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Posted on Aug 20, 2019

Daily Challenge #45 - Change Machine

#challenge

Today's challenge is to implement a change function that will accept an integer parameter that represents cents. The function should return the optimal change using the least number of coins.

The function should also return a key for each coin of US currency (specifically 25¢, 10¢, 5¢, and 1¢ coins). The value of each coin should represent the count of each coin in the change. The value for each coin that is not included should return 0.

Ex.
change(31)
{ 25 => 1, 10 => 0, 5 => 1, 1 => 1 }

Good luck!

This challenge comes from user Lordnibbler on CodeWars. Thank you to CodeWars, who has licensed redistribution of this challenge under the 2-Clause BSD License!

Want to propose a challenge for a future post? Email yo+challenge@dev.to with your suggestions!

Top comments (17)

Șerbu Vlad Gabriel • Aug 20 '19 • Edited

x86_64 assembly (System V ABI, GNU assembler), for those concerned about speed.

change.S:

    .global change

    .text
change:
    push %rbx
    xor %edx, %edx
    xor %ecx, %ecx
    mov %edi, %eax
    mov $table, %r9
loop:
    mov (%r9, %rcx, 4), %ebx
    div %ebx

    mov %eax, (%rsi, %rcx, 4)
    mov %edx, %eax
    xor %edx, %edx

    inc %ecx
    cmp $4, %ecx
    jne loop

    pop %rbx
    ret


    .section .rodata
table:
    .int 25
    .int 10
    .int 5
    .int 1

change.h:

/*
 * coins[0] = number of 25 cent coins
 * coins[1] = number of 10 cent coins
 * coins[2] = number of 5 cent coins
 * coins[3] = number of 1 cent coins
 */
unsigned int change(unsigned int n, unsigned int coins[4]);

Josh • Aug 21 '19 • Edited

I am concerned about speed

we need to get that stuff off the streets smh

causin' good folk to think they need to write stuff in assembly, fcol >.>

Amin • Aug 20 '19 • Edited

Hey there, thank you for your comment!

Don’t get me wrong, I like your version, but in the strict sense I’d argue it isn’t more “pure” than the original version.

We might think that the COINS variable declared with a const operator is indeed a constant but it is not. It is not possible to assign another array, but you can still update its values by targeting its indexes. This means that the function is inpure because its result is not predictable as it relies on an out-scope variable that can be updated outside of the function call, but used inside the function. The oustide world can update the array like so:

COINS[0] = "Hello world!";

The solution would be to declare the array as a freezed array in JavaScript:

const COINS = Object.freeze([1, 2, 3, ...]);

That way, no side-effects possible. This would also allow the function to be pure again. Another solution would be to declare the COINS variable inside the function, or even better, declare another parameter if the coins variable is something to be changed later.

const change = (cents, COINS = [1, 2, 3, ...]) => ...

Seiei Miyagi • Aug 20 '19 • Edited

ruby <3

def change(amount)
  result = { 25 => 0, 10 => 0, 5 => 0, 1 => 0 }
  case amount
  in 25.. then
    amount -= 25
    result[25] += 1
  in 10.. then
    amount -= 10
    result[10] += 1
  in 5.. then
    amount -= 5
    result[5] += 1
  in 1.. then
    amount -= 1
    result[1] += 1
  end until amount.zero?
  result
end

p change(31)

Amin • Aug 20 '19 • Edited

JavaScript

Pure variant of what has already been made in JavaScript.

function change(money) {
  return [25, 10, 5, 1].reduce(function([computedChange, remainingMoney], coin) {
    return [Object.assign(computedChange, {
      [coin]: Math.floor(remainingMoney / coin)
    }), remainingMoney % coin];
  }, [{}, money])[0];
}

Performance

I am curious about the performance in those kind of challenge. I am totally aware that JavaScript is not the fastest to do this kind of thing and the idea is not to bash any language of implementation. I'm just a guy curious about that. If some of you are willing to take the habit to post the performance for one milion iterations that would be cool to compare with others!

// one milion iterations
for (let index = 0; index < 1000000; index++) {
  change(31);
}

console.timeEnd("time");
// time: 21670.730ms

Source-Code

Available online.

Oleksii Filonenko • Aug 21 '19

Rust, iterative style:

use std::collections::HashMap;

const COINS: [u32; 4] = [25, 10, 5, 1];

fn change(mut money: u32) -> HashMap<u32, u32> {
    let mut change = HashMap::new();
    for &coin in &COINS {
        change.insert(coin, money / coin);
        money %= coin;
    }
    change
}

K.V.Harish • Oct 8 '19

My solution in js

const change = (amount) => {
  return {
    '25': Math.floor(amount/25),
    '10': Math.floor((a = amount%25)/10),
    '5': Math.floor((b = a%10)/5),
    '1': b % 5
  }
};

Vivek97 • Aug 21 '19 • Edited

public static void change(int number)
     {      
        int [] array = {25,10,5,1};
        number = 74;
        for(int m:array)
            {
                System.out.print(m+" =>"+number/m+" ,");
                number = number%m;      
            }
    }

Alvaro Montoro • Aug 21 '19

This is a nice solution in Java. But don't forget to remove the number = 74;, otherwise the function will always write the same results.

Jérémie Astor • Aug 21 '19

Just my two cents, in Gwion

fun void change(int cents) {
  cents / 25 => const int coin25;
  coin25 * 25 -=> cents;
  cents / 10 => const int coin10;
  coin10 *10 -=> cents;
  cents  /5 => const int coin5;
  coin5 * 5 -=> cents;

#! this is a comment
#! below is the debug print.

  <<<
    "{ ",
    "25 => ",   coin25,
    ", 10 => ", coin10,
    ", 5 => ",  coin5,
    ", 1 => ",  cents,
    " }"
  >>>;
}

31 => change;

prints

{ 25 => 1, 10 => 0, 5 => 1, 1 => 1 }

It probably could be written better, but looks like it gets the job done.

Chris Achard • Aug 20 '19

JavaScript with reduce:

const COINS = [25, 10, 5, 1]

const change = (cents) => {

  let remainingValue = cents

  const coinCount = COINS.reduce((count, coin) => {
    count[coin] = Math.floor(remainingValue / coin)
    remainingValue = remainingValue % coin
    return count
  }, {})

  return coinCount
}

Donald Feury • Aug 20 '19

Go - with tests:

change.go:

// Package change contains methods for interacting with US currency
package change

var coins = []int{25, 10, 5, 1}

// Change returns the fewest amount of each US coin to equal the amount of change indicated
func Change(amount int) map[int]int {
    usedCoins := map[int]int{25: 0, 10: 0, 5: 0, 1: 0}

    if amount == 0 {
        return usedCoins
    }

    remain := amount

    for _, coin := range coins {
        if remain == 0 {
            return usedCoins
        }

        mod := remain % coin
        if mod != remain {
            usedCoins[coin] = int(remain / coin)
            remain = mod
        }
    }

    return usedCoins
}

change_test.go

package change

import "testing"

func equal(actual map[int]int, expected map[int]int) bool {
    if len(actual) != len(expected) {
        return false
    }

    for k, v := range actual {
        expectedVal, present := expected[k]

        if !present || v != expectedVal {
            return false
        }
    }

    return true
}

func TestChange(t *testing.T) {
    for _, test := range changeTests {
        if actual := Change(test.input); !equal(actual, test.expected) {
            t.Fatalf(
                "FAIL: %s - Change(%d): %v, expected: %v \n",
                test.description,
                test.input,
                actual,
                test.expected,
            )
        }
        t.Logf("PASS: %s \n", test.description)
    }
}

change_test_cases.go

package change

var changeTests = []struct {
    description string
    input       int
    expected    map[int]int
}{
    {
        "no change",
        0,
        map[int]int{25: 0, 10: 0, 5: 0, 1: 0},
    },
    {
        "uses one coin",
        5,
        map[int]int{25: 0, 10: 0, 5: 1, 1: 0},
    },
    {
        "uses two coins",
        27,
        map[int]int{25: 1, 10: 0, 5: 0, 1: 2},
    },
    {
        "uses many coin",
        39,
        map[int]int{25: 1, 10: 1, 5: 0, 1: 4},
    },
}

View full discussion (17 comments)

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Daily Challenge #45 - Change Machine

Top comments (17)

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