How to use reduce function here?

Question

I am trying to find total of all the integers in a tuple

from  functools  import reduce
marks =  [("Jon" ,29 ), ("santi",35), ("anna",35)]

Total_marks = lambda x,y: x[1]

print(marks)
print (reduce(Total_marks,marks))

The above code can take the first value of integer, but i want to find the sum of all the integers, how to do it using reduce in python

Do you **have** to use `reduce`? `sum(score for name, score in marks)` is much more readable in my opinion. — lakshayg, Jul 09 '18 at 01:53
As suggested in other post, you can try `print(reduce(lambda x,y:x+y[1], marks,0))` https://stackoverflow.com/a/12218359/5916727 — niraj, Jul 09 '18 at 02:03

score 3 · Accepted Answer · answered Jul 09 '18 at 01:57

Change the definition of the adding function (there is no need to use lambda notation here):

def total_marks(x, y): 
    return x + y[1]

And tell reduce that the initial value is a number, not a tuple, by providing the third optional parameter:

reduce(total_marks, marks, 0)
#99

The same solution with lambda:

reduce(lambda x,y: x+y[1], marks, 0)

And one more solution that does not use reduce:

_, y = zip(*marks)
sum(y)
#99

lakshayg · Answer 2 · 2018-07-09T02:12:09.823

3

If using reduce is not necessary, a much more elegant solution is

marks =  [("Jon" ,29 ), ("santi",35), ("anna",35)]
total_marks = sum(score for _, score in marks)
print(total_marks)

edited Jul 09 '18 at 02:12

answered Jul 09 '18 at 01:58

lakshayg

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this is easy and can u post the full code – Karamzov Jul 09 '18 at 02:10
@Karamzov I have updated the code – lakshayg Jul 09 '18 at 02:12
How does the sum and for works here, how they knew to read the values ? – Karamzov Jul 09 '18 at 02:20
`sum` is a builtin function is python which can be used to sum iterables (lists, tuples, generators etc). The interesting part here is `score for _, score in marks`. This is what is called a list comprehension. A comment would be too short to explain what it does, please see this [blog](http://treyhunner.com/2015/12/python-list-comprehensions-now-in-color/). Googling "list comprehensions in python" will help too. – lakshayg Jul 09 '18 at 02:24
ok ... helpful .... – Karamzov Jul 09 '18 at 02:25
I changed the first set as `(2,29)` and run the code but still my result is 99 , not 101 , from your code how the list comprehension knows to sum the second value in list – Karamzov Jul 10 '18 at 08:06

score 2 · Answer 3 · answered Jul 09 '18 at 01:54

2

You would use operator.itemgetter(1) to get the numbers out of the tuple, and reduce with operator.add to sum them

from  functools  import reduce
from operator import itemgetter, add
marks =  [("Jon" ,29 ), ("santi",35), ("anna",35)]

print(reduce(add, map(itemgetter(1), marks)))
# 99

A simpler solution without reduce is sum(mark for name, mark in marks)

answered Jul 09 '18 at 01:54

Patrick Haugh

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Or even `sum(map(itemgetter(1), marks))` for the simpler solution. – RoadRunner Jul 09 '18 at 01:58
thanks patrick.... – Karamzov Jul 09 '18 at 08:47

score 1 · Answer 4 · answered Jul 09 '18 at 01:50

1

You can check if the first parameter (x) is a tuple:

from functools import reduce
marks = [("Jon" ,29 ), ("santi",35), ("anna",35)]
new_marks = reduce(lambda x, y:(x if isinstance(x, int) else x[-1])+y[-1], marks)
assert new_marks == sum(b for _, b in marks)
print(new_marks)

Output:

answered Jul 09 '18 at 01:50

Ajax1234

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Why the downvotes? – Ajax1234 Jul 09 '18 at 02:05
2

I feel this is an overly complicated use of `reduce` when you could instead provide the initial value: `reduce(lambda x, y: x + y[1], marks, 0)`. What happens otherwise is that `reduce` takes the first value as accumulator, which adds the unecessary complexity. – Olivier Melançon Jul 09 '18 at 02:06
good one.. thank you – Karamzov Jul 09 '18 at 02:12
@Karamzov Glad to help! – Ajax1234 Jul 09 '18 at 02:20

How to use reduce function here?

4 Answers4