Namespaces
Variants
Views
Actions

strtof, strtod, strtold

From cppreference.com
< c‎ | string‎ | byte
Defined in header <stdlib.h>
float       strtof( const char *restrict str, char **restrict str_end );
(since C99)
double      strtod( const char          *str, char          **str_end );
(until C99)
double      strtod( const char *restrict str, char **restrict str_end );
(since C99)
long double strtold( const char *restrict str, char **restrict str_end );
(since C99)

Interprets a floating point value in a byte string pointed to by str.

Function discards any whitespace characters (as determined by std::isspace()) until first non-whitespace character is found. Then it takes as many characters as possible to form a valid floating point representation and converts them to floating point value. The valid floating point value can be one of the following:

  • decimal floating point expression. It consists of the following parts:
  • (optional) plus or minus sign
  • nonempty sequence of decimal digits optionally containing decimal point character (as determined by the current C locale) (defines significand)
  • (optional) e or E followed with optional minus or plus sign and nonempty sequence of decimal digits (defines exponent)
  • binary floating point expression. It consists of the following parts:
  • (optional) plus or minus sign
  • 0x or 0X
  • nonempty sequence of hexadecimal digits optionally containing a decimal point character (as determined by the current C locale) (defines significand)
  • (optional) p or P followed with optional minus or plus sign and nonempty sequence of decimal digits (defines exponent)
  • infinity expression. It consists of the following parts:
  • (optional) plus or minus sign
  • INF or INFINITY ignoring case
  • not-a-number expression. It consists of the following parts:
  • (optional) plus or minus sign
  • NAN or NAN(char_sequence) ignoring case of the NAN part. char_sequence can only contain alphanumeric characters. The result is a quiet NaN floating point value.
  • any other expression that may be accepted by the currently installed C locale

The functions sets the pointer pointed to by str_end to point to the character past the last character interpreted. If str_end is NULL, it is ignored.

Contents

[edit] Parameters

str - pointer to the null-terminated byte string to be interpreted
str_end - pointer to a pointer to character.

[edit] Return value

Floating point value corresponding to the contents of str on success. If the converted value falls out of range of corresponding return type, range error occurs and HUGE_VAL, HUGE_VALF or HUGE_VALL is returned. If no conversion can be performed, 0 is returned.

[edit] Example

#include <stdio.h>
#include <errno.h>
#include <stdlib.h>
 
int main(void)
{
    const char *p = "111.11 -2.22 0X1.BC70A3D70A3D7P+6  1.18973e+4932zzz";
    printf("Parsing '%s':\n", p);
    char *end;
    for (double f = strtod(p, &end); p != end; f = strtod(p, &end))
    {
        printf("'%.*s' -> ", (int)(end-p), p);
        p = end;
        if (errno == ERANGE){
            printf("range error, got ");
            errno = 0;
        }
        printf("%f\n", f);
    }
}

Possible output:

Parsing '111.11 -2.22 0X1.BC70A3D70A3D7P+6  1.18973e+4932zzz':
'111.11' -> 111.110000
' -2.22' -> -2.220000
' 0X1.BC70A3D70A3D7P+6' -> 111.110000
'  1.18973e+4932' -> range error, got inf

[edit] References

  • C11 standard (ISO/IEC 9899:2011):
  • 7.22.1.3 The strtod, strtof, and strtold functions (p: 342-344)
  • C99 standard (ISO/IEC 9899:1999):
  • 7.20.1.3 The strtod, strtof, and strtold functions (p: 308-310)
  • C89/C90 standard (ISO/IEC 9899:1990):

[edit] See also

converts a byte string to a floating point value
(function) [edit]
(C99)(C95)(C99)
converts a wide string to a floating point value
(function) [edit]
C++ documentation for strtof, strtod, strtold