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Conditional Statements — switch & Ternary Operator

I. In-Class Exercises

Programming Exercises

Number to English: L1101
Most Economical Box Size: L1102
Maximum of Three Numbers: L1103

Sample Code

1#include <bits/stdc++.h>
2using namespace std;
3
4int main() {
5        int n;
6        cin >> n;
7        switch (n) {
8                case 1:
9                        cout << "one" << endl;
10                        break;
11                case 2:
12                        cout << "two" <<endl;
13                        break;
14                case 3:
15                        cout << "three" <<endl;
16                        break;
17                case 4:
18                        cout << "four" <<endl;
19                        break;
20                case 5:
21                        cout << "five" <<endl;
22                        break;
23                case 6:
24                        cout << "six" <<endl;
25                        break;
26                case 7:
27                        cout << "seven" <<endl;
28                        break;
29                case 8:
30                        cout << "eight" <<endl;
31                        break;
32                case 9:
33                        cout << "nine" <<endl;
34                        break;
35                default:
36                        cout << "out" << endl;
37        }
38        return 0;
39}

1#include <bits/stdc++.h>
2using namespace std;
3
4int main() {
5        int weight;
6        cin >> weight;
7        weight /= 10;
8//        weight = weight / 10;
9        switch (weight) {
10                case 0:
11                        cout << "A" << endl;
12                        break;
13                case 1:
14                        cout << "B" << endl;
15                        break;
16                case 2:
17                case 3:
18                        cout << "C" << endl;
19                        break;
20                case 4:
21                        cout << "D" << endl;
22                        break;
23                default:
24                        cout << "E" << endl;
25        }
26        return 0;
27}

1#include <bits/stdc++.h>
2using namespace std;
3
4int main() {
5        int a, b, c;
6        cin >> a >> b >> c;
7        int max = (a > b ? a : b) > c ? (a > b ? a : b) : c;
8//        max = max > c ? max : c;
9        cout << max << endl;
10        return 0;
11}

II. Knowledge Summary

✨ switch Structure

When to Use

When the variable being checked has a finite number of distinct values, the switch structure is suitable — for example, checking day of the week, month, grade level, etc.

Syntax

1switch (expression) {
2    case value1:
3        Operation 1;
4        break;
5    case value2:
6        Operation 2;
7        break;
8    case value3:
9        Operation 3;
10        break;
11    default:
12        Default operation;
13}

switch Flowchart

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Notes

Keep the following points in mind when using switch:

Values after case can only be numbers or constants, not variables or expressions containing variables
If two cases share the same operation, you cannot use an OR expression — you must write two cases (see the weekend example below)
Each case must have a break; otherwise execution will fall through to the next case
default is optional; break inside default is not required

Comprehensive Example: Day of the Week

Input an integer from 1-7, output the corresponding day.

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1int day;
2cin >> day;
3switch (day) {
4    case 1:
5        cout << "Monday" << endl;
6        break;
7    case 2:
8        cout << "Tuesday" << endl;
9        break;
10    case 3:
11        cout << "Wednesday" << endl;
12        break;
13    case 4:
14        cout << "Thursday" << endl;
15        break;
16    case 5:
17        cout << "Friday" << endl;
18        break;
19    case 6:
20    case 7:
21        cout << "Weekend" << endl;
22        break;
23    default:
24        cout << "Invalid input" << endl;
25}

✨ Ternary Operator

What is the Ternary Operator

The ternary operator (also called the conditional operator) is a concise way to write conditional logic in a single line, replacing simple if-else structures.

It uses the ? and : operators with three expressions, hence the name "ternary":

Left of ? is the condition expression
Left of : is the return value when the condition is true
Right of : is the return value when the condition is false

Syntax

1// Syntax: condition ? expression1 : expression2
2// If condition is true → returns expression1
3// If condition is false → returns expression2
4
5int a = 10, b = 20;
6int max_ab = a > b ? a : b; // a > b is false, returns b, so max_ab = 20

The ternary operator is essentially shorthand for if-else. The above code is equivalent to:

1int max_ab;
2if (a > b) {
3    max_ab = a;
4} else {
5    max_ab = b;
6}

Notes

Keep the following points in mind when using the ternary operator:

The ternary operator is suitable for simple conditional assignments; use if-else for complex logic to maintain readability
Ternary operators can be nested, but avoid too many levels of nesting
The ternary operator is an expression with a return value that can be used directly for assignment or output

Comprehensive Example: Check Odd or Even

int n;
cin >> n;
// Use ternary operator directly in output
cout << (n % 2 == 0 ? "Even" : "Odd") << endl;

Conditional Statements — switch & Ternary Operator

I. In-Class Exercises

Programming Exercises

Sample Code

II. Knowledge Summary

✨ switch Structure

When to Use

Syntax

switch Flowchart

Notes

Comprehensive Example: Day of the Week

✨ Ternary Operator

What is the Ternary Operator

Syntax

Notes

Comprehensive Example: Check Odd or Even

III. Homework

Programming Exercises

Knowledge Quiz