# std::atof

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< cpp‎ | string‎ | byte

Defined in header `<cstdlib>` |  |
double atof( const char *str ); |  |

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 digits, Latin letters, and underscores. The result is a quiet NaN floating-point value.

- any other expression that may be accepted by the currently installed C locale

### Parameters

str | - | pointer to the null-terminated byte string to be interpreted

### Return value

double value corresponding to the contents of `str` on success. If the converted value falls out of range of the return type, the return value is undefined. If no conversion can be performed, 0.0 is returned.

### Example

Run this code

    #include <cstdlib>
    #include <iostream>

    int main()
    {
        std::cout << std::atof("0.0000000123") << "\n"
                  << std::atof("0.012") << "\n"
                  << std::atof("15e16") << "\n"
                  << std::atof("-0x1afp-2") << "\n"
                  << std::atof("inF") << "\n"
                  << std::atof("Nan") << "\n";
    }

Output:

    1.23e-08
    0.012
    1.5e+17
    -107.75
    inf
    nan

### See also

strtof strtod strtold | converts a byte string to a floating point value (function)

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