C++ For Loop - Complete Guide

Understanding the For Loop

The for loop is a compact looping construct in C++ that repeats a statement or block for a controlled number of iterations.

It has three parts: initialization, condition, and iteration. The condition is checked before each iteration; the iteration expression runs after the loop body. Variables declared in the initialization part have scope limited to the loop.

Detailed Syntax Breakdown

Example

#include <iostream>
using namespace std;

int main() {
  for (int i = 0; i < 5; i++) {
    cout << "Value: " << i << '\n';
  }
}

Output

Value: 0
Value: 1
Value: 2
Value: 3
Value: 4

ℹ️ Note: The variable <code>i</code> is not visible outside this loop because it is declared in the initialization clause.

Full Syntax & Variations

General form: for ( init-statement ; condition ; iteration ) statement;

Each part is optional; omitting the condition makes it true.

⚠️ Warning: You can declare multiple variables in the init clause only if they share the same type. Use the comma operator in the iteration clause to update multiple values.

Example

// Infinite loop (use with care)
for (;;) { work(); }

// Omit initialization or iteration as needed
int i = 0;
for (; i < n; ) { /* ... */ ++i; }

// Declare multiple variables of the same type; update several with the comma operator
for (int i = 0, j = n - 1; i < j; ++i, --j) {
  // process from both ends
}

Range-Based For (C++11+)

For iterating containers and arrays, prefer the range-based for loop for clarity and safety.

Example

#include <vector>
#include <string>
#include <map>
#include <iostream>
using namespace std;

int main() {
  vector<int> v{1,2,3};

  // Read-only
  for (int x : v) { cout << x << ' '; }
  cout << '\n';

  // Modify elements in place
  for (int &x : v) { x *= 2; }

  // Read-only strings by const reference (avoids copies)
  vector<string> names{"Ann","Bob"};
  for (const string &s : names) { cout << s << ' '; }
  cout << '\n';

  // C++17 structured bindings (maps, pairs, tuples)
  map<string,int> freq{{"a",1},{"b",2}};
  for (auto &[key, val] : freq) { val++; }
}

ℹ️ Note: Use <code>const T&</code> for read-only elements to avoid copies; use <code>T&</code> to mutate elements.

Indexing Containers Safely

When indexing standard containers, prefer std::size_t (the type returned by size()) to avoid signed/unsigned comparison issues.

⚠️ Warning: Using <code>int i</code> with <code>i < v.size()</code> may trigger signed/unsigned warnings.

Example

#include <vector>
#include <cstddef>

std::vector<int> v{10,20,30};
for (std::size_t i = 0; i < v.size(); ++i) {
  // safe index-based access
}

Common Patterns

ℹ️ Note: Prefer pre-increment (<code>++i</code>) with iterators; for built-in integers both forms are equivalent in effect.

Erasing While Iterating (Containers)

Erasing elements invalidates iterators for many containers. Use the erase-return pattern or erase/remove_if.

⚠️ Warning: Do not increment the iterator after <code>erase</code> when using the returned iterator; doing so will skip elements.

Example

#include <vector>
#include <algorithm>

std::vector<int> v{1,2,3,4,5,6};
// Iterator-safe erase loop
for (auto it = v.begin(); it != v.end(); ) {
  if (*it % 2 == 0) it = v.erase(it); // returns next valid iterator
  else ++it;
}

// Or the erase-remove idiom (removes all evens)
v.erase(std::remove_if(v.begin(), v.end(), [](int x){ return x % 2 == 0; }), v.end());

Debugging Tips

🔹 Ensure the loop condition will eventually become false (avoid infinite loops).
🔹 Watch for off-by-one errors (< vs <=).
🔹 Initialize variables before use; keep loop state local to the loop when possible.
🔹 Avoid modifying the loop counter inside the body.
🔹 Avoid floating-point loop counters for index logic—precision can cause missed termination.
🔹 Prefer range-based for for containers/arrays when you don’t need the index.

C++ For Loop - Complete Guide

Understanding the For Loop

The for loop is a compact looping construct in C++ that repeats a statement or block for a controlled number of iterations.

Detailed Syntax Breakdown

Example

#include <iostream>
using namespace std;

int main() {
  for (int i = 0; i < 5; i++) {
    cout << "Value: " << i << '\n';
  }
}

Output

Value: 0
Value: 1
Value: 2
Value: 3
Value: 4

ℹ️ Note: The variable <code>i</code> is not visible outside this loop because it is declared in the initialization clause.

Full Syntax & Variations

General form: for ( init-statement ; condition ; iteration ) statement;

Each part is optional; omitting the condition makes it true.

⚠️ Warning: You can declare multiple variables in the init clause only if they share the same type. Use the comma operator in the iteration clause to update multiple values.

Example

// Infinite loop (use with care)
for (;;) { work(); }

// Omit initialization or iteration as needed
int i = 0;
for (; i < n; ) { /* ... */ ++i; }

// Declare multiple variables of the same type; update several with the comma operator
for (int i = 0, j = n - 1; i < j; ++i, --j) {
  // process from both ends
}

Range-Based For (C++11+)

For iterating containers and arrays, prefer the range-based for loop for clarity and safety.

Example

#include <vector>
#include <string>
#include <map>
#include <iostream>
using namespace std;

int main() {
  vector<int> v{1,2,3};

  // Read-only
  for (int x : v) { cout << x << ' '; }
  cout << '\n';

  // Modify elements in place
  for (int &x : v) { x *= 2; }

  // Read-only strings by const reference (avoids copies)
  vector<string> names{"Ann","Bob"};
  for (const string &s : names) { cout << s << ' '; }
  cout << '\n';

  // C++17 structured bindings (maps, pairs, tuples)
  map<string,int> freq{{"a",1},{"b",2}};
  for (auto &[key, val] : freq) { val++; }
}

ℹ️ Note: Use <code>const T&</code> for read-only elements to avoid copies; use <code>T&</code> to mutate elements.

Indexing Containers Safely

When indexing standard containers, prefer std::size_t (the type returned by size()) to avoid signed/unsigned comparison issues.

⚠️ Warning: Using <code>int i</code> with <code>i < v.size()</code> may trigger signed/unsigned warnings.

Example

#include <vector>
#include <cstddef>

std::vector<int> v{10,20,30};
for (std::size_t i = 0; i < v.size(); ++i) {
  // safe index-based access
}

Common Patterns

ℹ️ Note: Prefer pre-increment (<code>++i</code>) with iterators; for built-in integers both forms are equivalent in effect.

Erasing While Iterating (Containers)

Erasing elements invalidates iterators for many containers. Use the erase-return pattern or erase/remove_if.

⚠️ Warning: Do not increment the iterator after <code>erase</code> when using the returned iterator; doing so will skip elements.

Example

#include <vector>
#include <algorithm>

std::vector<int> v{1,2,3,4,5,6};
// Iterator-safe erase loop
for (auto it = v.begin(); it != v.end(); ) {
  if (*it % 2 == 0) it = v.erase(it); // returns next valid iterator
  else ++it;
}

// Or the erase-remove idiom (removes all evens)
v.erase(std::remove_if(v.begin(), v.end(), [](int x){ return x % 2 == 0; }), v.end());

Debugging Tips

🔹 Ensure the loop condition will eventually become false (avoid infinite loops).
🔹 Watch for off-by-one errors (< vs <=).
🔹 Initialize variables before use; keep loop state local to the loop when possible.
🔹 Avoid modifying the loop counter inside the body.
🔹 Avoid floating-point loop counters for index logic—precision can cause missed termination.
🔹 Prefer range-based for for containers/arrays when you don’t need the index.

C++ Basics

C++ Functions

C++ Classes

C++ Quiz

C++ For Loop - Complete Guide

Understanding the For Loop

Detailed Syntax Breakdown

Full Syntax & Variations

Range-Based For (C++11+)

Indexing Containers Safely

Common Patterns

Erasing While Iterating (Containers)

Debugging Tips

C++ For Loop - Complete Guide

Understanding the For Loop

Detailed Syntax Breakdown

Full Syntax & Variations

Range-Based For (C++11+)

Indexing Containers Safely

Common Patterns

Erasing While Iterating (Containers)

Debugging Tips