method

fetch

ruby latest stable - Class: Hash

fetch(*args)

public

Returns a value from the hash for the given key. If the key can’t be found, there are several options: With no other arguments, it will raise a KeyError exception; if default is given, then that will be returned; if the optional code block is specified, then that will be run and its result returned.

h = { "a" => 100, "b" => 200 }
h.fetch("a")                            #=> 100
h.fetch("z", "go fish")                 #=> "go fish"
h.fetch("z") { |el| "go fish, #{el}"}   #=> "go fish, z"

The following example shows that an exception is raised if the key is not found and a default value is not supplied.

h = { "a" => 100, "b" => 200 }
h.fetch("z")

produces:

prog.rb:2:in `fetch': key not found (KeyError)
 from prog.rb:2

static VALUE
rb_hash_fetch_m(int argc, VALUE *argv, VALUE hash)
{
    VALUE key;
    st_data_t val;
    long block_given;

    rb_check_arity(argc, 1, 2);
    key = argv[0];

    block_given = rb_block_given_p();
    if (block_given && argc == 2) {
        rb_warn("block supersedes default value argument");
    }
    if (!RHASH(hash)->ntbl || !st_lookup(RHASH(hash)->ntbl, key, &val)) {
        if (block_given) return rb_yield(key);
        if (argc == 1) {
            VALUE desc = rb_protect(rb_inspect, key, 0);
            if (NIL_P(desc)) {
                desc = rb_any_to_s(key);
            }
            desc = rb_str_ellipsize(desc, 65);
            rb_key_err_raise(rb_sprintf("key not found: %"PRIsVALUE, desc), hash, key);
        }
        return argv[1];
    }
    return (VALUE)val;
}

1Note

Fetch with default value

suzuki · Feb 4, 2016

You can specify default value as a fail-back if a hash doesn't have a given key.

{a: false}.fetch(:b, true) => true {a: false}.fetch(:a, true) => false

It is useful especially in the case where you want to set default value for a method.

def initialize(args) @foo = args.fetch(:foo, 1) @bar = args.fetch(:bar, 2)

This can take over the following way.

`@bar = args[:bar] || 2`

=> @bar is overwritten if caller specifies nil for :bar!

end