Loop backwards using indices

Looping Techniques

Programming

Indexing

Reverse Looping

Iteration

Loop backwards using indices

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Introduction

Looping backwards using indices means iterating from the last element to the first by decrementing an index variable. This is useful when removing elements during iteration (to avoid index shifting), processing data in reverse order, or implementing algorithms like reverse string comparison. Every major language supports backward loops through for loops with decrementing counters, and many provide built-in utilities like Python's reversed() or JavaScript's Array.reverse().

Python

Using range() with Negative Step

python

1items = ['a', 'b', 'c', 'd', 'e']
2
3# range(start, stop, step) — stop is exclusive
4for i in range(len(items) - 1, -1, -1):
5    print(f"Index {i}: {items[i]}")
6# Index 4: e
7# Index 3: d
8# Index 2: c
9# Index 1: b
10# Index 0: a

range(4, -1, -1) generates 4, 3, 2, 1, 0. The stop value -1 is exclusive, so it stops after 0.

Using reversed()

python

1# reversed() with enumerate for index + value
2for i, item in enumerate(reversed(items)):
3    original_index = len(items) - 1 - i
4    print(f"Index {original_index}: {item}")
5
6# Simpler: reversed(range(...))
7for i in reversed(range(len(items))):
8    print(f"Index {i}: {items[i]}")

Safe Removal During Backward Iteration

python

1numbers = [1, 2, 3, 4, 5, 6, 7, 8]
2
3# Remove even numbers — backward loop avoids index shifting
4for i in range(len(numbers) - 1, -1, -1):
5    if numbers[i] % 2 == 0:
6        del numbers[i]
7
8print(numbers)  # [1, 3, 5, 7]

Removing elements during a forward loop skips elements because indices shift. Backward iteration avoids this because removing an element only affects indices after the current position.

JavaScript

for Loop

javascript

1const items = ['a', 'b', 'c', 'd', 'e'];
2
3for (let i = items.length - 1; i >= 0; i--) {
4    console.log(`Index ${i}: ${items[i]}`);
5}
6// Index 4: e
7// Index 3: d
8// ...

Safe Removal with splice()

javascript

1const numbers = [1, 2, 3, 4, 5, 6, 7, 8];
2
3for (let i = numbers.length - 1; i >= 0; i--) {
4    if (numbers[i] % 2 === 0) {
5        numbers.splice(i, 1);
6    }
7}
8console.log(numbers); // [1, 3, 5, 7]

Java

java

1int[] items = {10, 20, 30, 40, 50};
2
3for (int i = items.length - 1; i >= 0; i--) {
4    System.out.println("Index " + i + ": " + items[i]);
5}

ArrayList Backward Removal

java

1import java.util.ArrayList;
2import java.util.List;
3
4List<Integer> numbers = new ArrayList<>(List.of(1, 2, 3, 4, 5, 6));
5
6// Remove even numbers backward
7for (int i = numbers.size() - 1; i >= 0; i--) {
8    if (numbers.get(i) % 2 == 0) {
9        numbers.remove(i);
10    }
11}
12System.out.println(numbers); // [1, 3, 5]

C / C++

1int arr[] = {10, 20, 30, 40, 50};
2int size = sizeof(arr) / sizeof(arr[0]);
3
4for (int i = size - 1; i >= 0; i--) {
5    printf("Index %d: %d\n", i, arr[i]);
6}

cpp

1#include <vector>
2#include <iostream>
3
4std::vector<int> vec = {10, 20, 30, 40, 50};
5
6// Using size_t (unsigned) — careful with underflow!
7for (size_t i = vec.size(); i-- > 0; ) {
8    std::cout << "Index " << i << ": " << vec[i] << "\n";
9}

C#

csharp

1int[] items = { 10, 20, 30, 40, 50 };
2
3for (int i = items.Length - 1; i >= 0; i--)
4{
5    Console.WriteLine($"Index {i}: {items[i]}");
6}
7
8// List backward removal
9var numbers = new List<int> { 1, 2, 3, 4, 5, 6 };
10for (int i = numbers.Count - 1; i >= 0; i--)
11{
12    if (numbers[i] % 2 == 0)
13        numbers.RemoveAt(i);
14}

Swift

swift

1let items = ["a", "b", "c", "d", "e"]
2
3for i in stride(from: items.count - 1, through: 0, by: -1) {
4    print("Index \(i): \(items[i])")
5}
6
7// Using reversed()
8for i in (0..<items.count).reversed() {
9    print("Index \(i): \(items[i])")
10}

Go

1items := []string{"a", "b", "c", "d", "e"}
2
3for i := len(items) - 1; i >= 0; i-- {
4    fmt.Printf("Index %d: %s\n", i, items[i])
5}

When to Use Backward Loops

Use Case	Why Backward
Removing elements during iteration	Prevents index shifting after removal
Reverse string/array processing	Natural order for palindrome checks, reverse printing
Stack-like processing (LIFO)	Process most recently added items first
Dependency resolution	Process dependencies before dependents

Common Pitfalls

Off-by-one error on the start index: The last valid index is length - 1, not length. Starting at length causes an IndexOutOfBoundsException or buffer overflow.
Using unsigned integers for the loop counter: In C/C++, size_t is unsigned. The condition i >= 0 is always true for unsigned types, creating an infinite loop. Use for (size_t i = size; i-- > 0; ) or cast to a signed type.
Removing elements during a forward loop: Forward iteration skips elements after removal because indices shift down. Always iterate backward when removing elements by index.
Forgetting that range() stop is exclusive in Python: range(4, 0, -1) produces 4, 3, 2, 1 — it stops before 0. To include index 0, use range(4, -1, -1).
Modifying the loop variable inside the body: Changing i inside a backward loop can cause skipped elements or infinite loops. Let the loop control statement handle decrementing.

Summary

Loop backward with for (int i = length - 1; i >= 0; i--) in most languages
In Python, use range(len(items) - 1, -1, -1) or reversed(range(len(items)))
Backward loops are essential for safely removing elements during iteration
Watch for off-by-one errors and unsigned integer underflow in C/C++
Prefer language-specific utilities (reversed(), .reversed(), stride()) for readability

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Work through 120+ system design problems with detailed solutions, from rate limiters to multi-region storage.

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Loop backwards using indices

System Design practice on Codemia

Introduction

Python

Using range() with Negative Step

Using reversed()

Safe Removal During Backward Iteration

JavaScript

for Loop

Safe Removal with splice()

Java

ArrayList Backward Removal

C / C++

C#

Swift

Go

When to Use Backward Loops

Common Pitfalls

Summary

Related reading

System Design Fundamentals

Track what you have practised

System Design practice on Codemia