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# 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 a 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.

### [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

Run this code

    #include <stdio.h>
    #include <errno.h>
    #include <stdlib.h>

    int main(void)
    {
        // parsing with error handling
        const char *p = "111.11 -2.22 Nan nan(2) inF 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);
        }

        // parsing without error handling
        printf("\"  -0.0000000123junk\"  -->  %g\n", strtod("  -0.0000000123junk", NULL));
        printf("\"junk\"                 -->  %g\n", strtod("junk", NULL));
    }

Possible output:

    Parsing '111.11 -2.22 Nan inF 0X1.BC70A3D70A3D7P+6  1.18973e+4932zzz':
    '111.11' -> 111.110000
    ' -2.22' -> -2.220000
    ' Nan' -> nan
    ' nan(2)' -> nan
    ' inF' -> inf
    ' 0X1.BC70A3D70A3D7P+6' -> 111.110000
    '  1.18973e+4932' -> range error, got inf
    "  -0.0000000123junk"  -->  -1.23e-08
    "junk"                 -->  0

### [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):

  - 4.10.1.4 The strtod function

### [edit] See also

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

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