Loops in C++

In programming, iteration statements (commonly known as loops) are used to execute a block of code repeatedly until a specific condition is met.

Using loops eliminates the need to write redundant code, reduces system memory footprint, and allows applications to process dynamic sequences or collections of data effectively.

Video Explanation​

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1. The for Loop​

The for loop is an entry-controlled loop structure. It is ideal when you know the exact number of times you need to iterate before entering the loop block.

Syntax​

for (initialization; condition; update) {
    // Code block to be executed repeatedly
}

Example​

#include <iostream>

int main() {
    for (int i = 0; i < 5; ++i) {
        std::cout << "Iteration: " << i << "\n";
    }
    return 0;
}

Execution Mechanics:​

1. Initialization: Executes exactly once when entering the loop. Usually sets a counter variable (e.g., int i = 0).
2. Condition: Evaluated before every iteration. If true, the loop body runs. If false, execution exits the loop.
3. Update: Executes at the absolute end of each iteration cycle (e.g., ++i), adjusting the counter variable before testing the condition again.

2. The Modern Range-Based for Loop​

Introduced in modern C++ (C++11 and later), this syntax provides a cleaner, safer way to iterate through entire arrays or standard collections without tracking index counters manually.

Syntax​

for (element_type variable : collection) {
    // Code block to execute for each element
}

Example​

#include <iostream>

int main() {
    int binarySequence[] = {1, 2, 4, 8, 16};

    for (int value : binarySequence) {
        std::cout << "Value: " << value << "\n";
    }
    return 0;
}

3. The while Loop​

The while loop is also an entry-controlled loop. It evaluates its expression before executing the inner code block. This is the optimal structure when the total number of iterations depends on runtime events rather than a fixed counter.

Syntax​

while (condition) {
    // Code block to execute while condition remains true
}

Example​

#include <iostream>

int main() {
    int counter = 0;

    while (counter < 5) {
        std::cout << "Count: " << counter << "\n";
        ++counter; // Essential statement to eventually invalidate the condition
    }
    return 0;
}

4. The do-while Loop​

The do-while loop is an exit-controlled loop structure. Unlike the preceding loops, it checks its condition at the end of an iteration cycle. Consequently, the loop body is guaranteed to execute at least once, regardless of whether the condition starts as true or false.

Syntax​

do {
    // Code block executes at least once
} while (condition); // Note the required trailing semicolon

Example​

#include <iostream>

int main() {
    int trackingId = 10;

    do {
        std::cout << "Processing ID: " << trackingId << "\n";
        ++trackingId;
    } while (trackingId < 5); // Condition starts as false

    return 0;
}

Output: Processing ID: 10 (The code inside ran once, even though $10 < 5$ is immediately false).

5. Nested Loops​

A nested loop structure is simply a loop inside another loop. For every single complete execution of the outer loop, the inner loop cycles through its entire execution run from scratch.

Example​

#include <iostream>

int main() {
    for (int row = 1; row <= 3; ++row) {
        for (int col = 1; col <= 2; ++col) {
            std::cout << "R:" << row << " C:" << col << " | ";
        }
        std::cout << "\n"; // Newline after completing inner row pass
    }
    return 0;
}

6. Jump Statements: break and continue​

C++ provides control statements to instantly alter execution behavior inside loop bodies mid-cycle.

A. The break Statement​

Instantly terminates the loop structure entirely and shifts program execution down to the statement immediately following the loop.

#include <iostream>

int main() {
    for (int i = 0; i < 10; ++i) {
        if (i == 4) {
            break; // Terminate loop entirely when i reaches 4
        }
        std::cout << "Value: " << i << "\n";
    }
    // Execution jumps here after break
    return 0;
}

B. The continue Statement​

Instantly skips the remainder of the statements in the current iteration cycle and shifts execution straight to the loop's next update step/evaluation phase.

#include <iostream>

int main() {
    for (int i = 0; i < 5; ++i) {
        if (i == 2) {
            continue; // Skip the rest of this iteration pass when i is 2
        }
        std::cout << "Processing index: " << i << "\n";
    }
    return 0;
}

7. Architectural Decisions: Loop Architecture Selection​