krx50200.c

answer of the c programming language second edition

/*
 * Exercise 5-2 from The C Programming Language, 2nd edition, by Kernighan
 * and Ritchie.
 *
 * "Write getfloat, the floating-point analog of getint. What type does
 * getfloat return as its function value?"
 */

/*
 * Here's the getint function, from section 5.2:
 */

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

int getch(void);
void ungetch(int);

/* getint:  get next integer from input into *pn */
int getint(int *pn)
{
        int c, sign;
        
        while (isspace(c = getch()))   /* skip white space */
                ;
        
        if (!isdigit(c) && c != EOF && c != '+' && c != '-') {
                ungetch(c);  /* it is not a number */
                return 0;
        }
        sign = (c == '-') ? -1 : 1;
        if (c == '+' || c == '-')
                c = getch();
        for (*pn = 0; isdigit(c); c = getch())
                *pn = 10 * *pn + (c - '0');
        *pn *= sign;
        if (c != EOF)
                ungetch(c);
        return c;
}

/*
 * The getch and ungetch functions, from section 4.3, are also required.
 */

#include <stdio.h>

#define BUFSIZE 100

char buf[BUFSIZE];      /* buffer for ungetch */
int bufp = 0;           /* next free position in buf */

int getch(void)         /* get a (possibly pushed-back) character */
{
        return (bufp > 0) ? buf[--bufp] : getchar();
}

void ungetch(int c)     /* push character back on input */
{
        if (bufp >= BUFSIZE)
                printf("ungetch: too many characters\n");
        else
                buf[bufp++] = c;
}

/*
 * The getfloat function.
 *
 * Reads the next number from input, and puts it into *fp.  Returns EOF for
 * end of file, zero if the next input is not a number, and a positive
 * value of the input contains a valid number.
 *
 * Based heavily on the getint function from K&R2.
 */

#include <ctype.h>
#include <math.h>

int getfloat(float *fp)
{
        int ch;
        int sign;
        int fraction;
        int digits;

        while (isspace(ch = getch()))   /* skip white space */
                ;

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

        sign = (ch == '-') ? -1 : 1;
        if (ch == '+' || ch == '-') {
                ch = getch();
                if (!isdigit(ch) && ch != '.') {
                        if (ch == EOF) {
                                return EOF;
                        } else {
                                ungetch(ch);
                                return 0;
                        }
                }
        }

        *fp = 0;
        fraction = 0;
        digits = 0;
        for ( ; isdigit(ch) || ch == '.' ; ch = getch()) {
                if (ch == '.') {
                        fraction = 1;
                } else {
                        if (!fraction) {
                                *fp = 10 * *fp + (ch - '0');
                        } else {
                                *fp = *fp + ((ch - '0') / pow(10, fraction));
                                fraction++;
                        }
                        digits++;
                }
        }

        *fp *= sign;

        if (ch == EOF) {
                return EOF;
        } else {
                ungetch(ch);
                return (digits) ? ch : 0;
        }
}

/*
 * Test module.
 */

#include <stdio.h>

int main(void)
{
        int ret;

        do {
                float f;

                fputs("Enter a number: ", stdout);
                fflush(stdout);
                ret = getfloat(&f);
                if (ret > 0) {
                        printf("You entered: %f\n", f);
                }
        } while (ret > 0);

        if (ret == EOF) {
                puts("Stopped by EOF.");
        } else {
                puts("Stopped by bad input.");
        }
        
        return 0;
}