How do I find the sum of a given list for x[i]≥ a fixed x value, for each x value?
Question:
The code block is supposed to do the following:

Find
sum(all y_values in the list if the index corresponding to that y value has an x_value which is ≥ a fixed x_value)
. Find this sum for all x_values in the order given by the list, and append it to a list of its own. 
The output should be a list with the same length as len(x_values).
For example:
input:
x_values = [1,2,3,3,4,5]
y_values = [9,4,3,1,2,1]
output should be:
[20,11,7,7,3,1]
Note: I’m using python3.
my_list = []
my_list2 = []
for j in range(len(x_values)):
for i in range(len(x_values)):
if x_values[i] >= x_values[j]: #x_values[j] is supposed to be fixed x_value
my_list.append(y_values[i])
my_list2.append(sum(my_list)) #my_list2 is supposed to be such that len(my_list2)=len(x_values)
Answers:
You shouldn’t be calculating sum(my_list)
every time through the inner loop. You should initialize my_list
each time through the outer loop, and calculate the sum after the inner loop.
However it can all be replaced with a single list comprehension and a generator in the sum()
argument.
Use zip()
to loop through x_values
and y_values
together, to get the y
values in the corresponding indexes as the x
values that are compared with fixed_x
.
result = [sum(y for x, y in zip(x_values, y_values) if x >= fixed_x) for fixed_x in x_values]