FormattedIO¶

Overview of Format Strings¶

In a manner similar to C's 'printf' and 'scanf', the IO package includes channel.writef and channel.readf functions. These functions take in a format string and some arguments. The string.format method is also available and is loosely equivalent to C's 'sprintf'. For example, one might do:

writef("My favorite %s is %i\n", "number", 7);

var s:string = "My favorite %s is %i".format("number", 7);
writeln(s);

// prints:
// My favorite number is 7
// My favorite number is 7

The following sections offer a tour through the conversions to illustrate the common cases. A more precise definition follows in the "Format String Syntax in Detail" section below.

In this file, we use "integral" to refer to the Chapel types int or uint and "floating-point" to refer to real, imaginary, or complex, of any bit width.

Formatted I/O for C Programmers¶

This table is designed to help C programmers understand the equivalent Chapel format specifiers.

Unlike in C, a value of the wrong type will be cast appropriately - so for example printing 2 (an int) with %.2dr will result in 2.00. Note that %n and %t are equivalent to %r for real conversions and %i for numeric conversions; so these are also equivalent to %i %d or %g in C. Also note that Chapel format strings includes many capabilities not available with C formatted I/O routines - including quoted strings, binary numbers, complex numbers, and raw binary I/O.

Generic Numeric Conversions¶

%{##.###}

decimal number padded on the left with spaces to 2 digits before the point, rounded to 3 after. Works with integral, real, imaginary, or complex arguments.

In all cases, the output is padded on the left to the total length of the conversion specifier (6 in this example). The output can be longer, when needed to accommodate the number.

%{##}

integral value padded out to 2 digits. Also works with real, imaginary or complex numbers by rounding them to integers. Numbers with more digits will take up more space instead of being truncated.

In both conversions above, an imaginary argument gets an 'i' afterwards and the entire expression is padded out to the width of ##### digits. For example:

writef("|${#####}|\n", 2.0i);
     // outputs:
     //   |   2i|

writef("|%{#####.#}|\n", 2.0i);
     // outputs:
     //   |   2.0i|

Complex arguments are printed in the format a + bi, where each of a and b is rounded individually as if printed under that conversion on its own. Then, the formatted complex number is padded to the requested size. For example:

writef("|%{#########}|\n", 1.0+2.0i);
     // outputs:
     //   |   1 + 2i|

writef("|%{#########.#}|\n", 1.0+2.0i);
     // outputs:
     //   | 1.0 + 2.0i|

See # Specifiers for more details on this conversion type.

%n

a "number" - equivalent to one of %i, %u, %r, %m, or %z below, depending on the type

%17n

a number padded out to 17 columns

%.4n

a number with 4 significant digits or a precision of 4

Integral Conversions¶

%i or %di

a signed integer in decimal, possibly negative (note - when reading an %i, - is allowed)

%u or %du

an unsigned decimal integer (note - when reading a %u, - is not allowed)

%bi

a binary signed integer

%bu

a binary unsigned integer

%@bu

a binary unsigned integer prefixed with 0b

%oi

an octal signed integer

%ou

an octal unsigned integer

%@ou

an octal unsigned integer prefixed with 0o

%xu

a hexadecimal unsigned integer

%xi

a hexadecimal signed integer

%@xu

a hexadecimal unsigned integer prefixed with 0x

%Xu

a hexadecimal unsigned integer in uppercase

%@Xu

a hexadecimal unsigned integer prefixed with 0X and uppercase

%17i

a decimal integer padded on the left with spaces to 17 columns (That is, it is right-justified in a 17-column field. Padding width is ignored when reading integers)

%*i

as with %17i but read the minimum width from the preceding argument

%017i

a decimal integer padded on the left with zeros to 17 columns

%-17i

a decimal integer left-justified (padded on the right) to 17 columns

%+i

a decimal integer showing + for positive numbers

% i

a decimal integer with a space for positive numbers

%|4i

output 4 raw, binary bytes of the passed integer in native endianness

%<4i

output 4 raw, binary bytes of the passed integer little endian

%>4i

output 4 raw, binary bytes of the passed integer big endian

%<8i

output 8 raw, binary bytes of the passed integer little endian (byte widths of 1, 2, 4, and 8 are supported for integral conversions)

Real Conversions¶

%r

a real number with decimal or exponential notation, where exponential is chosen if the decimal version would be too long

%6r

as with %r but padded on the left to 6 columns (ie right-justified)

%-6r

as with %r but padded on the right to 6 columns (ie left-justified)

%.4r

as with %r but with 4 significant digits

%.*r

as with %.4r but with significant digits read from preceding argument

%6.4r

as with %r but padded on the left to 6 columns and with 4 significant digits

%*.*r

as with %6.4r but read minimum width and significant digits from preceding arguments

%dr

a real number in decimal notation, e.g. 12.34

%6dr

a decimal number padded on the left to 6 columns (right-justified)

%.4dr

a decimal number with 4 digits after the radix point

%6.4dr

a decimal number padded on the left to 6 columns and with 4 digits after the radix point (width and precision are ignored when reading numbers in readf)

%er

a real number in exponential notation, e.g. 8.2e-23

%Er

like %er but with the 'e' in uppercase, e.g. 8.2E-23

%.4er

exponential notation with 4 digits after the period, e.g. 8.2000e-23

%xer

hexadecimal number using p to mark exponent e.g. 6c.3f7p-2a

%|4r

emit 4 raw, binary bytes of the passed number in native endianness

%<8r

emit 8 raw, binary bytes of the passed number in little endian

%<4r

emit 4 raw, binary bytes of the passed number in little endian (< | and > are supported for widths 4 or 8)

Complex and Imaginary Conversions¶

%m

an imaginary number, like a real with %r but ends with an i

%z

print complex number with %r for each part in the format a + bi

%@z

print complex number with %r for each part in the format (a,b)

%6z

as with %z but pad the entire complex number out to 6 columns

%6.4z

print a and b 4 significant digits and pad the entire complex number out to 6 columns

%dz

print a and b with %dr

%ez

print a and b with %er

%|4m

same as %|4r

%|8z

emit 8 raw, binary bytes of native-endian complex (a,b are each 4 bytes)

%<16z

emit 16 raw, binary bytes of little-endian complex (a,b each 8 bytes)

String and Bytes Conversions¶

%s

a string. When reading, read until whitespace. Note that if you want to be able to read your string back in, you should use one of the quoted or encoded binary versions (see below), since generally with %s it's not clear where the string ends.

%c

a single Unicode character (argument should be a string or an integral storing the character code)

%17s

• when writing - a string left padded (right justified) to 17 columns
• when reading - read up to 17 bytes or a whitespace, whichever comes first, rounding down to whole characters

%-17s

• when writing - a string right padded (left justified) to 17 columns

%.17s

• when writing - a string truncated to 17 columns. When combined with quoting strings, for example %.17"S, the conversion will print ... after a string if it was truncated. The truncation includes leaving room for the quotes and - if needed - the periods, so the shortest truncated string is ""... Generally, you won't be able to read these back in.
• when reading - read exactly 17 Unicode code points

%|17s

• when writing - emit string but cause runtime error if length does not match
• when reading - read exactly 17 bytes (error if we read < 17 bytes)

%|*s

as with %17s but the length is specified in the argument before the string.

%"S

use double-quotes to delimit string

%'S

use single-quotes to delimit string

%cS

use any character (c) to delimit string

%{(S)}

quoted string, starting with (, ending with ), where the parens could be replaced by arbitrary characters

%*S

quoted string, the arg before the string to specifies quote character

%|0S

write a string null-terminated or read bytes until a null-terminator

%|*S

means read bytes until a terminator byte. The terminator byte is read from the argument before the string.

%|1S %|2S %|4S and %|8S

work with encoded strings storing a length and then the string data. The digit before S is the number of bytes of length which is by default stored native endian. <, |, > can be used to specify the endianness of the length field, for example %<8S is 8 bytes of little-endian length and then string data.

%|vS

as with %|1S-%|8S but the string length is encoded using a variable-length byte scheme (which is always the same no matter what endianness). In this scheme, the high bit of each encoded length byte records whether or not there are more length bytes (and the remaining bits encode the length in a big-endian manner).

%|*vS or %|*0S

read an encoded string but limit it to a number of bytes read from the argument before the string; when writing cause a runtime error if the string is longer than the maximum.

%/a+/

where any regular expression can be used instead of a+ consume one or more 'a's when reading, gives an error when printing, and does not assign to any arguments (note - regular expression support is dependent on RE2 build; see Regexp)

%/(a+)/

consume one or more 'a's and then set the corresponding string argument to the read value

%17/a+/

match a regular expression up to 17 bytes (note that %.17/a+/, which would mean to match 17 characters, is not supported).

%/*/

next argument contains the regular expression to match

General Conversions¶

%t

read or write the object according to its readThis/writeThis routine

%jt

read or write an object in JSON format using readThis/writeThis

%ht

read or write an object in Chapel syntax using readThis/writeThis

%|t

read or write an object in binary native-endian with readThis/writeThis

%<t

read or write an object little-endian in binary with readThis/writeThis

%>t

read or write an object big-endian in binary with readThis/writeThis

Note About Whitespace¶

When reading, \n in a format string matches any zero or more space characters other than newline and then exactly one newline character. In contrast, " " matches at least one space character of any kind.

When writing, whitespace is printed from the format string just like any other literal would be.

Finally, space characters after a binary conversion will be ignored, so that a binary format string can appear more readable.

Format String Syntax in Detail¶

Chapel's format strings are simpler than those in C in one way: it is no longer necessary to specify the types of the arguments in the format string. For example, in C the l in %ld is specifying the type of the argument for integer (decimal) conversion. That is not necessary in Chapel since the compiler is able to use type information from the call.

Format strings in Chapel consist of:

• conversion specifiers e.g. "%xi" (described below)

• newline e.g. "\n"

  • when writing - prints a newline
  • when reading - reads any amount of non-newline whitespace and then exactly one newline. Causes the format string not to match if it did not read a newline.

• other whitespace e.g. " "

  • when writing - prints as the specified whitespace
  • when reading - matches at least one character of whitespace, possibly including newlines.

• other text e.g. "test"

  • when writing - prints the specified text
  • when reading - matches the specified text

# Specifiers¶

All # specifiers must be enclosed in %{} syntax, for example %{#} is the shortest one, and %{#.#} is a more typical one. The integer portion of the number will be padded out to match the number of # s before the decimal point, and the number of # s after the decimal point indicate how many digits to print after the decimal point. In other words, display how many digits to use when printing a floating-point number by using the # symbol to stand for digits. The fractional portion of the number will be rounded appropriately and extra space will be made if the integer portion is too small:

writef("n:%{###.###}\n", 1.2349);
     // outputs:
     // n:  1.235

This syntax also works for numbers without a decimal point by rounding them appropriately.

A # specifier may start with a ..

writef("%{.##}\n", 0.777);
     // outputs:
     //  0.78

% Specifiers¶

Specifiers starting with % offer quite a few options. First, some basic rules.

%%

means a literal %

\n

means a literal newline

\\

means a single backslash

%{}

curly braces can wrap a % or # conversion specifier. That way, even odd specifiers can be interpreted unambiguously. Some of the more complex features require the use of the %{} syntax, but it's always acceptable to use curly braces to make the format string clearer. Curly braces are required for # conversion specifiers.

In general, a % specifier consists of either text or binary conversions:

%
[optional endian flag (binary conversions only)]
[optional flags]
[optional field width or size in bytes]
[optional . then precision]
[optional base flag]
[optional exponential type]
[conversion type]

Going through each section for text conversions:

[optional flags]

@

means "alternate form". It means to print out a base when not using decimal (e.g. 0xFFF or 0b101011); and it will format a complex number with parens instead of as e.g. 1.0+2.0i

+

means to show a plus sign when printing positive numbers

0

means to pad numeric conversions with 0 instead of space

" "

(a space) leaves a blank before a positive number (in order to help line up with negative numbers)

-

left-justify the converted value instead of right-justifying. Note, if both 0 and - are given, the effect is as if only - were given.

~

when reading a record or class instance, skip over fields in the input not present in the Chapel type. This flag currently only works in combination with the JSON format. This flag allows a Chapel program to describe only the relevant fields in a record when the input might contain many more fields.

[optional field width]

When printing numeric or string values, the field width specifies the number of columns that the conversion should use to display the value. It can be *, which means to read the field width from an integral argument before the converted value.

For string conversions in readf (%s %" %' %//), the field width specifies the maximum number of bytes to read.

For numeric conversions in readf, the field width is ignored.

[optional . then precision]

When printing floating point values, the precision is used to control the number of decimal digits to print. For %r conversions, it specifies the number of significant digits to print; for %dr or %er conversions, it specifies the number of digits following the decimal point. It can also be *, which means to read the precision from an integral argument before the converted value.

For textual string conversions in writef, (%s %" %'), the precision indicates the maximum number of columns to print - and the result will be truncated if it does not fit. In readf for these textual string conversions, the precision indicates the maximum number of characters (e.g. Unicode code points) to input.

The precision is silently ignored for integral conversions (%i, %u, etc) and for %// conversions.

[optional base flag]

d

means decimal (and not exponential for floating-point)

x

means lower-case hexadecimal

X

means upper-case hexadecimal

o

means octal

b

means binary

j

means JSON-style strings, numbers, and structures

h

means Chapel-style strings, numbers, and structures

'

means single-quoted string (with \ and ')

"

means double-quoted string (with \ and ")

[optional exponential type]

e

means floating-point conversion printing exponential -12.34e+56

E

means floating-point conversion printing uppercase exponential -12.34E+56

[conversion type]

t

means type-based or thing - uses writeThis/readThis but ignores width and precision

n

means type-based number, allowing width and precision

i

means integral conversion

u

means unsigned integral conversion

r

means real conversion (e.g. 12.23)

m

means imaginary conversion with an i after it (e.g. 12.23i)

z

means complex conversion

s

means string conversion

S

means a quoted string conversion

{cS}

means string conversion with quote char c

{*S}

means string conversion with quote char in argument before the string

{xSy}

means string conversion with left and right quote chars x and y

/.../

means a regular expression (for reading only)

{/.../xyz}

means regular expression with flags xyz

c

means a Unicode character - either the first character in a string or an integral character code

For binary conversions:

[optional endian flag]

<

means little-endian

>

means big-endian

|

means native-endian

[optional size in bytes]

This is the number of bytes the format should read or write in this conversion. For integral conversions (e.g. %|i) it specifies the number of bytes in the integer, and 1, 2, 4, and 8 are supported. For real and imaginary conversions, 4 and 8 are supported. For complex conversions, 8 and 16 are supported. The size in bytes is required for binary integral and floating-point conversions.

The size can be *, which means that the number of bytes is read from the argument before the conversion.

For strings, if a terminator or length field is specified, exactly this number is the maximum size in bytes; if the terminator or length is not specified, the string must be exactly that size (and if the argument is not exactly that number of bytes it will cause an error even when writing).

[conversion type]

t

means type-based or thing - to read or write with readThis/writeThis

n

means type-based number (size is not mandatory)

i

means integral. Note that the size is mandatory for binary integral conversions

u

means unsigned integral. Note that the size is mandatory for binary integral conversions

r

means real. Note that the size is mandatory for binary real conversions

m

works the same as r for binary conversions

z

means complex. Note that the size is mandatory for binary complex conversions

s

• means string binary I/O
• %|17s means exactly 17 byte string

0S/1S/2S/4S/8S

• mean encoded string binary I/O:
• %|0S means null-terminated string
• %{|S*} means next-argument specifies string terminator byte
• %|1S means a one-byte length and then the string
• %|2S means a two-byte length and then the string
• %|4S means a four-byte length and then the string
• %|8S means an eight-byte length and then the string
• %|vS means a variable-byte-encoded length and then the string

c

means a Unicode character - either the first character in a string or an integral character code

Formatted I/O Examples¶

writef("%5i %5s %5r\n", 1, "test", 6.34);
     // outputs:
     //    1  test  6.34

writef("%2.4z\n", 43.291 + 279.112i);
     // outputs:
     // 43.29 + 279.1i

writef("%<4u", 0x11223344);
     // outputs:
     // (hexdump of the output)
     // 4433 2211
writef("%>4u", 0x11223344);
     // outputs:
     // (hexdump of the output)
     // 1122 3344
writef("%<4i %<4i", 2, 32);
     // outputs:
     // (hexdump of the output -- note that spaces after
     //  a binary format specifier are ignored)
     // 0200 0000 2000 0000


writef("%|0S\n", "test");
     // outputs:
     // (hexdump of the output)
     // 7465 7374 000a
writef("%|1S\n", "test");
     // outputs:
     // (hexdump of the output)
     // 0474 6573 740a
writef("%>2S\n", "test");
     // outputs:
     // (hexdump of the output)
     // 0004 7465 7374 0a
writef("%>4S\n", "test");
     // outputs:
     // (hexdump of the output)
     // 0000 0004 7465 7374 0a
writef("%>8S\n", "test");
     // outputs:
     // (hexdump of the output)
     // 0000 0000 0000 0004 7465 7374 0a
writef("%|vS\n", "test");
     // outputs:
     // (hexdump of the output)
     // 04 7465 7374 0a

writef('%"S\n', "test \"\" \'\' !");
     // outputs:
     // "test \"\" '' !"
writef("%'S\n", "test \"\" \'\' !");
     // outputs:
     // 'test "" \'\' !'
writef("%{(S)}\n", "test ()", "(", ")");
     // outputs:
     // (test (\))


writef("%40s|\n", "test");
writef("%-40s|\n", "test");
     // outputs:
     //                                     test|
     // test                                    |

writef("123456\n");
writef("%6.6'S\n", "a");
writef("%6.6'S\n", "abcdefg");
writef("%.3'S\n", "a");
writef("%.3'S\n", "abcd");
     // outputs:
     // 123456
     //    'a'
     // 'a'...
     // 'a'
     // ''...


var s:string;
var got = readf(" %c", s);
// if the input is " a", "\na", "  a", etc, s will contain "a"
// if the input is "b", got will be false and s will contain ""

var s:string;
var got = readf("\n%c", s);
// if the input is "\na", or " \na", s will contain "a"
// if the input is "b", got will be false and s will be ""

var got = readf("%/a+/");
// if the input is "a" or "aa" (and so on), got will return true
// if the input is "c" got will be false

var s:string;
var got = readf("%/a(b+)/", s);
// if the input is "c" got will be false and s will be ""
// if the input is "ab", got will be true and s will be "b"
// if the input is "abb", got will be true and s will be "bb"