Pairing digits from a number

Question

I wish to write a Python program that gets a number from the keyboard, and outputs a number built from the minimum of each pair of digits of the original number. For example:

Enter number please:  84372216
Original number: 84372216 
New number:   4321

I had an idea to transform the original number into a string, to create a list of digits (chars) in the odd locations and the even locations, and then to compare them by the index of the lists. This doesn't sound too efficient or easy to implement, as it involves too many conversions from int to string and vice versa. The advantage is that this solves easily the problem of a number with odd number of digits.

Is there a way of solving this problem without using strings ?

Thank you.

Answer 1

Use zip() on slices of the original number and min() on each tuple returned:

>>> number = input('Enter number please: ')
Enter number please: 84372216
>>> print('Original number: {}'.format(number))
Original number: 84372216
>>> new_number = ''.join(min(tup) for tup in zip(number[::2], number[1::2]))
>>> print('New number: {}'.format(new_number))
New number: 4321

Answer 2

num = 84372216
int(''.join(min(i) for i in zip(*[iter(str(num))]*2)))
#4321

list(zip(*[iter(str(num))]*2)) splits num into pairs as below

[('8', '4'), ('3', '7'), ('2', '2'), ('1', '6')]

Then use min and join combination to get the output

Answer 3

Transhuman's answer is cool, but here's a more readable version:

s = str(num)
r = int(''.join(min(s[i:i+2]) for i in range(0, len(s), 2)))

Another nifty trick is to use re.findall :

r = int(''.join(min(i) for i in re.findall('..', str(num))))

EDIT

I didn't notice you said "without using strings". In that case:

def f(n):
    return n if n < 10 else min(n % 10, n // 10 % 10) + 10 * f(n // 100)

Answer 4

It's quick and dirty, but it works:

def pairmin( n ):
        result = 0
        mult = 1
        while n > 0:
                if n > 10:
                        result =  min( n % 10, (n / 10) % 10 ) * mult + result
                else:
                        result = n * mult + result
                n = n / 100
                mult = mult * 10
        return result

print( pairmin( 84372216 ) )
print( pairmin( 8372216 ) )

Answer 5

A slight variation of what Vaultah posted in comment above. You can use map function, supply it with consecutive pairs and determine minimum from it.

 s = "84372216"
 "".join(list(map(lambda x,y: str(min([x,y])), s[::2],s[1::2])))
 #'4321'

Here map(lambda x, y: str(min([x,y])) takes two arguments from iterable s[::2],s[1::2] and determine what is the minimum value from the supplied args. Min result is being cast to str to join the final result. You can however cast the result back to int if you require further processing of result. Just add int() to overall expression.

To unserstand s[::2],s[1::2] , please check Understand Python Slice Notation . Slice notation works like s[start:end:step]