://ideone.com/wMtpEF). I need to assume that your real `gifts_cost` is an object created using a library that isn't specified in the question. — Charles Duffy, Jun 28 '20 at 01:21
@RafaelAlmeida, ...except this code _isn't_ a list comprehension. (I first closed the question as a duplicate of one on the subject, since that's the obvious assumption given the title and context -- but take a closer look; it's not, hence being reopened). — Charles Duffy, Jun 28 '20 at 01:25
I would suggest breaking the code into smaller portions, running them and seeing the output. The code seems self explainable. — pecey, Jun 28 '20 at 01:28
@Charles Duffy The initialization should be `gift_costs=[10, 20, 30, 40]`. You missed the commas and there is a syntax error. — xuhdev, Jun 28 '20 at 01:34
@Charles fair enough, my mistake. If those are numpy arrays it would make sense, but yes, the question should have the full code — Rafael Almeida, Jun 28 '20 at 02:12
@xuhdev, still fails later even with that correct, though, unless one uses numpy arrays. — Charles Duffy, Jun 28 '20 at 03:08

revliscano · Answer 1 · 2020-06-28T02:20:53.627

You are not explicitly saying it, but it's pretty much clear that you're using numpy arrays. So let's break that line down, taking some arbitrary array as an example:

>>> gift_costs = numpy.array([10, 22, 35, 41])
>>> total_price = (gift_costs[gift_costs < 25]).sum() * 1.08
>>> total_price
34.56

total_price = : What is on the right side of the expression is going to be assigned to the total_price variable.
gift_costs < 25 : Whenever you use a comparison operator (such as <) along with a numpy array, that conditional statement is going to be evaluated for each item of the array. Then, you're going to get a new array filled with boolean values, which are going to be True if a given value matches the condition, or False otherwise. So, for the above example, you would get the following: array([True, True, False, False])
gift_costs[gift_costs < 25] : Considering what we have just mentioned above, we are now asking gift_costs to return its values which position in the following sequence are True: [True, True, False, False]. Those elements are: 10 and 22, and the returned object is: array([10, 22])
.sum() : Numpy array objects have a method called sum(), which returns the summation (∑) of the array. In our example, this is 10 + 22 = 32.
* 1.08 : Finally, the value returned by the sum() method (32) is multiplied by 1.08, which equals to 34.56

Disclaimer: Excuse me for my English, I tried to make myself clear the best I could.

score 0 · Answer 2 · answered Jun 28 '20 at 02:32

We can also code in a line without numpy. Plz check this one.

l = [10, 22, 35, 41]

t = sum((x for x in l if x <25))*1.08

I used sum function and make a generator for the argument of the function.
The equation of generator is simple. Just read it as followings.
"take an x in list l if it is less than 25"
The generator gathering all the x in the condition and then transfer to the sum function...

Looks simple, doesn't it???

score 0 · Answer 3 · answered Jun 28 '20 at 02:53

It's basically the same thing. The for loop uses an if clause to identify cost <25 and subsequently applies to them the multiplication * 1.08. The one clause filters using the [gift_costs < 25] and subsequently applies the same multiplication. Another possibility (also more efficient in terms of computing time) is a list comprehension.

For loop code to one line code in python?

3 Answers3