练习4-1，练习4-2

练习4-1

#include <stdio.h>

#define MAX 100

int Strrindex(char *source, char t); //4-1
void GetLine(char *s); //get string of input

int main() {
  char input[MAX];//input string;
  int char_t_location;

  GetLine(input);
  char_t_location = Strrindex(input, '4');

  printf("%d\n", char_t_location);
}

void GetLine(char *s) {
  int c;
  while ((c = getchar()) != EOF) {
    *s = c;
    s++;
  }

  *s = '\0';
  /*
  array label is not variable,but pointer is variable
  */
}

//4-1
int Strrindex(char *s, char t) {
  int location = 1;
  int t_location = -1;

  while (*s != '\0') {
    if (*s == t) t_location = location;
    ++location;
    ++s;
  }

  return t_location;
}

练习4-2

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

#define MAX 20

void GetLine(char *s);
double Atof(char s[]);

int main(void) {
  double num_output;
  char string_input[MAX];

  GetLine(string_input);
  num_output = Atof(string_input);

  printf("%6.3f\n", num_output);
  return 0;
}

void GetLine(char *s) {
  int c;
  while ((c = getchar()) != EOF) {
    *s = c;
    s++;
  }

  *s = '\0';
  /*
  array label is not variable,but pointer is variable
  */
}

//4-2
double Atof(char s[]) {
  double val, power;
  int i, sign, index;
  char index_sign = '+';

  for (i = 0; isspace(s[i]); i++) {}

  sign = (s[i] == '-') ? -1 : 1;

  if (s[i] == '+' || s[i] == '-')
    i++;
  for (val = 0.0; isdigit(s[i]); i++)
    val = 10.0 * val + (s[i] - '0');
  if (s[i] == '.')
    i++;
  for (power = 1.0; isdigit(s[i]); i++) {
    val = 10.0 * val + (s[i] - '0');
    power *= 10.0;
  }

  if (s[i] == 'e' || s[i] == 'E') {
    ++i;
    if ( !(isdigit(s[i])) ) {
      index_sign = (s[i] == '-') ? '-' : '+';
      ++i;
    }

    for (index = 0; isdigit(s[i]); i++)
      index = 10 * index + (s[i] - '0');

    if (index_sign == '+')
      return sign * val / power * pow(10.0, index);
    else
      return sign * val / power / pow(10.0, index);
  }

  return sign * val / power;
}