How can I write a program that reads an input string and converts the string into a float number.

I'm a bit stuck for the function:

#include <stdlib.h>
#include <string.h>
double convert_to_double (char *);
int main(void)
{
 char *s;
s = malloc(10 * sizeof(char));
printf("Enter text: ");
fgets(str, 10, stdin);
printf("The number is %lf", convert_to_double (str));
return 0;
}

double convert_to_double (char *str) {
 double convert_to_double (char *str) {
char *s;
double result;
result = strtod(str, &s);


if (s != NULL) {
    char *anotherEnd;
    double anotherResult = strtod(s, &anotherEnd);
}
if (isalnum(s) == 0){  
    printf("Wrong digit entered..");
}

return result;

}

CodePudding user response：

If you want to strictly validate the input string, please try:

double convert_to_double(char *str)
{
    char *e;
    double d = strtod(str, &e);
    if (*e != '\n') {
        fprintf(stderr, "input error %c\n", *e);
        exit(1);
    }
    return d;
}

The character *e is the first character remaining after the valid number representation is converted; usually a newline in this usage. If you do not have to check the input string so strictly, just say:

double convert_to_double(char *str)
{
    double d = strtod(str, (char **)NULL);
    return d;
}

You may not have to functionize it.

CodePudding user response：

You are doing the wrong way for strtod :

double convert_to_double (char *str) {
{
    char *s;
    double result;
    // first parameter is the string you want to convert
    // second pram is a pointer to the first char which was not converted.
    // this can be used if you have more than one data to convert in the same string
    result = strtod(str, &s);
    // example of using returned pointer :
    if (s != NULL) {
        char *anotherEnd;
        double anotherResult = strtod(s, &anotherEnd);
    }
    return result;
}

CodePudding user response：

How do you convert a string to double?

if (s != NULL) is not a useful test. The end pointer s is never NULL when attempting to convert a string.

Use strtod(). Test for potential errors.

double convert_to_double (const char *str) {
  // Optional test of str, which suppose to point to a string
  if (str == NULL) {
    fprintf(stderr, "Null pointer\n");
    return NAN;
  }

  char *endptr;  // Location of end of conversion.
  errno = 0;
  double result = strtod(str, &endptr);

  if (str == endptr) {
    fprintf(stderr, "No conversion of <%s>\n", str);
    return NAN;
  }

  // Consume trailing white-space. 
  while (isspace((unsigned char) *endptr)) {
    endptr  ;
  }
  if (*endptr) {
    fprintf(stderr, "Trailing junk <%s>\n", str);
    return result;
  }

  if (errno) {
    perror("Error:");
    return result;
  }
  return result;
}

Pedantically the last test should be more forgiving as underflow is usually not a real problem.

  if (errno) {
    if (errno != ERANGE && fabs(result > DBL_MIN)) { 
      perror("Error:");
      return result;
    }
  }

A double may be hundreds of digits in text form. Be generous in buffer size.

// s = malloc(10 * sizeof(char));
#define DBL_STR_SIZE 2000
str = malloc(sizeof *str * DBL_STR_SIZE);
if (str) {
  printf("Enter text: ");
  if (fgets(str, DBL_STR_SIZE, stdin)) {
    // process str