Python program find sum of consecutive number values equal to number n?

Question

I want to find consecutive digits in a string that sum to a given number.

Example:

a="23410212" number is=5 — output 23,41,410,0212,212.

This code is not working. What do I need to fix?

def find_ten_sstrsum():
    num1="2825302"
    n=0;
    total=0;
    alist=[];
    ten_str="";
    nxt=1;
    for n in range(len(num1)):
        for n1 in range(nxt,len(num1)):
            print(total)
            if(total==0):
                total=int(num1[n])+int(num1[n1])
                ten_str=num1[n]+num1[n1]
            else:
                total+=int(num1[n1])
                ten_str+=num1[n1]
            if(total==10):
                alist.append(ten_str)
                ten_str=""
                total=0
                nxt+=1
                break
            elif(total<10):
                nxt+=1
     return alist

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user2390182 · Accepted Answer · 2017-01-25T09:28:39.187

This (sort-of) one-liner will work:

def find_ten_sstrsum(s, n):
  return list(  # list call only in Python 3 if you don't want an iterator
    filter(
      lambda y: sum(map(int, y))==n, 
      (s[i:j] for i in range(len(s)) for j in range(i+1, len(s)+1)))
  )

>>> find_ten_sstrsum('23410212', 5)
['23', '41', '410', '0212', '212']

This uses a nested generator expression over all possible slices and filters out the ones with the correct digit-sum. This is, of course, far from optimal (especially for long strings) because the inner loop should be stopped as soon as the digit-sum exceeds n, but should give you an idea.

A more performant and readable solution would be a generator function:

def find_ten_sstrsum(s, n):
  for start in range(len(s)):
    for end in range(start+1, len(s)+1):
      val = sum(map(int, s[start:end]))
      if val > n:
        break
      if val == n:
        yield s[start:end]

>>> list(find_ten_sstrsum('23410212', 5))
['23', '41', '410', '0212', '212']

Definitely read up on sum and map, as well.

score 0 · Answer 2 · answered Jan 24 '17 at 22:31

Your function has several problems. Most of all, you have no way to give it different data to work on. You have it hard-coded to handle one particular string and a total of 10. You've even written variable names with "ten" in them, even though your example uses n=5. Your function should have two input parameters: the given string and the target sum.

NOTE: schwobaseggl just posted a lovely, Pythonic solution. However, I'll keep writing this, in case you need a function closer to your present learning level.

You have several logic paths, making it hard to follow how you handle your data. I recommend a slightly different approach, so that you can treat each partial sum cleanly:

for start in range(len(num1)):
    total = 0     # Start at 0 each time you get a new starting number.
    sum_str = ""
    for last in num1[start:]:
        print(total)
        # Don't create separate cases for the first and other additions.
        this_digit = num1[last]
        total += int(this_digit)
        ten_str += this_digit

        # If we hit the target, save the solution and
        #   start at the next digit
        if(total == target):
            alist.append(ten_str)
            break

        # If we passed the target, just
        #   start at the next digit
        elif(total > target):
            break

 return alist

Now, this doesn't solve quite all of your problems, and I haven't done some of the accounting work for you (variable initializations, def line, etc.). However, I think it moves you in the right direction and preserves the spirit of your code.

Python program find sum of consecutive number values equal to number n?

2 Answers2