Check If A Large Number Is Divisible By 3 Or Not

I saw this exercise from GeeksforGeeks [1], and it is a good example for type casting/conversion between int and string in Go.

The question is: Given a number, check if it is divisible by 3 or not.

The answer is: If a number is divisible by 3 if the sum of its digits is divisible by 3. For example, 12345 is divisible by 3, because 1+2+3+4+5=15, and 15 is divisible by 3.

Since Go is strongly-typed, we assume the number can be passed as either int or string type.

Number as int

Run Code on Go Playground

import (
      "strconv"
)

func IsIntDivisibleBy3(n int) bool {
      digits := strconv.Itoa(n)
      sumOfDigits := 0
      for _, digit := range digits {
              d, _ := strconv.Atoi(string(digit))
              sumOfDigits += d
      }

      return (sumOfDigits % 3) == 0
}

Example:

import (
      "testing"
)

func TestIsIntDivisibleBy3(t *testing.T) {
      if IsIntDivisibleBy3(123456758933312) != false {
              t.Error(123456758933312)
      }
      if IsIntDivisibleBy3(769452) != true {
              t.Error(769452)
      }
}

Number as string

Run Code on Go Playground

import (
      "strconv"
)

func IsStrDivisibleBy3(n string) (bool, error) {
      sumOfDigits := 0
      for _, digit := range n {
              d, err := strconv.Atoi(string(digit))
              if err != nil {
                      return false, err
              }
              sumOfDigits += d
      }

      return (sumOfDigits % 3) == 0, nil
}

Example:

import (
      "testing"
)

func TestIsStrDivisibleBy3(t *testing.T) {
      result, err := IsStrDivisibleBy3("3635883959606670431112222")
      if err != nil {
              t.Error(err)
      }
      if result != true {
              t.Error("3635883959606670431112222")
      }

      result, err = IsStrDivisibleBy3("123456758933312")
      if err != nil {
              t.Error(err)
      }
      if result != false {
              t.Error("123456758933312")
      }

      result, err = IsStrDivisibleBy3("769452")
      if err != nil {
              t.Error(err)
      }
      if result != true {
              t.Error("769452")
      }
}

Tested on:

Ubuntu Linux 17.10, Go 1.10.1
Go Playground

References:

[1]	Check if a large number is divisible by 3 or not - GeeksforGeeks