How to Add Key-Value to Hash in Ruby: Simple Guide

In Ruby, you add a key-value pair to a hash by using the syntax hash[key] = value. This assigns the value to the specified key in the hash, creating the pair if it doesn't exist or updating it if it does.

Syntax

To add or update a key-value pair in a Ruby hash, use the syntax:

• hash[key] = value: Assigns the value to the key in the hash.
• hash: The hash variable you want to modify.
• key: The key where you want to store the value (can be a symbol, string, or other object).
• value: The data you want to associate with the key.

ruby

hash = {}
hash[:name] = "Alice"

Example

This example shows how to create a hash and add a new key-value pair to it. It also updates an existing key's value.

ruby

person = { age: 30 }
person[:name] = "Alice"
puts person
person[:age] = 31
puts person

Output

{:age=>30, :name=>"Alice"}
{:age=>31, :name=>"Alice"}

Common Pitfalls

Some common mistakes when adding key-value pairs to hashes include:

• Using parentheses instead of square brackets, like hash(key) = value, which causes errors.
• Forgetting that keys are case-sensitive and must match exactly.
• Trying to add a key to a frozen hash, which raises an error.

ruby

hash = {}
# Wrong way - causes error:
# hash(:name) = "Alice"

# Right way:
hash[:name] = "Alice"

Quick Reference

Action	Syntax	Description
Add or update key-value	hash[key] = value	Assigns value to key in hash
Access value by key	hash[key]	Retrieves value for key
Check if key exists	hash.key?(key)	Returns true if key is present
Delete key-value pair	hash.delete(key)	Removes key and its value

Key Takeaways

Use square brackets with the key to add or update a hash value: hash[key] = value.

Keys can be symbols, strings, or other objects but must be consistent when accessing.

Avoid syntax errors by not using parentheses when assigning values to hash keys.

You cannot modify a frozen hash; it will raise an error if you try to add keys.

Use hash.key?(key) to check if a key exists before adding or updating.