# mruby-sprintf This mrbgem provides `sprintf` functionality for mruby. It allows you to format strings according to a specified format string, similar to the `sprintf` function in C or other languages. ## Functionality - Provides the `sprintf` kernel function. - Adds the `%` operator to the `String` class as an alias for `sprintf`. ## Usage You can use `sprintf` or the `String#%` operator for string formatting: ```ruby # Using sprintf str = sprintf("Hello, %s! You are %d years old.", "World", 30) puts str # Output: Hello, World! You are 30 years old. # Using String#% str2 = "The value is %.2f" % 12.3456 puts str2 # Output: The value is 12.35 # Using String#% with an array of arguments str3 = "%d + %d = %d" % [1, 2, 1+2] puts str3 # Output: 1 + 2 = 3 ``` ## Format Specifiers This mrbgem supports a wide range of format specifiers, flags, width, and precision options. For a comprehensive list and detailed explanation of all supported format specifiers and their behavior, please refer to the extensive comments within the C source file: [`src/sprintf.c`](src/sprintf.c). ## Format Specifiers Details The `sprintf` function in this mrbgem supports a variety of field type characters that control how arguments are interpreted and formatted. ### Integer Formats | Field | Description | | :---- | :---------------------------------------------------------------------------------------------------------------------- | | `b` | Convert argument as a binary number. Negative numbers will be displayed as a two's complement prefixed with `..1`. | | `B` | Equivalent to `b`, but uses an uppercase `0B` for prefix in the alternative format (when `#` flag is used). | | `d` | Convert argument as a decimal number. | | `i` | Identical to `d`. | | `o` | Convert argument as an octal number. Negative numbers will be displayed as a two's complement prefixed with `..7`. | | `u` | Identical to `d`. (In many `sprintf` implementations, `u` is for unsigned decimal, but here it behaves like `d`). | | `x` | Convert argument as a hexadecimal number. Negative numbers will be displayed as a two's complement prefixed with `..f`. | | `X` | Equivalent to `x`, but uses uppercase letters for hexadecimal digits (e.g., `0XFF` vs `0xff`). | ### Float Formats | Field | Description | | :---- | :------------------------------------------------------------------------------------------------------------------------------------------------------ | | `e` | Convert floating-point argument into exponential notation (e.g., `[-]d.dddddde[+-]dd`). Precision specifies digits after the decimal point (default 6). | | `E` | Equivalent to `e`, but uses an uppercase `E` for the exponent. | | `f` | Convert floating-point argument as `[-]ddd.dddddd`. Precision specifies digits after the decimal point. | | `g` | Converts a floating-point number using `e` or `f` form based on exponent and precision. Precision specifies significant digits. | | `G` | Equivalent to `g`, but uses an uppercase `E` in exponent form if applicable. | ### Other Formats | Field | Description | | :---- | :-------------------------------------------------------------------------------------------------------------- | | `c` | Argument is treated as the numeric code for a single character or a single character string itself. | | `p` | Argument's `inspect` method is called, and the result is substituted. | | `s` | Argument is a string to be substituted. If precision is specified, at most that many characters will be copied. | | `%` | A literal percent sign (`%`) itself will be displayed. No argument is taken. | ### Flags Flags modify the behavior of the format specifiers: | Flag | Applies to | Meaning | | :--------- | :---------------------------------- | :-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | `space` | Numeric Formats (bBdiouxX, aAeEfgG) | Leave a space at the start of non-negative numbers. For `o`, `x`, `X`, `b`, `B`, uses a minus sign with absolute value for negative values. | | `(digit)$` | All | Specifies the absolute argument number for this field (e.g., `%2$d` uses the second argument). Cannot be mixed with relative argument numbers. | | `#` | bBoxX, aAeEfgG | Use an alternative format. For `o`, increases precision to make the first digit `0` if not formatted as complement. For `x`, `X`, `b`, `B` (non-zero), prefixes with `0x`, `0X`, `0b`, `0B`. For `e`, `E`, `f`, `g`, `G`, forces a decimal point. For `g`, `G`, do not remove trailing zeros. | | `+` | Numeric Formats | Add a leading plus sign (`+`) to non-negative numbers. For `o`, `x`, `X`, `b`, `B`, uses a minus sign with absolute value for negative values. | | `-` | All | Left-justify the result of this conversion. | | `0` (zero) | Numeric Formats | Pad with zeros, not spaces. For `o`, `x`, `X`, `b`, `B`, radix-1 is used for negative numbers formatted as complements. | | `*` | All | Use the next argument as the field width. If negative, left-justifies. If `*` is followed by a number and `$`, use that argument as width (e.g., `%*1$d`). | ## Further Examples (from C source comments) Here are more examples illustrating the use of flags, width, and precision, adapted from the comments in `src/sprintf.c`. ### Examples of Flags ```ruby # '+' and space flag specifies the sign of non-negative numbers. puts sprintf("%d", 123) #=> "123" puts sprintf("%+d", 123) #=> "+123" puts sprintf("% d", 123) #=> " 123" # '#' flag for 'o' increases number of digits to show '0'. # '+' and space flag changes format of negative numbers. puts sprintf("%o", 123) #=> "173" puts sprintf("%#o", 123) #=> "0173" puts sprintf("%+o", -123) #=> "-173" puts sprintf("%o", -123) #=> "..7605" puts sprintf("%#o", -123) #=> "..7605" # '#' flag for 'x' add a prefix '0x' for non-zero numbers. # '+' and space flag disables complements for negative numbers. puts sprintf("%x", 123) #=> "7b" puts sprintf("%#x", 123) #=> "0x7b" puts sprintf("%+x", -123) #=> "-7b" puts sprintf("%x", -123) #=> "..f85" puts sprintf("%#x", -123) #=> "0x..f85" # Note: mruby's output might differ slightly for complements with # puts sprintf("%#x", 0) #=> "0" # '#' for 'X' uses the prefix '0X'. puts sprintf("%X", 123) #=> "7B" puts sprintf("%#X", 123) #=> "0X7B" # '#' flag for 'b' add a prefix '0b' for non-zero numbers. # '+' and space flag disables complements for negative numbers. puts sprintf("%b", 123) #=> "1111011" puts sprintf("%#b", 123) #=> "0b1111011" puts sprintf("%+b", -123) #=> "-1111011" puts sprintf("%b", -123) #=> "..10000101" puts sprintf("%#b", -123) #=> "0b..10000101" # Note: mruby's output might differ slightly for complements with # puts sprintf("%#b", 0) #=> "0" # '#' for 'B' uses the prefix '0B'. puts sprintf("%B", 123) #=> "1111011" puts sprintf("%#B", 123) #=> "0B1111011" # '#' for 'e' forces to show the decimal point. puts sprintf("%.0e", 1) #=> "1e+00" puts sprintf("%#.0e", 1) #=> "1.e+00" # '#' for 'f' forces to show the decimal point. puts sprintf("%.0f", 1234) #=> "1234" puts sprintf("%#.0f", 1234) #=> "1234." # '#' for 'g' forces to show the decimal point. # It also disables stripping lowest zeros. puts sprintf("%g", 123.4) #=> "123.4" puts sprintf("%#g", 123.4) #=> "123.400" puts sprintf("%g", 123456) #=> "123456" puts sprintf("%#g", 123456) #=> "123456." ``` ### Examples of Width The width specifies the minimum number of characters that will be written. Padding is typically done with spaces, or with zeros if the `0` flag is used. ```ruby # padding is done by spaces, width=20 # 0 or radix-1. <------------------> puts sprintf("%20d", 123) #=> " 123" puts sprintf("%+20d", 123) #=> " +123" puts sprintf("%020d", 123) #=> "00000000000000000123" puts sprintf("%+020d", 123) #=> "+0000000000000000123" puts sprintf("% 020d", 123) #=> " 0000000000000000123" puts sprintf("%-20d", 123) #=> "123 " puts sprintf("%-+20d", 123) #=> "+123 " puts sprintf("%- 20d", 123) #=> " 123 " puts sprintf("%020x", -123) #=> "..ffffffffffffffff85" # Output for negative hex with 0-padding can vary ``` ### Examples of Precision For numeric fields, precision controls the number of decimal places or minimum number of digits. For strings, it determines the maximum number of characters. ```ruby # precision for 'd', 'o', 'x' and 'b' is minimum number of digits # <------> puts sprintf("%20.8d", 123) #=> " 00000123" puts sprintf("%20.8o", 123) #=> " 00000173" puts sprintf("%20.8x", 123) #=> " 0000007b" puts sprintf("%20.8b", 123) #=> " 01111011" puts sprintf("%20.8d", -123) #=> " -00000123" puts sprintf("%20.8o", -123) #=> " ..777605" # Complemented output puts sprintf("%20.8x", -123) #=> " ..ffff85" # Complemented output puts sprintf("%20.8b", -11) #=> " ..110101" # Complemented output # "0x" and "0b" for '#x' and '#b' is not counted for precision, # but "0" for '#o' is counted. <------> puts sprintf("%#20.8d", 123) #=> " 00000123" puts sprintf("%#20.8o", 123) #=> " 00000173" # '#' with 'o' and precision puts sprintf("%#20.8x", 123) #=> " 0x0000007b" puts sprintf("%#20.8b", 123) #=> " 0b01111011" puts sprintf("%#20.8d", -123) #=> " -00000123" puts sprintf("%#20.8o", -123) #=> " ..777605" # Complemented output puts sprintf("%#20.8x", -123) #=> " 0x..ffff85" # Complemented output with # puts sprintf("%#20.8b", -11) #=> " 0b..110101" # Complemented output with # # precision for 'e' is number of digits after the decimal point # <------> puts sprintf("%20.8e", 1234.56789) #=> " 1.23456789e+03" # precision for 'f' is number of digits after the decimal point # <------> puts sprintf("%20.8f", 1234.56789) #=> " 1234.56789000" # precision for 'g' is number of significant digits # <-------> puts sprintf("%20.8g", 1234.56789) #=> " 1234.5679" # <-------> puts sprintf("%20.8g", 123456789) #=> " 1.2345679e+08" # precision for 's' is maximum number of characters # <------> puts sprintf("%20.8s", "string test") #=> " string t" ``` ### General Examples ```ruby puts sprintf("%d %04x", 123, 123) #=> "123 007b" puts sprintf("%08b '%4s'", 123, 123) #=> "01111011 ' 123'" # Note: mruby output for %s with number might differ puts sprintf("%1$*2$s %2$d %1$s", "hello", 8) #=> " hello 8 hello" puts sprintf("%1$*2$s %2$d", "hello", -8) #=> "hello -8" puts sprintf("%+g:% g:%-g", 1.23, 1.23, 1.23) #=> "+1.23: 1.23:1.23" puts sprintf("%u", -123) #=> "-123" ``` ### Named References For more complex formatting, Ruby supports a reference by name. `%s` style uses format style, but `%{name}` style doesn't (in standard Ruby, mruby might vary). ```ruby puts sprintf("%d : %f", { :foo => 1, :bar => 2.0 }) # Using a float for %f #=> "1 : 2.000000" # The %{name} style might behave differently or not be supported in the same way as CRuby. # Standard CRuby example: # puts sprintf("%{foo}f", { :foo => 1 }) # => "1f" (This specific syntax might not apply or work in mruby) # For mruby, stick to %type for named arguments: puts sprintf("%s", { :foo => "test" }) #=> "test" ```