Exercise 5.2 - get next float from input to *pn

Question

Write getfloat, the floating-point analog of getint. What type does getfloat return as its function value?

/*Write a program to get next float from input to *pn */

#include<stdio.h>
#include<ctype.h>

#define SIZE 1000

int getch(void);

void ungetch(int);

int getfloat(float *);

int main(void)
{
    int n;
    float array[SIZE];

    for(n=0;n<SIZE && getfloat(&array[n]) !=EOF;n++)
    {
        for(;n>=0;n--)
            printf("%f\n",array[n]);
    }
    return 0;
}

int getfloat(float *pn) {
    int c, sign;
    float power;

    while (isspace(c = getch()));

    if (!isdigit(c) && c != EOF && c != '+' && c != '-' && c != '.') {
        ungetch(c);
        return 0;
    }

    sign = (c == '-') ? -1 : 1;

    if (c == '+' || c == '-')
        c = getch();

    for (*pn = 0.0; isdigit(c); c = getch())
        *pn = 10.0 * *pn + (c - '0');
    if (c == '.')
        c = getch();

    for (power = 1.0; isdigit(c); c = getch()) {
        *pn = 10.0 * *pn + (c - '0');    /* fractional part */
        power *= 10.0;
    }

    *pn *= sign / power;

    if (c != EOF)
        ungetch(c);

    return c;
}

#define BUFSIZE 100

char buf[BUFSIZE];
int bufp = 0;

int getch(void) {
    return (bufp > 0) ? buf[--bufp] : getchar();
}

void ungetch(int c) {
    if (bufp >= BUFSIZE)
        printf("ungetch: too many characters\n");
    else
        buf[bufp++] = c;
}

Explanation

The function getfloat is similar to the getint function, wherein instead of getting an integer, we get a float value.

We declare an array, float array[SIZE] in which we store the float values and the the float value is got from the function int getfloat(float *);

The function getfloat, sends a pointer to the float and stores a float value in the array. The mechanism of how it gets a float is defined in the getfloat function.

The curx of how the float is got is in this snippet. The function reads all the characters and converts to a float and stores them in the *pn pointer. Then it reads the decimal part and for all the digits after the decimal point, it increments the power by 10. For e.g. if the decimal was 0.123 for 3 decimal digits, the power will be 10 * 10 * 10 = 1000.

for(*pn = 0.0 ; isdigit(c);c=getch())
    *pn = 10.0 * *pn + (c - '0');
if( c == '.')
    c = getch();

for(power=1.0;isdigit(c);c=getch())
{
    *pn = 10.0 * *pn + (c - '0');   /* fractional part */
    power *= 10.0;
}

*pn  *= sign / power;

Finally to get the decimal representation along with sign, then the number in pn is divided by power and multiplied by sign, i.e, *pn = ((*pn * sign) / power) and thus the correct float value is obtained.

The float value is stored in the array index that was sent from the main program.