Standard Numeric Format Strings

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Scanf format string which stands for " scan f ormatted" refers to a control parameter used by a class of functions in the string-processing libraries of various programming languages. The format string specifies a method for reading a string into an arbitrary number of varied data type parameter s. The input string is by default read from the standard inputbut variants exist that read the input from other sources. The term "scanf" is due to the C language, which popularized this type of function, but these functions predate C, and other names are used, such as "readf" in ALGOL Scanf format strings, which provide formatted input parsingare complementary to printf format stringswhich provide formatted output templating.

In both cases these provide simple functionality and fixed format compared to more sophisticated and flexible parsers or template engines, but are sufficient for many purposes. The scanf function, which is found in Creads input for numbers and other datatypes from standard input often a command line interface or similar kind of a text user interface. The following shows code in C that reads a variable number of unformatted decimal integers from the standard input stream and prints out each of them on a separate line:.

Otherwise, the function will not perform correctly because it will be attempting to c format specifiers binary options the wrong sections of memory, rather than pointing to the memory location of the variable you are attempting to get input for. C format specifiers binary options fact needs to be kept in mind when assigning scanf output to strings. As scanf c format specifiers binary options designated to read only from standard input, many programming languages with interfacessuch as PHPhave derivatives such as sscanf and fscanf but not scanf itself.

The formatting placeholders in scanf are more or less the same as that in printfits reverse function. There are rarely constants i. The exception is one or more whitespace characters, which discards all whitespace characters in the input. The above can be used in compound with numeric modifiers and the lL modifiers c format specifiers binary options stand for "long" in between the percent symbol and the letter.

There can also be numeric values between the percent symbol and the letters, preceding the long modifiers if any, that specifies the number of characters to be scanned. No argument behind the format string should be included for this dropped variable. The ff modifier in printf is not present in scanf, causing differences between modes of input and output. The ll and c format specifiers binary options modifiers are not present in the C90 standard, but are present in the C99 standard.

The above format string scans the first seven characters as a decimal integer, then reads the remaining as a string until a space, new line or tab is found, then consumes whitespace until the first non-whitespace character is found, then consumes that character, and finally scans the remaining characters as a double.

Therefore a robust program must check whether the scanf call succeeded and take appropriate action. If the input was c format specifiers binary options in the correct format, the erroneous data will still be on the input stream and must discarded before new input can be read.

An alternative method, which avoids this, is to use fgets and then examine the string read in. The last step can be done by sscanffor example. Great care should be taken to ensure that the formatting string includes limitations for string and array sizes.

In most cases the input string size from a user is arbitrary; it can not be determined before the scanf function is executed. Another potential problem is to allow dynamic formatting strings, for example formatting strings stored in configuration files or c format specifiers binary options user controlled files. In this case the allowed input length of string sizes can not be specified unless the formatting string is checked beforehand and limitations are enforced.

Related to this are additional or mismatched formatting placeholders which do not match the actual vararg list. These placeholders might be partially extracted from the stack, contain undesirable or even insecure pointers depending on the particular implementation of varargs.

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Standard numeric format strings are used to format common numeric types. A standard numeric format string takes the form Axx , where:. A is a single alphabetic character called the format specifier. Any numeric format string that contains more than one alphabetic character, including white space, is interpreted as a custom numeric format string.

For more information, see Custom Numeric Format Strings. The precision specifier ranges from 0 to 99 and affects the number of digits in the result. Note that the precision specifier controls the number of digits in the string representation of a number. It does not round the number itself. To perform a rounding operation, use the Math. Floor , or Math. When precision specifier controls the number of fractional digits in the result string, the result strings reflect numbers that are rounded away from zero that is, using MidpointRounding.

The precision specifier determines the number of digits in the result string. To pad a result string with leading or trailing spaces, use the composite formatting feature and define an alignment component in the format item. Some overloads of the ToString method of all numeric types. For example, you can supply a numeric format string to the Int ToString String and Int Format method, and the StringBuilder. The composite format feature allows you to include the string representation of multiple data items in a single string, to specify field width, and to align numbers in a field.

For more information, see Composite Formatting. Interpolated strings in C and Visual Basic, which provide a simplified syntax when compared to composite format strings. You can download the Formatting Utility , an application that enables you to apply format strings to either numeric or date and time values and displays the result string.

The following table describes the standard numeric format specifiers and displays sample output produced by each format specifier. See the Notes section for additional information about using standard numeric format strings, and the Example section for a comprehensive illustration of their use. A standard numeric format string can be used to define the formatting of a numeric value in one of two ways:.

It can be passed to an overload of the ToString method that has a format parameter. The following example formats a numeric value as a currency string in the current culture in this case, the en-US culture.

It can be supplied as the formatString argument in a format item used with such methods as String. WriteLine , and StringBuilder. The following example uses a format item to insert a currency value in a string.

Optionally, you can supply an alignment argument to specify the width of the numeric field and whether its value is right- or left-aligned. The following example left-aligns a currency value in a character field, and it right-aligns a currency value in a character field. It can be supplied as the formatString argument in an interpolated expression item of an interpolated string. For more information, see the String interpolation topic in the C reference or the Interpolated strings topic in the Visual Basic reference.

The following sections provide detailed information about each of the standard numeric format strings. The "C" or currency format specifier converts a number to a string that represents a currency amount.

The precision specifier indicates the desired number of decimal places in the result string. If the precision specifier is omitted, the default precision is defined by the NumberFormatInfo. If the value to be formatted has more than the specified or default number of decimal places, the fractional value is rounded in the result string.

If the value to the right of the number of specified decimal places is 5 or greater, the last digit in the result string is rounded away from zero.

The result string is affected by the formatting information of the current NumberFormatInfo object. The following table lists the NumberFormatInfo properties that control the formatting of the returned string. The following example formats a Double value with the currency format specifier.

The "D" or decimal format specifier converts a number to a string of decimal digits , prefixed by a minus sign if the number is negative. This format is supported only for integral types. The precision specifier indicates the minimum number of digits desired in the resulting string. If required, the number is padded with zeros to its left to produce the number of digits given by the precision specifier. If no precision specifier is specified, the default is the minimum value required to represent the integer without leading zeros.

As the following table shows, a single property affects the formatting of the result string. The following example formats an Int32 value with the decimal format specifier. The exponential "E" format specifier converts a number to a string of the form "-d.

The string starts with a minus sign if the number is negative. Exactly one digit always precedes the decimal point. The precision specifier indicates the desired number of digits after the decimal point. If the precision specifier is omitted, a default of six digits after the decimal point is used. The case of the format specifier indicates whether to prefix the exponent with an "E" or an "e". The exponent always consists of a plus or minus sign and a minimum of three digits.

The exponent is padded with zeros to meet this minimum, if required. The following example formats a Double value with the exponential format specifier. The fixed-point "F" format specifier converts a number to a string of the form "-ddd. The precision specifier indicates the desired number of decimal places. If the precision specifier is omitted, the current NumberFormatInfo. NumberDecimalDigits property supplies the numeric precision. The following table lists the properties of the NumberFormatInfo object that control the formatting of the result string.

The following example formats a Double and an Int32 value with the fixed-point format specifier. The general "G" format specifier converts a number to the more compact of either fixed-point or scientific notation, depending on the type of the number and whether a precision specifier is present.

The precision specifier defines the maximum number of significant digits that can appear in the result string. If the precision specifier is omitted or zero, the type of the number determines the default precision, as indicated in the following table.

Fixed-point notation is used if the exponent that would result from expressing the number in scientific notation is greater than -5 and less than the precision specifier; otherwise, scientific notation is used. The result contains a decimal point if required, and trailing zeros after the decimal point are omitted. If the precision specifier is present and the number of significant digits in the result exceeds the specified precision, the excess trailing digits are removed by rounding.

However, if the number is a Decimal and the precision specifier is omitted, fixed-point notation is always used and trailing zeros are preserved. If scientific notation is used, the exponent in the result is prefixed with "E" if the format specifier is "G", or "e" if the format specifier is "g". The exponent contains a minimum of two digits. This differs from the format for scientific notation that is produced by the exponential format specifier, which includes a minimum of three digits in the exponent.

Note that, when used with a Double value, the "G17" format specifier ensures that the original Double value successfully round-trips. This is because Double is an IEEE compliant double-precision binary64 floating point number that gives up to 17 significant digits of precision.

We recommend its use instead of the "R" format specifier , since in some cases "R" fails to successfully round-trip double-precision floating point values. The following example illustrates one such case. When used with a Single value, the "G9" format specifier ensures that the original Single value successfully round-trips. This is because Single is an IEEE compliant single-precision binary32 floating point number that gives up to nine significant digits of precision. We recommend its use instead of the "R" format specifier , since in some cases "R" fails to successfully round-trip single-precision floating point values.

The following table lists the NumberFormatInfo properties that control the formatting of the result string. The numeric "N" format specifier converts a number to a string of the form "-d,ddd,ddd. If the precision specifier is omitted, the number of decimal places is defined by the current NumberFormatInfo. The percent "P" format specifier multiplies a number by and converts it to a string that represents a percentage.

If the precision specifier is omitted, the default numeric precision supplied by the current PercentDecimalDigits property is used. The round-trip "R" format specifier attempts to ensure that a numeric value that is converted to a string is parsed back into the same numeric value. This format is supported only for the Single , Double , and BigInteger types. For Double and Single values, the "R" format specifier in some cases fails to successfully round-trip the original value and also offers relatively poor performance.

Instead, we recommend that you use the "G17" format specifier for Double values and the "G9" format specifier to successfully round-trip Single values. When a BigInteger value is formatted using this specifier, its string representation contains all the significant digits in the BigInteger value.

Although you can include a precision specifier, it is ignored. Round trips are given precedence over precision when using this specifier. The following example formats a BigInteger value with the round-trip format specifier. See the following paragraph for more information. The following example uses the "R" format string with a Double value that does not round-trip successfully, and also uses the "G17" format string to successfully round-trip the original value.

The hexadecimal "X" format specifier converts a number to a string of hexadecimal digits. The case of the format specifier indicates whether to use uppercase or lowercase characters for hexadecimal digits that are greater than 9. The result string is not affected by the formatting information of the current NumberFormatInfo object.