I got stuck on that:
- a list
y = (1,2,3...) - a function
func(A,B) - a constant
C
How can I express this situation with loops?
B1 = func(C , y[0])
B2 = func(B1 , y[1])
B3 = func(B2 , y[2]) #.... and so on.
2 Answers
The first argument is just the return value of the previous call, starting with C:
result = C
for yval in y:
result = func(result, yval)
As pointed out in the comments, this pattern is captured by the often overlooked reduce function. (Overlooked, in part, because it was demoted from the built-in namespace to the functools module in Python 3.)
from functools import reduce
result = reduce(func, y, C)
Comments
There are a few ways to do this:
First of all, regular loop:
C = ... # the constant
b = C
for i in y:
b = func(b, i)
But using a reduce like this, is my preferred way of doing this:
from functools import reduce
b = reduce(func, y, C) # the last arg being the initial item used
You could also use the walrus notation, which is only useful (IMO) when saving the intermediate states.
bs = [(b := func(b, yi)) for yi in y)]
b # b being the end result
1 Comment
chepner
For the third case, there is
itertools.accumulate(y, func, initial=C).
functools.reducedoes. You're looking forreduce(func, [C] + y). (You need to first dofrom functools import reduce)