Mappings in Blockchain / Solidity - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a mapping in blockchain smart contracts?

A mapping is like a digital dictionary that links one value (a key) to another value. It helps store and find data quickly in smart contracts.

beginner

How do you declare a mapping in Solidity?

You write: mapping(KeyType => ValueType) public name; where KeyType is the type of the key and ValueType is the type of the value stored.

intermediate

Can you iterate over all keys in a mapping directly?

No, mappings do not store keys or allow iteration. You must keep a separate list of keys if you want to loop through them.

beginner

What happens if you access a mapping with a key that does not exist?

You get the default value for the value type (like 0 for numbers or false for booleans), because mappings always return a value even if the key was never set.

beginner

Why are mappings useful in blockchain smart contracts?

They let you store and quickly find data linked by keys, like user balances or permissions, without using lots of storage or complex searching.

How do you declare a mapping from addresses to unsigned integers in Solidity?

A mapping(uint, address) public balances;

B mapping(uint => address) public balances;

C mapping(address => uint) public balances;

D mapping(address, uint) public balances;

The correct syntax uses '=>' to link key type to value type: mapping(address => uint).

What value do you get when accessing a mapping with a key that was never set?

A An error is thrown

B The default value of the value type

C Null

D Random data

Mappings return the default value for the value type if the key does not exist.

Can you loop through all keys in a mapping directly?

A No, mappings do not store keys for iteration

B Yes, mappings support iteration

C Only if keys are numbers

D Only in test environments

Mappings do not keep track of keys, so you cannot loop through them directly.

What is a common use of mappings in smart contracts?

A Running loops

B Creating user interfaces

C Compiling code

D Storing user balances

Mappings are often used to store balances or permissions linked to user addresses.

Which of these is NOT a valid key type for a mapping in Solidity?

A mapping

B uint

C string

D address

Mappings cannot have another mapping as a key type.

Explain what a mapping is and how it works in blockchain smart contracts.

Think of it like a digital dictionary that stores data linked by keys.

Describe how you would store and retrieve user balances using a mapping in Solidity.

Focus on how keys and values relate and how to get a balance.