Apr 18, 2023

Assorted Ruby Details

The command line

irb starts an interactive Ruby interpreter ruby starts Ruby, with input from the command line

End with an end-of-file character (^D or F6) Not actually very useful

ruby filename.rb executes the given file ruby -e quotedString executes the quoted string

Example: ruby -e 'puts "The time is #{Time.now}" '

ruby -v tells you Ruby’s version number On Unix, if the first line of a Ruby file is

#!/usr/bin/ruby (or wherever ruby is located), the file can be executed by just entering the file name

Adding and removing methods

def adds a method; undef removes a method The only parameter to undef is the method name

To add an instance method, first “open” the class Example: class String; def nchars; length; end; end

There are several ways to add a class method to a class def Person.species; 'human'; end Within the Person class, you can use def self.species

You can add a method to individual objects def oscar.mood; 'grouchy' ; end

Numbers

Numbers may be written in decimal, hexadecimal, octal, or binary Decimal: 3405691582 Hex: 0xCAFEBABE or 0XCAFEBABE Octal: 031277535276 or 0o31277535276 Binary: 0b11001010111111101011101010111110

or 0Betc. For readability, numbers may contain (but not begin or

end with) underscores Examples: 3_405_691_582, 0b_111_101_101

Integers may be indexed to retrieve their bits Example: 5.step(0, -1) { |i| print 6[i] } 000110

printf and friends

printf format_string, value, …, value Formats are % length code for most things,

% length.fractional_digits code for floats %d decimal, %o octal, %x hex, %b binary,

%f float, %s string Negative lengths mean left justified Various other controls

Example: printf "pi = %8.4f", 3.141592 pi.=…3.1416

The (equivalent) methods sprintf and format take the same parameters as printf, but return the resultant string rather than printing it

Some File < IO methods gets – get a line of text getc – get a character of text (as ASCII; use .chr) ungetc – put back a character pos – the current character position in the input stream lineno – the number of times gets has been called pos= – move to the given position in the file rewind – move to the beginning of the file readlines – read the stream as an array of strings write(string), print(string), <<(string) – write at the current

position eof? – test if at the end of file closed? – test if the file has been closed

Some File methods rename(oldname, newname) – rename a file read(filename) – read the entire file as a single string readlines(filename) – read the entire file as an array of strings open(filename) –

with no block, a synonym for File.new with a block, the file is passed to the block, and automatically closed when

the block finishes exists?(filename) – test if a file with that name exists writable?(filename) – test if the file can be written directory?(filename) – test if the file is a directory zero?(filename) – test if the file is empty size(filename) – returns the size of the file mtime(filename) – returns the modification time of the file

Streams

The following constants refer to standard I/O streams: STDIN, STDOUT, STDERR

The following variables are initially set to the corresponding constants: $stdin, $stdout, $stderr

In addition, $defout (initially equal to $stdout) is where output with no specified destination is sent

Some String methods

ljust(length), center(length), rjust(length) – left justify, center, or right justify the string by padding with spaces

downcase, upcase, swap, capitalize – modify capitalization

include?(s_or_c) – tests whether the string includes the given string or character

index(s_or_c [, offset]) – returns the index after offset(or nil) at which the gives string starts

rindex(s_or_c [, limit]) – returns the last index (before limit), or nil, at which the string starts

Some more String methods

strip – remove leading and trailing spaces chop – remove the last character (also chop! is

destructive) chomp – remove the last character if it is a newline

(also chomp!) tr(chars, replacement) – replace the characters in

chars with the corresponding characters in replacement; accepts ch1-ch2 notation

Some Array methods

min, max – return the smallest or largest element uniq – return an array with no duplicate elements compact – return an array with no nil elements sort – return a sorted array & – perform an intersection (only elements in both) | – perform a union (elements in either) grep(regexp) – return elements matching the pattern push(element) – add the element to the end of the array pop – remove and return the last element shift – remove and return the first element

Chaining

Nondestructive methods can usually be chained Example: x = gets.chomp.strip.downcase

Many destructive methods return nil if they make no changes in the receiver, hence cannot be chained Example: x = gets.chomp!.strip!.downcase! will

result in a runtime error

Iterators

In Ruby, loops are considered low-level, to be used only when there is no appropriate iterator

collection.each – step through every element n.times – do a block n times n.downto(limit) – step from n down to and including

limit n.upto(limit) – step from n up to and including limit string.each_line – get each line from a string string.each_char – get each character (as an integer)

from a string

More iterators

collection.each_index – iterate over the indices of a collection collection.each_with_index – iterate over the values in a

collection, along with their indices Example: lineup.each_with_index { |man, pos| print pos, man }

hash.each_key – iterate over keys hash.each_value – iterate over values hash.each_pair – iterate over key-value pairs collection.select { |v| condition } – choose only items that meet

the condition collection.map { |v| transformation } – create a new collection

with the transformation applied to each item

The End