Modified Coin Change

November 19, 2019

This is a trick question.

In the general coin change problem, dynamic programming is required because the greedy algorithm doesn’t work for some sets of coins. But with the coins restricted to powers of 5, the greedy algorithm does work. And, in fact, there is an even simpler algorithm; express the number in base 5 and sum its digits:

(define (f n) (sum (digits n 5)))
> (f 42)
6
> (f 142)
6

 

That’s 25+5+5+5+1+1 for the first problem, and 125+5+5+5+1+1 for the second problem. You can run the program at https://ideone.com/bgOJvY.

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Posted by programmingpraxis
Filed in Exercises
2 Comments »

2 Responses to “Modified Coin Change”

  1. Daniel said
    November 22, 2019 at 1:43 AM

    Here’s a solution in Python.

    def modified_cc(n):
    out = 0
    while n > 0:
        n, r = divmod(n, 5)
        out += r
    return out

print(modified_cc(42))
print(modified_cc(142))

    Output:

    6
6
  2. Daniel said
    November 22, 2019 at 2:06 AM

    Here’s a solution in C that’s similar to the Python solution I posted earlier.

    #include <assert.h>
#include <stdio.h>
#include <stdlib.h>

int modified_cc(int n) {
  int out = 0;
  while (n > 0) {
    out += n % 5;
    n /= 5;
  }
  return out;
}

int main(int argc, char* argv[]) {
  assert(argc == 2);
  int n = atoi(argv[1]);
  printf("%d\n", modified_cc(n));
  return EXIT_SUCCESS;
}

    Example Usage:

    $ ./a.out 42
6

$ ./a.out 142
6

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