I. In-Class Exercises

Programming Exercises

1. Simple Comparison: L1081
2. Triangle Validity Check: L1082
3. Check Divisibility by 2, 3, 5, 7: L1083

Sample Code

1#include <bits/stdc++.h>
2using namespace std;
3
4int main() {
5        int a, b, c;
6        cin >> a >> b >> c;
7        if (a + b > c) {
8                cout << "yes" << endl;
9        } else {
10                cout << "no" << endl;
11        }
12        return 0;
13}

1#include <bits/stdc++.h>
2using namespace std;
3
4int main() {
5        int a, b, c;
6        cin >> a >> b >> c;
7        if (a + b > c && a + c > b && b + c > a) {
8                cout << "Yes" << endl;
9        } else {
10                cout << "No" << endl;
11        }
12
13        return 0;
14}

1#include <bits/stdc++.h>
2using namespace std;
3
4int main() {
5        int n;
6        cin >> n;
7        if (0 == n % 2) {
8                cout << "2" << endl;
9        }
10        if (0 == n % 3) {
11                cout << 3 << endl;
12        }
13        if (0 == n % 5) {
14                cout << 5 << endl;
15        }
16        if (0 == n % 7) {
17                cout << 7 << endl;
18        }
19        return 0;
20}

II. Knowledge Summary

✨ What is Conditional Judgment

In daily life, we often need to make different choices based on different situations. For example:

• If it rains today, bring an umbrella; otherwise, don't
• If the exam score is 60 or above, it's a pass; otherwise, it's a fail

It's the same in programs. Programs execute line by line from top to bottom by default, but sometimes we want the program to execute different code based on conditions — this is a conditional statement.

✨ Conditional Control Flowchart

A conditional control flowchart includes branching structures with condition checks.

In a conditional control flowchart:

• Condition check: represented by a diamond
• Start/End: represented by an oval, rounded rectangle, or circle
• Input/Output: represented by a parallelogram
• Operation steps: represented by a rectangle

✨ Simple if-else Structure

if-else Syntax

The if-else structure checks a condition: if true, it executes the code after if; if false, it executes the code after else.

if (boolean_expression) {
    Operation 1
} else {
    Operation 2
}

if-else Flowchart

if-else Usage Notes

Keep the following points in mind when using if-else:

1. The if structure can be used alone, but the else structure must follow an if structure
2. The boolean expression after if must be enclosed in parentheses
3. If multiple lines of code need to be executed after if-else, they must be enclosed in curly braces

✨ Comprehensive Examples

Example 1: Check Odd or Even

Input an integer and determine whether it's even or odd using the modulo operation.

1int n;
2cin >> n;
3if (n % 2 == 0) {
4    cout << "Even" << endl;
5} else {
6    cout << "Odd" << endl;
7}

Example 2: Pass or Fail

Input a score; 60 or above is a pass, otherwise fail.

1int score;
2cin >> score;
3if (score >= 60) {
4    cout << "Pass" << endl;
5} else {
6    cout << "Fail" << endl;
7}

III. Homework

Programming Exercises

1. Absolute Value: L1084
2. Compare Two Numbers: L1085
3. Check if Two-Digit Number: L1086
4. Luggage Shipping Price: L1087

Knowledge Quiz

• Simple if-else Structure - Quiz