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Loops — while & do-while

I. In-Class Exercises

Programming Exercises

Sum Problem 1: L1111
Sum Problem 2: L1112
Sum Problem 3: L1113

Sample Code

1// while loop drawing a regular 16-sided polygon
2#include "CTurtle.hpp"
3#include <bits/stdc++.h>
4using namespace std;
5using namespace cturtle;
6
7int main() {
8    // Create a canvas screen
9    TurtleScreen scr;
10    // Create a drawing turtle
11    Turtle turtle(scr);
12
13    int sides = 16;
14    int sideLength = 20;
15    double angle = 360.0 / sides;
16
17    int count = 0;
18    while (count < sides) {
19            turtle.forward(sideLength);
20            turtle.right(angle);
21            ++count;
22    }
23
24    // Pause the program
25    system("pause");
26    // End of program
27    return 0;
28}

1// do-while loop drawing a regular 16-sided polygon
2#include "CTurtle.hpp"
3#include <bits/stdc++.h>
4using namespace std;
5using namespace cturtle;
6
7int main() {
8    // Create a canvas screen
9    TurtleScreen scr;
10    // Create a drawing turtle
11    Turtle turtle(scr);
12
13    int sides = 16;
14    int sideLength = 20;
15    double angle = 360.0 / sides;
16
17    int count = 0;
18    do {
19            turtle.forward(sideLength);
20            turtle.right(angle);
21            ++count;
22    } while (count < sides);
23
24    // Pause the program
25    system("pause");
26    // End of program
27    return 0;
28}

1#include <bits/stdc++.h>
2using namespace std;
3
4int main() {
5        int n;
6        cin >> n;
7        int sum = 0;
8        int i = n;
9        while (i > 0) {
10                sum += i--;
11        }
12        cout << sum << endl;
13        return 0;
14}

1#include <bits/stdc++.h>using namespace std;
2
3int main() {
4        int n;
5        cin >> n;
6        double sum = 0;
7        int i = 1;
8        while (i <= n) {
9                sum += 1.0 / i;
10                ++i;
11        }
12        printf("%.3f", sum);
13        return 0;
14}

1#include <bits/stdc++.h>using namespace std;
2
3int main() {
4        int n;
5        cin >> n;
6        int sum = 0;
7        int i = 1;
8        while (i <= n) {
9                sum += i;
10                i += 2;
11        }
12        cout << sum << endl;
13        return 0;
14}

II. Knowledge Summary

✨ What is a Loop

In daily life, many things need to be done repeatedly. For example:

When running, you go around the track lap after lap until you've completed the required number of laps
When doing homework, you solve problems one by one until they're all done

It's the same in programs. When we need to repeatedly execute a piece of code, we use loop structures. Loop structures allow programs to repeat automatically without writing the same code multiple times.

A loop typically consists of three parts:

Initialize the loop variable: Preparation before the loop begins
Loop condition: Determines whether to continue looping
Update the loop variable: Changes the variable's value after each iteration so the loop eventually ends

✨ while Loop

Syntax

The while loop checks the condition first; if true, it executes the loop body; if false, it exits the loop.

1while (loop_condition) {
2    // If condition is true, execute code inside braces
3    Operation
4    Update loop variable
5}
6// If condition is false, exit loop and run subsequent code

while Loop Flowchart

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Comprehensive Example: Sum from 1 to n

Input an integer n, calculate 1+2+3+...+n.

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1int n;
2cin >> n;
3int sum = 0;
4int i = 1;
5while (i <= n) {
6    sum += i;
7    ++i;
8}
9cout << sum << endl;

✨ do-while Loop

Syntax

The do-while loop executes the loop body first, then checks the condition; if true, it continues; if false, it exits.

1do {
2    // Execute code inside braces first
3    Operation
4    Update loop variable
5} while (loop_condition); // Note the semicolon here
6// If condition is false, exit loop and run subsequent code

do-while Loop Flowchart

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Comprehensive Example: Input Validation

Ask the user to input an integer between 1 and 100. If the input is invalid, prompt for re-entry until the input is correct. This scenario requires at least one input, making it perfect for do-while.

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1int n;
2do {
3    cout << "Enter an integer from 1-100: ";
4    cin >> n;
5} while (n < 1 || n > 100);
6cout << "Valid input: " << n << endl;

✨ Difference Between while and do-while

	while	do-while
Check timing	Checks condition first, then executes body	Executes body first, then checks condition
Minimum executions	0 times (doesn't execute if condition is false initially)	1 time (always executes at least once)
Use case	Unknown iteration count, may not execute at all	Scenarios requiring at least one execution (e.g., input validation)

1int i = 10;
2
3// while loop: condition is false, body doesn't execute, no output
4while (i < 5) {
5    cout << i << endl;
6    ++i;
7}
8
9// do-while loop: executes body first, outputs 10
10do {
11    cout << i << endl;
12    ++i;
13} while (i < 5);

Loops — while & do-while

I. In-Class Exercises

Programming Exercises

Sample Code

II. Knowledge Summary

✨ What is a Loop

✨ while Loop

Syntax

while Loop Flowchart

Comprehensive Example: Sum from 1 to n

✨ do-while Loop

Syntax

do-while Loop Flowchart

Comprehensive Example: Input Validation

✨ Difference Between while and do-while

III. Homework

Programming Exercises

Knowledge Quiz