Loop: Definition, Syntax, and Examples

A loop is a programming construct that repeats a block of code as long as a specific condition is met. Loops allow developers to automate repetitive tasks—like processing lists, reading files, or displaying elements—without writing the same code multiple times.

How Loops Work

At their core, loops check a condition before or during execution. If the condition is true, the code inside the loop runs; when it becomes false, the loop stops.

This pattern saves time and reduces code duplication. For example, instead of printing ten numbers manually, a loop can do it automatically:

for i in range(10):
    print(i)

Each iteration changes the value of i, allowing the loop to run until the range ends.

Common Types of Loops

While syntax differs slightly between languages, most programming environments provide these essential loop types:

• For loop: Repeats code a known number of times.
• While loop: Continues as long as a condition remains true.
• Do-while loop: Executes at least once before checking the condition (not available in all languages).

Loop Syntax Across Languages

Python

Python’s for loop iterates through items in a sequence.

names = ["Luna", "Milo", "Nina"]
for name in names:
    print("Hello,", name)

A while loop keeps running while its condition stays true:

count = 0
while count < 3:
    print("Counting:", count)
    count += 1

JavaScript / TypeScript

JavaScript loops give more control over counters and conditions.

for (let i = 0; i < 5; i++) {
  console.log("Step", i);
}

A while loop works similarly:

let count = 0;
while (count < 3) {
  console.log(count);
  count++;
}

In TypeScript, type-safe counters make loops more predictable:

for (let i: number = 0; i < 5; i++) {
  console.log(`Index: ${i}`);
}

Swift

for number in 1...3 {
    print("Number:", number)
}

Swift’s repeat-while loop ensures at least one execution:

var tries = 0
repeat {
    print("Try:", tries)
    tries += 1
} while tries < 3

SQL

SQL isn’t a procedural language, but looping is possible in stored procedures or scripts.

DECLARE @counter INT = 1;
WHILE @counter <= 3
BEGIN
    PRINT CONCAT('Iteration ', @counter);
    SET @counter = @counter + 1;
END

This pattern processes records or executes repetitive database operations.

When to Use Loops

Loops appear in almost every kind of program. Here are some everyday use cases:

1. Processing Collections

Iterating through lists or arrays to read or modify data:

numbers = [2, 4, 6, 8]
for n in numbers:
    print(n * 2)

2. Generating Dynamic Web Output (React)

In React, loops often appear inside render functions to display repeated elements:

const items = ["Home", "About", "Contact"];
function Nav() {
  return (
    <ul>
      {items.map((item, index) => <li key={index}>{item}</li>)}
    </ul>
  );
}

3. Validating Input (JavaScript)

Loops make it easy to check every item in an array for errors:

const emails = ["test@", "user@example.com", ""];
for (const email of emails) {
  if (!email.includes("@")) {
    console.log("Invalid:", email);
  }
}

4. Automating Repetitive Tasks (Swift)

for day in ["Mon", "Tue", "Wed"] {
    print("Meeting scheduled on \(day)")
}

Loop Control Statements

Most languages include ways to control loop flow:

• break – exits the loop immediately
• continue – skips to the next iteration

Example in TypeScript:

for (let i = 0; i < 5; i++) {
  if (i === 2) continue;  // Skip 2
  if (i === 4) break;     // Stop early
  console.log(i);
}

These controls give flexibility when managing complex iterations.

Nested Loops

A loop inside another loop is called a nested loop. It’s often used to work with two-dimensional data such as matrices or tables.

matrix = [[1, 2], [3, 4]]
for row in matrix:
    for value in row:
        print(value)

Nested loops should be used sparingly, as they can slow down programs when processing large data sets.

Common Mistakes

Even experienced developers can make errors when working with loops:

• Infinite loops: Forgetting to change the condition so it never becomes false.
• Off-by-one errors: Running one iteration too few or too many.
• Incorrect indexing: Accessing elements outside an array’s range.
• Modifying arrays during iteration: Can cause skipped elements or crashes.

For example, this infinite loop never ends:

let i = 0;
while (i < 5) {
  console.log(i);
  // forgot to increment i
}

Best Practices

To write efficient, readable loops:

• Initialize counters clearly.
• Keep conditions simple and easy to understand.
• Avoid deep nesting whenever possible.
• Use built-in iteration methods like .map() or .forEach() for clarity.
• Add comments or descriptive variable names to clarify purpose.

Good loop design makes your code more predictable and easier to maintain.

Loops in Real-World Programming

Loops power many everyday processes:

• Data analysis: Iterating through rows in a dataset.
• User interfaces: Displaying repeating elements dynamically.
• Automation: Processing files, emails, or logs.
• APIs: Sending multiple requests until all results are retrieved.

They’re a key part of how programs think—repeating actions until goals are met.

Summary

A loop repeats instructions until a specific condition is no longer true. It’s one of the most fundamental patterns in computer programming, enabling automation, iteration, and efficient data processing.

Understanding how loops work prepares you to handle everything from rendering UI lists in React to processing datasets in Python or automating reports in SQL.

Mastering loops isn’t just about repetition—it’s about control, precision, and writing programs that think ahead.