Greedy.c outputting the wrong number and I have no idea why? (CS50 Code)

Question

I am working on a program to work out the least number of coins for an amount of change given by the user.

It is meant to check for values of 0.25, 0.10, 0.05 and 0.01. If the user's value is greater than, or equal to one of these (in a set of successive if statements) an extra coin is added to the coin count and that amount is taken away from the user's original number.

Then, when the user's number is less than 0.01, the program is supposed to end and print the coin count.

I have read this code through several times and tried to fix it but I can't see a problem, can you help me with it?

0.25 shows 1 coin (correctly) but 0.26 shows 1 coin (incorrectly), for some reason missing my later if statement.

The 2 blocks of code controlling 0.25 and 0.01 are copied and pasted except or the coin value, so what's going on?

Thanks, Raisu

#include <stdio.h>
#include <cs50.h>
int main (void)
{
    /**  
    * Declare all variables
    */
    float UserInput  =  0.00;
    float Coin1      =  0.25;
    float Coin2      =  0.10;
    float Coin3      =  0.05;
    float Coin4      =  0.01;
    int   CoinCount  =  0;
    /**
    * Ask user for change amount and check it is more than 0, then print it
    */ 
    do
    {
        printf("Please put the amount of change, in the format 0.00\r\n\r\n");
        UserInput = GetFloat();// Get a float, check format
    }
    while (UserInput <0 || UserInput == 0);

    printf ("You have entered %f\r\n\r\n",UserInput);
    /**
     * If userinput is over or equal to 0.25, take off 0.25 and add 1 to the coin count
     */ 
    if (UserInput>=0.25) 
    {
        do
        {
            UserInput -= Coin1;
            CoinCount +=1;
        }
        while (UserInput >= Coin1);
        printf ("\r\n%i of %f Pieces\r\n",CoinCount,Coin1);
    }
    /**
    * If userinput is over or equal to 0.10, take off 0.10 and add 1 to the coin count
    */ 
    if (UserInput >=0.10)
    {
        do
        {
            UserInput -= Coin2;
            CoinCount +=1;
        }
        while (UserInput >= Coin2);
        printf ("\r\n%i of %f Pieces\r\n",CoinCount,Coin2);
    } 
    /** 
     * If userinput is over or equal to 0.05, take off 0.05 and add 1 to the coin count
     */ 
    if (UserInput >=0.05) 
    {
        do
        {
        UserInput -= Coin3;
        CoinCount +=1;
        }
        while (UserInput >= Coin3);
        printf ("\r\n%i of %f Pieces\r\n",CoinCount,Coin3);
    }
    /**
     * If userinput is over or equal to 0.01, take off 0.01 and add 1 to the coin count
     */
    if (UserInput >=0.01) 
    {
        do
        {
            UserInput -= Coin4;
            CoinCount +=1;
        }
        while (UserInput >= Coin4);
        printf ("\r\n%i of %f Pieces\r\n",CoinCount,Coin4);

    } 
    printf("\r\nTotal Coins Needed: %i\r\n*********************\r\n\r\n\r\n",CoinCount);
}

I suggest using `int` types and working in cents, please [see this](http://stackoverflow.com/questions/588004/is-floating-point-math-broken) — Weather Vane, Feb 20 '16 at 18:45
Just an observation: `if (UserInput >=0.10)` should be `if (UserInput >=Coin2)` and so on. — Weather Vane, Feb 20 '16 at 18:47
Do as Weater Vane suggests: treat the amount as integer number of cents, not as fractional number that represents dollars. — M Oehm, Feb 20 '16 at 18:58

score 1 · Answer 1 · answered Feb 20 '16 at 19:15

I wrote this little program to illustrate the problem that floating point format cannot store all values precisely.

#include <stdio.h>

int main(void){
    float UserInput;
    int UserCents;
    printf("Please put the amount of change, in the format 0.00\n");
    if (scanf("%f", &UserInput) != 1)
        return 1;
    UserCents = (int)(UserInput * 100 + 0.5);
    printf("Dollars = %.15f\n", UserInput); 
    printf("Cents   = %d\n", UserCents); 
    return 0;
}

Program session:

Please put the amount of change, in the format 0.00
12.34
Dollars = 12.340000152587891
Cents   = 1234

So you have two avenues to go, 1) switch to using int, or 2) compare your floating point values to a tolerance. Never compare them for equality.

Sudipta Kumar Sahoo · Answer 2 · 2016-02-20T19:23:26.723

I have modified your code. which is working fine. (i tried with few inputs).

int main (void)
{
    /**  
    * Declare all variables
    */
    double UserInput  =  0.00;
    double Coin1      =  0.25;
    double Coin2      =  0.10;
    double Coin3      =  0.05;
    double Coin4      =  0.01;
    int   TotalCoinCount  =  0;
    int CoinCount = 0;
    /**
    * Ask user for change amount and check it is more than 0, then print it
    */ 
    do
    {
        printf("Please put the amount of change, in the format 0.00\r\n\r\n");
        //UserInput = GetFloat();// Get a float, check format
        cin >> UserInput;
    }
    while (UserInput <0 || UserInput == 0);

    printf ("You have entered %f\r\n\r\n",UserInput);
    UserInput = floorf(UserInput * 100) / 100;
    /**
     * If userinput is over or equal to 0.25, take off 0.25 and add 1 to the coin count
     */     
    if (UserInput>=0.25) 
    {
        do
        {
            UserInput -= Coin1;
            UserInput = floorf(UserInput * 100) / 100;
            CoinCount +=1;
        }
        while (UserInput >= Coin1);
        printf ("\r\n%i of %f Pieces\r\n",CoinCount,Coin1);
    }
    TotalCoinCount += CoinCount;
    CoinCount = 0;
    /**
    * If userinput is over or equal to 0.10, take off 0.10 and add 1 to the coin count
    */ 
    if (UserInput >=0.10)
    {
        do
        {
            UserInput -= Coin2;
            UserInput = floorf(UserInput * 100) / 100;
            CoinCount +=1;
        }
        while (UserInput >= Coin2);
        printf ("\r\n%i of %f Pieces\r\n",CoinCount,Coin2);
    } 
    TotalCoinCount += CoinCount;
    CoinCount = 0;
    /** 
     * If userinput is over or equal to 0.05, take off 0.05 and add 1 to the coin count
     */ 
    if (UserInput >=0.05) 
    {
        do
        {
        UserInput -= Coin3;
        UserInput = floorf(UserInput * 100) / 100;
        CoinCount +=1;
        }
        while (UserInput >= Coin3);
        printf ("\r\n%i of %f Pieces\r\n",CoinCount,Coin3);
    }
    TotalCoinCount += CoinCount;
    CoinCount = 0;
    /**
     * If userinput is over or equal to 0.01, take off 0.01 and add 1 to the coin count
     */
    if (UserInput >=0.01) 
    {
        do
        {
            UserInput -= Coin4;
            UserInput = floorf(UserInput * 100) / 100;
            CoinCount +=1;
        }
        while (UserInput >= Coin4);
        printf ("\r\n%i of %f Pieces\r\n",CoinCount,Coin4);      
    } 
    TotalCoinCount += CoinCount;
    CoinCount = 0;
    printf("\r\nTotal Coins Needed: %i\r\n*********************\r\n\r\n\r\n",TotalCoinCount);
}

I have changed float to double and used floorf() to avoid any double data precision corruption.

`double d = 0.1; printf("%.18f\n", d);` outputs `0.100000000000000010`. Using `double` does not make the issue go away. — Weather Vane, Feb 20 '16 at 18:57
Anyway, this is a C++ answer. The C compiler (see question tag) will not compile it. — Weather Vane, Feb 20 '16 at 21:23

Greedy.c outputting the wrong number and I have no idea why? (CS50 Code)

2 Answers2