CODEWITHSUNDEEP
cluafloating-pointgreedy
Beri
Beri

Reputation: 245

Numbers acting strange in both Lua and C

I have written this program in Lua and C which is a greedy algorithm.

Greedy = function(num)
local q = 0;
local d = 0;
local n = 0;
local p = 0;

local x = 1;

while x == 1 do 
    if (num >= 0.25) then
        q = q + 1;
        num = num - 0.25;
        print("q");
    elseif (num >= 0.10) then
        d = d + 1;
        num = num - 0.10;
        print("d");
    elseif (num >= 0.05) then
        n = n + 1;
        num = num - 0.05;
        print("n");
    elseif (num >= 0.01) then
        p = p + 1;
        num = num - 0.01;
        print("p");
    end

    if (num == 0) then
        x = 0;
    end
end

if (x == 0) then
    local all = q+d+n+p;
    print(all);
end


end

 Greedy(1);

This code works fine with some numbers but if I try to calculate numbers like 0.90,0.04 or 0.12 it wont work and I have the same code written in C too. But I have the same problem with it.

#include <stdio.h>

int greedy(void){
    int quarter = 0;
    int dime = 0;
    int nickel = 0;
    int penny = 0;

float num = 0.40;

int x = 1;

while(x == 1){
     if (num >= 0.25){
         quarter = quarter + 1;
         num = num - 0.25;
         printf("q\n");
     }else if(num >= 0.10){
         dime = dime + 1;
         num = num - 0.10;
         printf("d\n");
     }else if(num >= 0.05){
         nickel = nickel + 1;
         num = num - 0.05;
         printf("n\n");
     }else if(num >= 0.01){
         penny = penny + 1;
         num = num - 0.01;
         printf("p\n");
     };

     if(num == 0){
         x = 0;
     };
};

if(x == 0){
    int all = quarter + dime + nickel + penny;
    printf("%i\n", all);
};
return 0;
};

int main(void){
   greedy();
}

What did I do wrong?

Upvotes: 1

Views: 88

Answers (2)

Alar
Alar

Reputation: 780

Seeing your scope is counting your money, rewrite your code this way:

Greedy = function(num)
  local q = 0;
  local d = 0;
  local n = 0;
  local p = 0;

  local x = 1;

  num=num*100
  while x == 1 do 
   if (num >= 25) then
      q = q + 1;
      num = num - 25;
      print("q");
   elseif (num >= 10) then
      d = d + 1;
      num = num - 10;
      print("d");
   elseif (num >= 5) then
      n = n + 1;
      num = num - 5;
      print("n");
   elseif (num >= 1) then
      p = p + 1;
      num = num - 1;
      print("p");
   end

  if (num == 0) then
      x = 0;
  end
  end

  if (x == 0) then
      local all = q+d+n+p;
      print(all);
  end

end 

Greedy(1);

Upvotes: 1

Yu Hao
Yu Hao

Reputation: 122493

The problem is that, many floating-point numbers can't be represented (using double or float) precisely, try this in Lua:

> print(0.9 == 0.9 - 0.2 + 0.1 + 0.1)
false

It should be equal in maths, but not here. The same for the C code.

Upvotes: 2

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