# struct Char::Reader

- Char::Reader
- Struct
- Value
- Object

##
Overview

A `Char::Reader` allows iterating a `String` by Chars.

As soon as you instantiate a `Char::Reader` it will decode the first char in the `String`, which can be accessed by invoking `#current_char`. At this point `#pos`, the current position in the string, will equal zero. Successive calls to `#next_char` return the next chars in the string, advancing `#pos`.

NOTE The null character `'\0'` will be returned in `#current_char` when the end is reached (as well as when the string is empty). Thus, `#has_next?` will return `false` only when `#pos` is equal to the string's bytesize, in which case `#current_char` will always be `'\0'`.

NOTE For performance reasons, `Char::Reader` has value semantics, so care must be taken when a reader is declared as a local variable and passed to another method:

    def lstrip(reader)
      until reader.current_char.whitespace?
        reader.next_char
      end
      reader
    end

    # caller's internal state is untouched
    reader = Char::Reader.new("   abc")
    lstrip(reader)
    reader.current_char # => ' '

    # to modify caller's internal state, the method must return a new reader
    reader = lstrip(reader)
    reader.current_char # => 'a'

##
Included Modules

- Enumerable(Char)

##
Defined in:

char/reader.cr

##
Constructors

- .new(string : String, pos = 0)

  Creates a reader with the specified string positioned at byte index pos.

- .new(*, at_end string : String)

  Creates a reader that will be positioned at the last char of the given string.

##
Instance Method Summary

- #current_char : Char

  Returns the current character, or `'\0'` if the reader is at the end of the string.

- #current_char? : Char | Nil

  Returns the current character.

- #current_char_width : Int32

  Returns the size of the `#current_char` (in bytes) as if it were encoded in UTF-8.

- #each(&) : Nil

  Yields successive characters from `#string` starting from `#pos`.

- #error : UInt8 | Nil

  If there was an error decoding the current char because of an invalid UTF-8 byte sequence, returns the byte that produced the invalid encoding.

- #has_next? : Bool

  Returns `true` if the reader is not at the end of the string.

- #has_previous? : Bool

  Returns `true` if the reader is not at the beginning of the string.

- #next_char : Char

  Reads the next character in the string.

- #next_char? : Char | Nil

  Tries to read the next character in the string.

- #peek_next_char : Char

  Returns the next character in the `#string` without incrementing `#pos`.

- #pos : Int32

  Returns the byte position of the current character.

- #pos=(pos : Int32)

  Sets `#pos` to pos.

- #previous_char : Char

  Reads the previous character in the string.

- #previous_char? : Char | Nil

  Tries to read the previous character in the string.

- #string : String

  Returns the reader's String.

### Instance methods inherited from module `Enumerable(Char)`

accumulate(initial : U) : Array(U) forall U
accumulate : Array(T)
accumulate(initial : U, &block : U, T -> U) : Array(U) forall U
accumulate(&block : T, T -> T) : Array(T) accumulate, all?(& : T -> ) : Bool
all?(pattern) : Bool
all? : Bool all?, any?(& : T -> ) : Bool
any?(pattern) : Bool
any? : Bool any?, chunks(&block : T -> U) forall U chunks, compact_map(& : T -> _) compact_map, count(& : T -> ) : Int32
count(item) : Int32 count, cycle(n, & : T -> ) : Nil
cycle(& : T -> ) : Nil cycle, each(& : T -> ) each, each_cons(count : Int, reuse = false, &) each_cons, each_cons_pair(& : T, T -> ) : Nil each_cons_pair, each_slice(count : Int, reuse = false, &) each_slice, each_step(n : Int, *, offset : Int = 0, & : T -> ) : Nil each_step, each_with_index(offset = 0, &) each_with_index, each_with_object(obj : U, & : T, U -> ) : U forall U each_with_object, empty? : Bool empty?, find(if_none = nil, & : T -> ) find, find!(& : T -> ) : T find!, first(&)
first(count : Int) : Array(T)
first : T first, first? : T | Nil first?, flat_map(& : T -> _) flat_map, group_by(& : T -> U) forall U group_by, in_groups_of(size : Int, filled_up_with : U = nil) forall U
in_groups_of(size : Int, filled_up_with : U = nil, reuse = false, &) forall U in_groups_of, in_slices_of(size : Int) : Array(Array(T)) in_slices_of, includes?(obj) : Bool includes?, index(& : T -> ) : Int32 | Nil
index(obj) : Int32 | Nil index, index!(& : T -> ) : Int32
index!(obj) : Int32 index!, index_by(& : T -> U) : Hash(U, T) forall U index_by, join(io : IO, separator = "") : Nil
join(separator, io : IO) : Nil
join(separator = "") : String
join(io : IO, separator = "", & : T, IO -> )
join(separator, io : IO, &)
join(separator = "", & : T -> ) join, map(& : T -> U) : Array(U) forall U map, map_with_index(offset = 0, & : T, Int32 -> U) : Array(U) forall U map_with_index, max(count : Int) : Array(T)
max : T max, max? : T | Nil max?, max_by(& : T -> U) : T forall U max_by, max_by?(& : T -> U) : T | Nil forall U max_by?, max_of(& : T -> U) : U forall U max_of, max_of?(& : T -> U) : U | Nil forall U max_of?, min(count : Int) : Array(T)
min : T min, min? : T | Nil min?, min_by(& : T -> U) : T forall U min_by, min_by?(& : T -> U) : T | Nil forall U min_by?, min_of(& : T -> U) : U forall U min_of, min_of?(& : T -> U) : U | Nil forall U min_of?, minmax : Tuple(T, T) minmax, minmax? : Tuple(T | Nil, T | Nil) minmax?, minmax_by(& : T -> U) : Tuple(T, T) forall U minmax_by, minmax_by?(& : T -> U) : Tuple(T, T) | Tuple(Nil, Nil) forall U minmax_by?, minmax_of(& : T -> U) : Tuple(U, U) forall U minmax_of, minmax_of?(& : T -> U) : Tuple(U, U) | Tuple(Nil, Nil) forall U minmax_of?, none?(& : T -> ) : Bool
none?(pattern) : Bool
none? : Bool none?, one?(& : T -> ) : Bool
one?(pattern) : Bool
one? : Bool one?, partition(& : T -> ) : Tuple(Array(T), Array(T))
partition(type : U.class) forall U partition, present? : Bool present?, product(initial : Number)
product
product(initial : Number, & : T -> )
product(& : T -> _) product, reduce(memo, &)
reduce(&) reduce, reduce?(&) reduce?, reject(& : T -> )
reject(type : U.class) forall U
reject(pattern) : Array(T) reject, sample(n : Int, random : Random = Random::DEFAULT) : Array(T)
sample(random : Random = Random::DEFAULT) : T sample, select(& : T -> )
select(type : U.class) : Array(U) forall U
select(pattern) : Array(T) select, size : Int32 size, skip(count : Int) skip, skip_while(& : T -> ) : Array(T) skip_while, sum(initial)
sum
sum(initial, & : T -> )
sum(& : T -> ) sum, take_while(& : T -> ) : Array(T) take_while, tally(hash)
tally : Hash(T, Int32) tally, tally_by(hash, &)
tally_by(&block : T -> U) : Hash(U, Int32) forall U tally_by, to_a : Array(T)
to_a(& : T -> U) : Array(U) forall U to_a, to_h
to_h(& : T -> Tuple(K, V)) forall K, V to_h, to_set : Set(T)
to_set(&block : T -> U) : Set(U) forall U to_set, zip(*others : Indexable | Iterable | Iterator, &)
zip(*others : Indexable | Iterable | Iterator) zip, zip?(*others : Indexable | Iterable | Iterator, &)
zip?(*others : Indexable | Iterable | Iterator) zip?

### Class methods inherited from module `Enumerable(Char)`

element_type(x) element_type

### Instance methods inherited from struct `Struct`

==(other) : Bool ==, hash(hasher) hash, inspect(io : IO) : Nil inspect, pretty_print(pp) : Nil pretty_print, to_s(io : IO) : Nil to_s

### Instance methods inherited from struct `Value`

==(other : JSON::Any)
==(other : YAML::Any)
==(other) ==, dup dup

### Instance methods inherited from class `Object`

! : Bool !, !=(other) !=, !~(other) !~, ==(other) ==, ===(other : JSON::Any)
===(other : YAML::Any)
===(other) ===, =~(other) =~, as(type : Class) as, as?(type : Class) as?, class class, dup dup, hash(hasher)
hash hash, in?(collection : Object) : Bool
in?(*values : Object) : Bool in?, inspect(io : IO) : Nil
inspect : String inspect, is_a?(type : Class) : Bool is_a?, itself itself, nil? : Bool nil?, not_nil!(message)
not_nil! not_nil!, pretty_inspect(width = 79, newline = "\n", indent = 0) : String pretty_inspect, pretty_print(pp : PrettyPrint) : Nil pretty_print, responds_to?(name : Symbol) : Bool responds_to?, tap(&) tap, to_json(io : IO) : Nil
to_json : String to_json, to_pretty_json(indent : String = " ") : String
to_pretty_json(io : IO, indent : String = " ") : Nil to_pretty_json, to_s(io : IO) : Nil
to_s : String to_s, to_yaml(io : IO) : Nil
to_yaml : String to_yaml, try(&) try, unsafe_as(type : T.class) forall T unsafe_as

### Class methods inherited from class `Object`

from_json(string_or_io, root : String)
from_json(string_or_io) from_json, from_yaml(string_or_io : String | IO) from_yaml

### Macros inherited from class `Object`

class_getter(*names, &block) class_getter, class_getter!(*names) class_getter!, class_getter?(*names, &block) class_getter?, class_property(*names, &block) class_property, class_property!(*names) class_property!, class_property?(*names, &block) class_property?, class_setter(*names) class_setter, def_clone def_clone, def_equals(*fields) def_equals, def_equals_and_hash(*fields) def_equals_and_hash, def_hash(*fields) def_hash, delegate(*methods, to object) delegate, forward_missing_to(delegate) forward_missing_to, getter(*names, &block) getter, getter!(*names) getter!, getter?(*names, &block) getter?, property(*names, &block) property, property!(*names) property!, property?(*names, &block) property?, setter(*names) setter

##
Constructor Detail

def self.new(string : String, pos = 0) #

Creates a reader with the specified string positioned at byte index pos.

def self.new(*, at_end string : String) #

Creates a reader that will be positioned at the last char of the given string.

##
Instance Method Detail

def current_char : Char #

Returns the current character, or `'\0'` if the reader is at the end of the string.

    reader = Char::Reader.new("ab")
    reader.current_char # => 'a'
    reader.next_char
    reader.current_char # => 'b'
    reader.next_char
    reader.current_char # => '\0'

def current_char? : Char | Nil #

Returns the current character.

Returns `nil` if the reader is at the end of the string.

def current_char_width : Int32 #

Returns the size of the `#current_char` (in bytes) as if it were encoded in UTF-8.

    reader = Char::Reader.new("aé")
    reader.current_char_width # => 1
    reader.next_char
    reader.current_char_width # => 2

def each(&) : Nil #

Yields successive characters from `#string` starting from `#pos`.

    reader = Char::Reader.new("abc")
    reader.next_char
    reader.each do |c|
      puts c.upcase
    end

    B
    C

def error : UInt8 | Nil #

If there was an error decoding the current char because of an invalid UTF-8 byte sequence, returns the byte that produced the invalid encoding. Returns `0` if the char would've been out of bounds. Otherwise returns `nil`.

def has_next? : Bool #

Returns `true` if the reader is not at the end of the string.

NOTE This only means `#next_char` will successfully increment `#pos`; if the reader is already at the last character, `#next_char` will return the terminating null byte because there isn't really a next character.

    reader = Char::Reader.new("ab")
    reader.has_next? # => true
    reader.next_char # => 'b'
    reader.has_next? # => true
    reader.next_char # => '\0'
    reader.has_next? # => false

def has_previous? : Bool #

Returns `true` if the reader is not at the beginning of the string.

def next_char : Char #

Reads the next character in the string.

If the reader is at the end of the string after incrementing `#pos`, returns `'\0'`. If the reader is already at the end beforehand, raises `IndexError`.

    reader = Char::Reader.new("abc")
    reader.next_char # => 'b'
    reader.next_char # => 'c'
    reader.next_char # => '\0'
    reader.next_char # raise IndexError

def next_char? : Char | Nil #

Tries to read the next character in the string.

If the reader is at the end of the string before or after incrementing `#pos`, returns `nil`.

    reader = Char::Reader.new("abc")
    reader.next_char?   # => 'b'
    reader.next_char?   # => 'c'
    reader.next_char?   # => nil
    reader.current_char # => '\0'

def peek_next_char : Char #

Returns the next character in the `#string` without incrementing `#pos`.

Returns `'\0'` if the reader is at the last character of the string. Raises `IndexError` if the reader is at the end.

    reader = Char::Reader.new("ab")
    reader.peek_next_char # => 'b'
    reader.current_char   # => 'a'

def pos : Int32 #

Returns the byte position of the current character.

    reader = Char::Reader.new("ab")
    reader.pos # => 0
    reader.next_char
    reader.pos # => 1

def pos=(pos : Int32) #

Sets `#pos` to pos.

    reader = Char::Reader.new("abc")
    reader.next_char
    reader.next_char
    reader.pos = 0
    reader.current_char # => 'a'

def previous_char : Char #

Reads the previous character in the string.

Raises `IndexError` if the reader is already at the beginning of the string. Otherwise decrements `#pos`.

    reader = Char::Reader.new(at_end: "abc")
    reader.previous_char # => 'b'
    reader.previous_char # => 'a'
    reader.previous_char # raises IndexError

def previous_char? : Char | Nil #

Tries to read the previous character in the string.

Returns `nil` if the reader is already at the beginning of the string. Otherwise decrements `#pos`.

    reader = Char::Reader.new(at_end: "abc")
    reader.previous_char? # => 'b'
    reader.previous_char? # => 'a'
    reader.previous_char? # => nil

def string : String #

Returns the reader's String.