*Source code available on GitHub.*

Ah, an interesting-looking one!

73167176531330624919225119674426574742355349194934 96983520312774506326239578318016984801869478851843 85861560789112949495459501737958331952853208805511 12540698747158523863050715693290963295227443043557 66896648950445244523161731856403098711121722383113 62229893423380308135336276614282806444486645238749 30358907296290491560440772390713810515859307960866 70172427121883998797908792274921901699720888093776 65727333001053367881220235421809751254540594752243 52584907711670556013604839586446706324415722155397 53697817977846174064955149290862569321978468622482 83972241375657056057490261407972968652414535100474 82166370484403199890008895243450658541227588666881 16427171479924442928230863465674813919123162824586 17866458359124566529476545682848912883142607690042 24219022671055626321111109370544217506941658960408 07198403850962455444362981230987879927244284909188 84580156166097919133875499200524063689912560717606 05886116467109405077541002256983155200055935729725 71636269561882670428252483600823257530420752963450

So, immediately we want to get this big number and open it in a text file. In Python, while you could just open the file, use it and then close it again, there’s a handy construct called `with`

–`as`

which is perfect for dealing with text files. We copy and paste that big number into a file called `input.txt`

and then do:

with open('input.txt') as f:

We do what we want to the text `f`

and as soon as we leave the `with`

statement, Python closes the file down and cleans up after us.

We want to read the file with `f.read()`

and get rid of all the new line characters (`\n`

). This does it:

with open('input.txt') as f: data = f.read().replace('\n','')

Now `data`

contains a big string of our number and nothing else. We happen to know the number is 1000 digits long and we want 13 adjacent digits at a time, but these numbers could be anything so we’ll be general. The following `for`

loop will iterate through all the possible 13 adjacent digits, find their product, and store it if it’s the biggest yet.

for i in range(len(data)-digitsLength): digits = [int(j) for j in data[i:i+digitsLength]] product = prod(digits) if product > maxProduct: maxProduct = product

You may notice that `prod()`

isn’t a built-in function: we’ll define that. It just finds the products of all the elements in an iterable variable like a list, tuple or set. It’s a simple definition:

def prod(iterable): product = 1 for i in iterable: product *= i return product

Fine, that’s all done. Our program is as follows:

def prod(iterable): product = 1 for i in iterable: product *= i return product with open('input.txt') as f: data = f.read().replace('\n','') digitsLength = 13 # The number of adjacent digits we want to find the product of maxProduct = 0 for i in range(len(data)-digitsLength): digits = [int(j) for j in data[i:i+digitsLength]] product = prod(digits) if product > maxProduct: maxProduct = product print(maxProduct)