Merge and update methods in Ruby - Step-by-Step Execution

Concept Flow - Merge and update methods

Start with Hash A and Hash B

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Call merge or update on Hash A with Hash B

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For each key in Hash B

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If key exists in A?

Yes→Replace value in A

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Add key-value to A

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Return new Hash (merge) or modify A (update)

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End

Start with two hashes. For each key in the second hash, add or replace the key-value in the first hash. Merge returns a new hash; update changes the original.

Execution Sample

Ruby

a = {x: 1, y: 2}
b = {y: 3, z: 4}
c = a.merge(b)
a.update(b)

Shows how merge returns a new hash combining a and b, while update changes a itself.

Execution Table

Step	Operation	Hash A	Hash B	Result	Notes
1	Initial Hashes	{:x=>1, :y=>2}	{:y=>3, :z=>4}	-	Start with two hashes
2	Call a.merge(b)	{:x=>1, :y=>2}	{:y=>3, :z=>4}	{:x=>1, :y=>3, :z=>4}	Returns new hash with b's keys merged, y replaced
3	a after merge	{:x=>1, :y=>2}	-	-	a is unchanged after merge
4	Call a.update(b)	{:x=>1, :y=>2}	{:y=>3, :z=>4}	{:x=>1, :y=>3, :z=>4}	a is changed in place, y replaced, z added
5	a after update	{:x=>1, :y=>3, :z=>4}	-	-	a now includes changes from b

💡 All keys from b processed; merge returns new hash; update modifies a in place.

Variable Tracker

Variable	Start	After merge	After update	Final
a	{:x=>1, :y=>2}	{:x=>1, :y=>2}	{:x=>1, :y=>3, :z=>4}	{:x=>1, :y=>3, :z=>4}
b	{:y=>3, :z=>4}	{:y=>3, :z=>4}	{:y=>3, :z=>4}	{:y=>3, :z=>4}
c	-	{:x=>1, :y=>3, :z=>4}	-	{:x=>1, :y=>3, :z=>4}

Key Moments - 3 Insights

Why does the hash 'a' not change after calling 'merge'?

Because 'merge' returns a new hash with combined keys and values but does not modify the original hash 'a' (see execution_table step 3).

How does 'update' differ from 'merge' in terms of modifying the original hash?

'update' changes the original hash 'a' by adding or replacing keys from 'b' directly (see execution_table step 4 and 5), unlike 'merge' which returns a new hash.

What happens to keys that exist in both hashes during merge or update?

The value from the second hash 'b' replaces the value in the first hash 'a' for those keys (see execution_table steps 2 and 4).

Visual Quiz - 3 Questions

Look at the execution table, what is the value of 'a' after the merge operation?

A {:y=>3, :z=>4}

B {:x=>1, :y=>3, :z=>4}

C {:x=>1, :y=>2}

D nil

💡 Hint

Check the 'a after merge' row in the execution_table.

At which step does 'a' get modified in place?

A Step 3

B Step 4

C Step 2

D Step 5

💡 Hint

Look for the step where 'a.update(b)' is called in the execution_table.

If we changed 'b' to {y: 5, w: 6}, what would be the value of 'a' after update?

A {:x=>1, :y=>5, :w=>6}

B {:x=>1, :y=>3, :z=>4}

C {:x=>1, :y=>2}

D {:y=>5, :w=>6}

💡 Hint

Refer to variable_tracker and how update replaces and adds keys from 'b' to 'a'.

Concept Snapshot

Hash#merge returns a new hash combining keys from both hashes.
Keys in the second hash replace those in the first.
Hash#update modifies the original hash in place.
Use merge when you want a new hash; update to change existing.
Both handle key conflicts by replacing with second hash's value.

Full Transcript

This visual trace shows how Ruby's Hash methods merge and update work. We start with two hashes, a and b. When calling a.merge(b), Ruby creates a new hash combining keys from both, replacing any duplicate keys with values from b, but leaves a unchanged. When calling a.update(b), Ruby changes a itself by adding or replacing keys from b. The execution table tracks each step, showing that after merge, a stays the same, but after update, a includes b's keys and values. Key moments clarify that merge returns a new hash while update modifies the original. The visual quiz tests understanding of these differences and how keys are replaced or added. The snapshot summarizes the key points for quick recall.