[Golang] Largest Product in a Series - Problem 8 - Project Euler


Go solution to largest product in a series - Problem 8 - Project Euler. [1]

Problem:

The four adjacent digits in the 1000-digit number that have the greatest product are 9 × 9 × 8 × 9 = 5832.

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

Find the thirteen adjacent digits in the 1000-digit number that have the greatest product. What is the value of this product?

Solution:

Index at 197 (index starts from 0)

5 X 5 X 7 X 6 X 6 X 8 X 9 X 6 X 6 X 4 X 8 X 9 X 5 = 23514624000

If you use int type, integer overflow will happen. Use int64 instead.

Run Code on Go Playground

package main

import (
      "fmt"
      "strconv"
)

func Get1000Digits() (digits []int64) {
      num1000 := `7316717653133062491922511967442657474235534919493496983520312774506326239578318016984801869478851843858615607891129494954595017379583319528532088055111254069874715852386305071569329096329522744304355766896648950445244523161731856403098711121722383113622298934233803081353362766142828064444866452387493035890729629049156044077239071381051585930796086670172427121883998797908792274921901699720888093776657273330010533678812202354218097512545405947522435258490771167055601360483958644670632441572215539753697817977846174064955149290862569321978468622482839722413756570560574902614079729686524145351004748216637048440319989000889524345065854122758866688116427171479924442928230863465674813919123162824586178664583591245665294765456828489128831426076900422421902267105562632111110937054421750694165896040807198403850962455444362981230987879927244284909188845801561660979191338754992005240636899125607176060588611646710940507754100225698315520005593572972571636269561882670428252483600823257530420752963450`
      for _, runeValue := range num1000 {
              i, err := strconv.ParseInt(string(runeValue), 10, 0)
              if err != nil {
                      panic(err)
              }
              digits = append(digits, i)
      }
      return
}

func AdjacentDigitsProduct(index, howMany int64, digits []int64) int64 {
      product := int64(1)
      for i := int64(0); i < howMany; i++ {
              product *= digits[index+i]
      }
      return product
}

func main() {
      digits := Get1000Digits()
      var max, index int64 = 0, 0
      for i := int64(0); i < int64(len(digits)-13); i++ {
              product := AdjacentDigitsProduct(i, 13, digits)
              if product > max {
                      max = product
                      index = i
              }
      }
      fmt.Print("Index at ")
      fmt.Println(index)
      for i := int64(0); i < 13; i++ {
              fmt.Print(digits[index+i])
              if i != 12 {
                      fmt.Print(" X ")
              }
      }
      fmt.Print(" = ")
      fmt.Println(max)
}

Tested on: Go Playground


References:

[1]Largest product in a series - Problem 8 - Project Euler
[2][Golang] Iterate Over UTF-8 Strings (non-ASCII strings)
[3][Golang] Type Conversion between String and Integer