addition of specific elements of a list python

Question

A "pyschool" exercise :

"Define a function calls addFirstAndLast(x) that takes in a list of numbers and returns the sum of the first and last numbers."

Here is the best solution I have come up with. Is there a more elegant way to write this function that also only uses only built in functions?

def addFirstAndLast(x):
    sum_list = []
    if len(x) == 0:
        return 0
    elif len(x) == 1 :
        return int(x[0])
    elif len(x) > 1 :
        sum_list.append(x[0]) 
        sum_list.append(x[-1])
    return sum(sum_list)

return x[0] + x[-1] for the last case, There is no need to create a new list as sum_list and then apply sum() if you really have only 2 numbers to add. — ZdaR
– ZdaR, Commented May 31, 2015 at 11:57

Kasravnd · Accepted Answer · 2015-05-31 14:27:57Z

2

>>> def addFirstAndLast(x):
...   return (x[0]+x[-1])/(1/len(x)+1) if x else 0
... 
>>> addFirstAndLast([])
0
>>> addFirstAndLast([1])
1
>>> addFirstAndLast([1,3])
4

Note 1 : only when the length of list is 1 the result of (1/len(x)+1) is 2 so you divide the sum of first and last elements by 2 else it divide by 1.

Note 2 : if you are in python 3 use // for division instead /.

edited May 31, 2015 at 14:27

answered May 31, 2015 at 12:01

Kasravnd

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5 Comments

ZdaR Over a year ago

This handles all the test cases., great :)

PM 2Ring Over a year ago

Somewhat convoluted logic there... and shouldn't you be using // for integer division?

Kasravnd Over a year ago

@PM2Ring In python 3 yes but in this case there is no need to that because OP has accepted the answer!

PM 2Ring Over a year ago

Answers on Stack Exchange sites are also for the benefit of future readers, not just the OP.

Kasravnd Over a year ago

@PM2Ring Yes its true :)

Peter · Accepted Answer · 2015-05-31 12:06:11Z

2

Kasra's answer is a lot better but this is a slightly different way to go about it. Since x[0] is always returned if it exists, you can just check if x[-1] should be added too.

def firstAndLast(x):
    if x:
        value = x[0]
        if len(x)>1:
            value += x[-1]
        return value
    return 0

answered May 31, 2015 at 12:06

Peter

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1 Comment

PM 2Ring Over a year ago

Your code is easier to understand than Kasra's, and doesn't perform 2 mysterious divisions.

skies457 · Accepted Answer · 2015-05-31 12:03:58Z

0

def addFirstAndLast(x):
    if x:
        return sum(zip(*filter(lambda (i,s): i == 0 or i == len(x) - 1, enumerate(x)))[1])
    else:
        return 0

Enumerate elements in the list, filter, unzip and then return the sum.

answered May 31, 2015 at 12:03

skies457

1166 bronze badges

Comments

PM 2Ring · Accepted Answer · 2015-05-31 13:50:14Z

Use extended slice notation, with max to handle an empty list.

for i in range(5):
    a = range(10, 10 + i)
    b = a[::max(1, len(a)-1)]
    print a, b, sum(b)

output

[] [] 0
[10] [10] 10
[10, 11] [10, 11] 21
[10, 11, 12] [10, 12] 22
[10, 11, 12, 13] [10, 13] 23

And here's a version that does it in a function.

def add_first_and_last(x):
    return sum(x[::max(1, len(x)-1)])

for i in range(5):
    a = range(10, 10 + i)
    print a, add_first_and_last(a)

output

[] 0
[10] 10
[10, 11] 21
[10, 11, 12] 22
[10, 11, 12, 13] 23

At Kasra's prompting I've done some timing tests of the main existing answers. I've also added a modified version of Peter's algorithm that's slightly faster.

from timeit import Timer

def addFirstAndLast_Sam(x):
    sum_list = []
    if len(x) == 0:
        return 0
    elif len(x) == 1 :
        return int(x[0])
    elif len(x) > 1 :
        sum_list.append(x[0]) 
        sum_list.append(x[-1])
    return sum(sum_list)

def add_first_and_last_PM2Ring_slow(x):
    return sum(x[::max(1, len(x)-1)])

def add_first_and_last_PM2Ring_fast(x):
    return x[0] + x[-1] if len(x) > 1 else x[0] if x else 0

def firstAndLast_Peter(x):
    if x:
        value = x[0]
        if len(x)>1:
            value += x[-1]
        return value
    return 0

def addFirstAndLast_Kasra(x):
    return (x[0] + x[-1]) // (1 // len(x) + 1) if x else 0

funcs = (
    add_first_and_last_PM2Ring_fast,
    firstAndLast_Peter,
    addFirstAndLast_Kasra,
    add_first_and_last_PM2Ring_slow,
    addFirstAndLast_Sam,
)

num = 10
lists = [range(10, 10 + i) for i in range(num + 1)]

def verify():
    ''' Verify that the functions actually perform as intended '''
    print 'Verifying...'
    for func in funcs:
        fname = func.func_name
        print '\n%s' % fname
        for a in lists:
            print a, func(a)

def time_test(loops, reps):
    ''' Print timing stats for all the functions '''
    print '\nTiming tests\nLoops = %d, Repetitions = %d' % (loops, reps)

    for func in funcs:
        fname = func.func_name
        print '\n%s' % fname
        setup = 'from __main__ import lists, %s' % fname
        t = Timer('[%s(a) for a in lists]' % fname, setup)
        r = t.repeat(reps, loops)
        r.sort()
        print r

verify()
time_test(loops=10000, reps=3)

output

Verifying...

add_first_and_last_PM2Ring_fast
[] 0
[10] 10
[10, 11] 21
[10, 11, 12] 22
[10, 11, 12, 13] 23
[10, 11, 12, 13, 14] 24
[10, 11, 12, 13, 14, 15] 25
[10, 11, 12, 13, 14, 15, 16] 26
[10, 11, 12, 13, 14, 15, 16, 17] 27
[10, 11, 12, 13, 14, 15, 16, 17, 18] 28
[10, 11, 12, 13, 14, 15, 16, 17, 18, 19] 29

firstAndLast_Peter
[] 0
[10] 10
[10, 11] 21
[10, 11, 12] 22
[10, 11, 12, 13] 23
[10, 11, 12, 13, 14] 24
[10, 11, 12, 13, 14, 15] 25
[10, 11, 12, 13, 14, 15, 16] 26
[10, 11, 12, 13, 14, 15, 16, 17] 27
[10, 11, 12, 13, 14, 15, 16, 17, 18] 28
[10, 11, 12, 13, 14, 15, 16, 17, 18, 19] 29

addFirstAndLast_Kasra
[] 0
[10] 10
[10, 11] 21
[10, 11, 12] 22
[10, 11, 12, 13] 23
[10, 11, 12, 13, 14] 24
[10, 11, 12, 13, 14, 15] 25
[10, 11, 12, 13, 14, 15, 16] 26
[10, 11, 12, 13, 14, 15, 16, 17] 27
[10, 11, 12, 13, 14, 15, 16, 17, 18] 28
[10, 11, 12, 13, 14, 15, 16, 17, 18, 19] 29

add_first_and_last_PM2Ring_slow
[] 0
[10] 10
[10, 11] 21
[10, 11, 12] 22
[10, 11, 12, 13] 23
[10, 11, 12, 13, 14] 24
[10, 11, 12, 13, 14, 15] 25
[10, 11, 12, 13, 14, 15, 16] 26
[10, 11, 12, 13, 14, 15, 16, 17] 27
[10, 11, 12, 13, 14, 15, 16, 17, 18] 28
[10, 11, 12, 13, 14, 15, 16, 17, 18, 19] 29

addFirstAndLast_Sam
[] 0
[10] 10
[10, 11] 21
[10, 11, 12] 22
[10, 11, 12, 13] 23
[10, 11, 12, 13, 14] 24
[10, 11, 12, 13, 14, 15] 25
[10, 11, 12, 13, 14, 15, 16] 26
[10, 11, 12, 13, 14, 15, 16, 17] 27
[10, 11, 12, 13, 14, 15, 16, 17, 18] 28
[10, 11, 12, 13, 14, 15, 16, 17, 18, 19] 29

Timing tests
Loops = 10000, Repetitions = 3

add_first_and_last_PM2Ring_fast
[0.15383195877075195, 0.15486502647399902, 0.18578314781188965]

firstAndLast_Peter
[0.16625690460205078, 0.16726803779602051, 0.17240190505981445]

addFirstAndLast_Kasra
[0.19242000579833984, 0.19251012802124023, 0.21216797828674316]

add_first_and_last_PM2Ring_slow
[0.38388991355895996, 0.39070892333984375, 0.39607501029968262]

addFirstAndLast_Sam
[0.38914084434509277, 0.38966894149780273, 0.41235685348510742]

As you can see, my original code is definitely not the fastest, but it is the shortest. :)

There is no need to use sum and max and len for this simple task!!! you just make it very inefficient!
@Kasra: I agree that this approach is slower than yours. OTOH, Peter's code is faster than both of ours. I'll post some timing tests.

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