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Solution: Project Euler Problem 8

The Problem:

Find the greatest product of five consecutive digits in the 1000-digit number.

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

My Solution (in Java):


// Author: William Clausen
//
// Date: Jan 7, 2013
//
// This program solves Problem 8 from Project Euler.

public class Problem8 {

	// Datamembers to hold the number of numbers to be multiplied
	// and the number itself.
	public int numMult;
	public String nums;

	Problem8(int mult, String numbers)
	{
		numMult = mult;
		nums = numbers;
	}

	// This method solves the problem.
	public int solve()
	{
		// Integers to store the largest product seen so far and
		// the most recent product.
		int bigProd = 1;
		int newProd = 1;

		// An array to keep track of which numbers are being multiplied together.
		int[] multiples = new int[numMult];

		// Now, loop through the whole number (which is a string) and store the last five
		// numbers seen in an array. Calculate the product of those five numbers
		// and check if it's the biggest product seen so far. If the new product is the
		// largest seen so far, store it.
		for (int i = 0; i < nums.length(); i++) {
			if (multiples[i % numMult] != 0) {
				newProd /= multiples[i % numMult];
			}
			// Here is some safety work to make sure only numbers are being multiplied.
			if (Character.isDigit(nums.charAt(i))) {
				multiples[i % numMult] = Integer.parseInt(nums.substring(i, i+1));
			} else {
				// If a non-number is encountered, then I decided the product should be
				// 0 for as long as that character would have been in the array of
				// stored numbers.
				for (int j = 0; j < numMult; j++) {
					multiples[j] = 0;
				}
			}
			newProd = calcProd(multiples);
			if (newProd > bigProd) {
				bigProd = newProd;
			}
		}

		return bigProd;
	}

	// Method that calculates the product of numbers stored in an array.
	public int calcProd(int[] nums)
	{
		int prod = 1;

		for (int i = 0; i < nums.length; i++) {
			if (nums[i] == 0) {
				return 0;
			} else {
				prod *= nums[i];
			}
		}

		return prod;
	}
}

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