Yashotantra Academy logo
← Solidity theorytheory 0/50 · 0%
level 1
Curious · 0 XP
streak
0d
perfect
0
badges
0

100 XP to Apprentice

Types · easy

9. Mappings

Key/value state, defaults and why you cannot iterate.

1 min read3 questionsup to 50 XPtopic 9/50

Read in another language

AI translates the explanation and quiz text while preserving code and answer scoring.

A mapping is a hash table from key to value. Every possible key exists and returns the zero value until written, so there is no length and no way to iterate.

solidity
mapping(address => uint256) public balanceOf;
mapping(address => mapping(address => uint256)) public allowance;

If you need to enumerate holders, keep a parallel array or emit events and index them off chain.

How to study this topic

  1. 1. Read the explanation once end to end, skipping the code.
  2. 2. Re-read the code snippets and say out loud what each line does.
  3. 3. Answer the quiz without scrolling back — that is the real check.
  4. 4. Read every explanation, even for the answers you got right.

Scoring

  • · Up to 30 XP from quiz accuracy.
  • · +15 XP for a perfect first attempt.
  • · +5 XP while a 3-day streak is running.
  • · 75% or higher marks the topic complete.

Check your understanding

  1. 1. What does an unwritten mapping key return?

  2. 2. Can you iterate a mapping in Solidity?

  3. 3. How do you track holders for enumeration?

Badges

First light
Pass your first topic
Sharp shooter
3 perfect first-try quizzes
On a roll
Study 3 days in a row
Week warrior
Study 7 days in a row
Grinder
Earn 500 XP
Deep diver
Earn 1500 XP