math.h and ceil explanation

[slee8251]

#define taxRate .06#include <math.h>


float legalInput( float minimum );
float roundUpToPenny( float amount );
void  makeChange ( const float denoms[], int n, float tend, float cost );




int main ( void ) {
    const float denoms[] = {20, 10, 5, 1, .25, .10, .05, .01};
    const int nDenoms = 8;
    
    float price;        // Amount on price tag
    float tendered;     // Money from customer
    float total;        // price + tax
    
   
    printf("Making Change Please enter price and amount tendered, when prompted.\n" );
    price = legalInput( 0.01 );
    total = roundUpToPenny( price * (1 + taxRate));  
    tendered = legalInput( total ); 
    printf( "\nPrice = %.2f + tax = %.2f\n", price, total );
    printf( "Amount tendered = %.2f\n", tendered );
    makeChange ( denoms, nDenoms, total, tendered );
    
    return 0;
}


// -----------------------------------------------------------------------------
// Round an amount up to the nearest penny.
float 
roundUpToPenny( float amount ){
    return ceil( amount*100 ) / 100;
}


// -----------------------------------------------------------------------------
// Prompt user to input a dollar amount greater than or equal to the minimum.
float 
legalInput( float minimum ) {
    float input;
    // puts("legalInput called.");
    for (;;) {
        printf ("Enter value >= %.2f: ", minimum );
        scanf ("%f", &input );
        if (input >= minimum) break;
        printf ("Try again; enter a number >%f: ", minimum );    
    }
    return input;
}




// -----------------------------------------------------------------------------
// Given a price and an amount tendered, 
// output a list of coins and bills needed to make change.
void 
makeChange ( const float denoms[], int n, float cost, float tend )
{
    //puts("\n makeChange called.");
    float change;        // Money returned to customer
    float remaining;    // Amount of change not yet printed
    int k = 0;
    remaining = change = tend - cost;
    if (change < 0) {
        printf( "Please give me $%.2f more.\n", -change );
    }
    else printf( "Total change = %.2f\n", change );
    while (remaining > 0 && k<8) {
        if (remaining >= denoms[k]-.001) {
            //if (diff > .001) {
            remaining -= denoms[k];
            printf("\t%5.2f\n", denoms[k] );
        }
        else ++k;
    }
}

if i input 5.00, why is the tax 31 cents and not 30 cents?

thanks in advance

[stahta01]

I think you are starting to find out why most calculations dealing with money is done using integer penny values or on BCD hardware.

What Every Computer Scientist Should Know About Floating-Point Arithmetic

Tim S.