Web3 & DApp Development Fundamentals · Lesson

Loop and Calldata Tricks

Cheaper operations.

Loop and Calldata Tricks is a free Web3 & DApp Development Fundamentals lesson on CoddyKit — lesson 3 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Web3 & DApp Development Fundamentals learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Loops Multiply Cost

Loops are dangerous for gas because every operation inside runs once per iteration. A single expensive call inside a loop can blow your gas budget or hit the block gas limit.

The goal is to push as much work as possible outside the loop and make each iteration as cheap as possible.

Cache the Length

Reading array.length on a storage array costs an SLOAD each time the loop condition is checked. Cache the length in a local variable before the loop.

// BAD: SLOAD on every iteration
for (uint256 i = 0; i < items.length; i++) { }

// GOOD: one SLOAD
uint256 len = items.length;
for (uint256 i = 0; i < len; i++) { }

Avoid Storage Inside Loops

Never read or write the same storage slot repeatedly inside a loop. Accumulate into a local variable and write the result once after the loop ends.

// GOOD: single storage write after the loop
uint256 total = 0;
for (uint256 i = 0; i < len; i++) {
    total += amounts[i];
}
balance = total; // one SSTORE

Unchecked Increment

Since Solidity 0.8, arithmetic has built-in overflow checks that cost extra gas. A loop counter that can never realistically overflow can be incremented inside an unchecked block.

for (uint256 i = 0; i < len;) {
    // loop body
    unchecked { ++i; }
}

Pre-increment over Post-increment

++i is marginally cheaper than i++ because post-increment must produce a temporary copy of the old value. The saving is tiny per operation but adds up across many iterations.

Prefer ++i in loop counters where the return value is unused.

for (uint256 i = 0; i < len; ++i) {
    // ++i avoids a temporary copy
}

Calldata over Memory

For external functions, declaring array and struct parameters as calldata instead of memory avoids copying the data into memory. Calldata is read-only and cheaper to access.

// Cheaper: no memory copy
function process(uint256[] calldata data) external {
    for (uint256 i = 0; i < data.length; ++i) { }
}

Shrink Calldata

Each non-zero calldata byte costs 16 gas. Smaller payloads mean cheaper transactions. Techniques include:

Packing multiple values into one uint256
Using smaller types where the ABI allows
Passing indices instead of full structs when the data is already on-chain

Short-Circuit Conditions

Logical operators && and || short-circuit: the second operand is only evaluated if needed. Order conditions so the cheapest or most-likely-decisive check comes first.

// Check the cheap flag before the expensive storage read
if (isEnabled && expensiveStorageCheck()) {
    // ...
}

Avoid Redundant Reads

If you access the same calldata or memory element multiple times in a loop body, cache it in a local variable. While calldata access is cheaper than storage, repeated access still adds up.

for (uint256 i = 0; i < len; ++i) {
    uint256 v = data[i]; // read once
    total += v;
    if (v > max) max = v;
}

Bound Your Loops

Unbounded loops over user-controlled arrays can exceed the block gas limit, making the function permanently uncallable (a denial-of-service risk).

Always cap iteration counts or use a pull-based pattern where users process their own entries, keeping each transaction within safe gas bounds.

require(ids.length <= 100, "too many");
for (uint256 i = 0; i < ids.length; ++i) { }

Putting It Together

An optimized loop typically: caches the length, accumulates into locals, uses calldata inputs, increments with unchecked { ++i; }, and stays bounded.

Each trick is small alone, but combined they can cut loop gas by a large fraction.

function batch(uint256[] calldata vals) external {
    uint256 len = vals.length;
    uint256 total;
    for (uint256 i = 0; i < len;) {
        total += vals[i];
        unchecked { ++i; }
    }
    sum = total;
}

Quick Check

Why is using calldata instead of memory for an external function's array parameter cheaper?

Recap

You learned loop and calldata gas tricks:

Cache array length before looping
Accumulate in locals, write storage once
Use unchecked { ++i; } for safe counters
Prefer calldata parameters and shrink payloads
Always bound loops to avoid gas-limit DoS

Next we measure these gains with profiling tools.

Frequently asked questions

Is the “Loop and Calldata Tricks” lesson free?

Yes — the full text of “Loop and Calldata Tricks” is free to read here on the web, and the Web3 & DApp Development Fundamentals course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Web3 & DApp Development Fundamentals course, upgrade to CoddyKit PRO.

What will I learn in “Loop and Calldata Tricks”?

Cheaper operations. You practise Web3 & DApp Development Fundamentals with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Web3 & DApp Development Fundamentals?

No prior experience is required. Web3 & DApp Development Fundamentals on CoddyKit is structured for beginners through advanced learners; this is — lesson 3 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “Loop and Calldata Tricks” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this Web3 & DApp Development Fundamentals lesson?

Yes. Every Web3 & DApp Development Fundamentals lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

All lessons in this course

← Back to Web3 & DApp Development Fundamentals