Summing first 2 elements in a Python list when the length of the list is unknown

python arrays list sum conditional-statements

16,354

Solution 1

There is. Two elements of the solution - builtin function sum and lists's slices:

>>> sum([1,2,3][:2])
3
>>> sum([1,1,1,1][:2])
2
>>> sum([1,1][:2])
2
>>> sum([1][:2])
1
>>> sum([][:2])
0

Solution 2

If you can't use sum, one possible solution uses exceptions:

totalsum = 0
try:
  totalsum += nums[0]
  totalsum += nums[1]
except IndexError:
  pass
return totalsum

Catch the error and short-circuit the summation if an element doesn't exist. Easier to ask forgiveness than permission, as they say.

Solution 3

try this:

def sum2(nums):
  if len(nums) == 1:
    return nums[0]
  else:
    return sum(nums [:2])

16,354

Author by

MyCarta

I am a Geophysicists from Canada (I grew up in Italy). I have a Honors degree in Geology from the University of Rome, Italy, and a Masters degree in Geophysics from the University of Calgary, Alberta. I am also a would-be data scientist, blogger, and independent researcher through my own company, MyCarta, which is involved in visualization and digital mapping of geoscience data. I like to explore methods from other disciplines (at the top of the list medical and forensic image analysis) and adapt them to geo-data. I like Python.

Updated on June 18, 2022

Comments

MyCarta almost 2 years
I am working on the following Python list exercise from codingbat.com:
Given an array of ints, return the sum of the first 2 elements in the array. If the array length is less than 2, just sum up the elements that exist, returning 0 if the array is length 0. Examples:
```
sum2([1, 2, 3]) → 3 

sum2([1, 1]) → 2

sum2([1, 1, 1, 1]) → 2
```
My solution below works:
```
def sum2(nums):
  if len(nums)>=2:
    return nums[0] + nums[1]
  elif len(nums)==1:
    return nums[0]
  return 0
```
But I wonder if there's any way to solve the problem with fewer conditional statements.
- jonrsharpe almost 10 years
  
  return sum(nums[:2])?
- MyCarta almost 10 years
  
  +1 as it is same solution as answer accepted
jonrsharpe almost 10 years

Those aren't "ranges", they're "slices".
Roman Bodnarchuk almost 10 years

@jonrsharpe That is true, was remembering the right term.
jonrsharpe almost 10 years

Better to use except IndexError - a "naked" except can catch all kinds of other things (e.g. a TypeError because nums can't be indexed at all or doesn't only contain numbers).
MyCarta almost 10 years

why do I have to wait 10 minutes to accept the answer?
wnnmaw almost 10 years

@MyCarta to make sure you have time to review all the answers that pop up in that time
MyCarta almost 10 years

Cannot use it because the exercise is structured so that I have to use sum but very interesting, did not know anything exceptions, thank you.
MyCarta almost 10 years

OK, thank you for the clarification. That makes sense.
jonrsharpe almost 10 years

@MyCarta your first answer wasn't using sum! EAFP is a common Python coding style, worth being aware of.
MyCarta almost 10 years

OK I tried the code suggested by @TheSoundDefense and it works. Really interesting and I agree worth learning. I could've choosen either answer.