C++ <cstdlib> - strtof() Function

The C++ <cstdlib> strtof() function is used to interpret a floating-point value in a byte string pointed to by str.

The function first discards any whitespace characters (as determined by isspace()) until the first non-whitespace character is found. Then, starting from this character, takes an optional initial plus or minus sign followed by as many characters as possible to form a valid floating-point representation and converts them to a floating-point value.

The valid floating-point value for this function using the "C" locale is formed by an optional sign character (+ or -), followed by one of:

• A sequence of digits, optionally containing a decimal-point character (.), optionally followed by an exponent part (an e or E character followed by an optional sign and a sequence of digits).
• A 0x or 0X prefix, then a sequence of hexadecimal digits (as in isxdigit()) optionally containing a period which separates the whole and fractional number parts. Optionally followed by a power of 2 exponent (a p or P character followed by an optional sign and a sequence of hexadecimal digits).
• INF or INFINITY (ignoring case).
• NAN or NANsequence (ignoring case), where sequence is a sequence of characters, where each character is either an alphanumeric character (as in isalnum) or the underscore character (_).

If str_end is not a null pointer, the function also sets the value of str_end to point to the first character after the number. If str_end is a null pointer, it is ignored.

If the first sequence of non-whitespace characters in str is not a valid floating-point number, or if no such sequence exists (because either empty or contains only whitespaces), no conversion is performed and zero is returned.

Syntax

• C++11

float strtof (const char* str, char** str_end);

Parameters

Return Value

On success, the function returns the converted floating-point number as a value of type float. If no conversion can be performed, 0.0 is returned. If the converted value falls out of the range, a positive or negative HUGE_VAL is returned, and errno is set to ERANGE.

Example 1:

The example below shows the usage of <cstdlib> strtof() function.

#include <cstdio>
#include <cstdlib>

int main (){
  char str1[] = "123";
  char str2[] = "10.55";
  char str3[] = "100 some words";
  char str4[] = "some words 555";
  char str5[] = "inF";
  char str6[] = "Nan(2)";

  char *end;

  float num1 = strtof(str1, &end);
  float num2 = strtof(str2, &end);
  float num3 = strtof(str3, &end);
  float num4 = strtof(str4, NULL);
  float num5 = strtof(str5, NULL);
  float num6 = strtof(str6, NULL);

  //displaying the result
  printf("strtof(\"%s\") = %.2f\n", str1, num1);
  printf("strtof(\"%s\") = %.2f\n", str2, num2);
  printf("strtof(\"%s\") = %.2f\n", str3, num3);
  printf("strtof(\"%s\") = %.2f\n", str4, num4);
  printf("strtof(\"%s\") = %.2f\n", str5, num5);
  printf("strtof(\"%s\") = %.2f\n", str6, num6);

  return 0;
}

The output of the above code will be:

strtof("123") = 123.00
strtof("10.55") = 10.55
strtof("100 some words") = 100.00
strtof("some words 555") = 0.00
strtof("inF") = inf
strtof("Nan(2)") = nan

Example 2:

Consider one more example where the string contains multiple floating-point values. The second parameter if not a null pointer then this function sets this parameter to value which points to the first character after the interpreted number. This feature can be used to interpret multiple floating-point values from the string.

#include <cstdio>
#include <cstdlib>

int main (){
  char str[] = "123 10.55 555.89";
  char *pEnd;

  float val1 = strtof(str, &pEnd);
  float val2 = strtof(pEnd, &pEnd);
  float val3 = strtof(pEnd, &pEnd);

  //displaying the result
  printf("val1 = %.2f\n", val1);
  printf("val2 = %.2f\n", val2);
  printf("val3 = %.2f\n", val3);

  return 0;
}

The output of the above code will be:

val1 = 123.00
val2 = 10.55
val3 = 555.89