Find the greatest product of five consecutive digits in the 1000-digit number. – PROJECT EULER #8

This is both a completely straight forward problem and a nice way to learn about scanning through strings.

In the Racket case, we’ll want to use nested for loops. The outer loop will use for/fold to keep track of the best result so far. The inner loop will take an offset into the string and calculate the product. Assuming that the string n contains the number given in the problem:

(for/fold ([best 0])
          ([i (in-range (- (string-length n) 5))])
  (max best
       (for/product ([j (in-range 5)])
         (string->number (string (string-ref n (+ i j)))))))

The code in the Python version is pretty much the same:

best = 0
for i in range(len(n) - 5):
    best = max(best, int(n[i + 0]) * 
                     int(n[i + 1]) * 
                     int(n[i + 2]) * 
                     int(n[i + 3]) * 
                     int(n[i + 4]))
print best

If you want to get a bit fancy about it, you can take advantage of the reduce function to rewrite that code in a much more compact (albeit less readable fashion):

from operator import mul
print reduce(max, [reduce(mul, map(int, n[i:i+5]), 1) for i in range(len(n) - 5)], 0)

If you work from the inside out:

• n[i:i+5] will pull out five digits starting at i as a string
• map(int, n[i:i+5]) will convert that into a list of five actual integers
• reduce(operator.mul, ..., 1) will basically combine that list into a single number using multiplication as the glue (* isn’t a function in Python, ergo operator.mul)
• [reduce(...) for i in range(len(n) - 5)] will generate all such products
• reduce(max, ..., 0) will calculate the maximum of all of these numbers

Each of these versions will give you the expected value of 40824 (and all in less than a millisecond). None too shabby for a day’s work.

As always, you can download my code for this or any Project Euler problem I’ve uploaded here.