LESSON 13/53Making Decisions

switch and ? :

GOALWrite a clean choice when one value is compared against many.

White light switch — Santeri Viinamäki · CC BY-SA 4.0

When one variable is compared against many exact values, a chain of else if gets long. switch is made for exactly that.

ON THIS PAGE

The example
Line by line
Watch it run
When to reach for switch
The ternary operator ? :
Common mistakes
Key words
Quick check
Now you try

SHOW EXAMPLE IN

//THE EXAMPLE

main.cpp

#include <iostream>
using namespace std;

int main() {
    int day = 7;

    switch (day) {
        case 6:
        case 7:
            cout << "Weekend" << endl;
            break;
        case 1:
            cout << "Monday again" << endl;
            break;
        default:
            cout << "School day" << endl;
    }

    int age = 20;
    cout << (age >= 18 ? "adult" : "child") << endl;

    return 0;
}

> what you see

Weekend
adult

Try this code

//LINE BY LINE

switch (day) {

Compares the value of day against each case below.

case 6:case 7:

case 6 has no break, so 6 and 7 both fall through to the same result.

break;

Leaves the switch. Forget it and every case below runs as well.

default:

Runs when no case matched. It is the else of a switch.

age >= 18 ? "adult" : "child"

The ternary operator: "condition ? if-true : if-false". A one-line if/else.

Watch it run

n = 1. The switch matches n against each case, looking for the one that fits.

1 / 6

When to reach for `switch`

Good fit: one variable compared against many exact values (a menu, a day number, a letter grade).
Poor fit: ranges (score > 90), decimals, or comparing strings.

The ternary operator `? :`

When there are only two outcomes and both are short, they fit on one line.

string label = (age >= 18) ? "adult" : "child";

// ижил утгатай:
string label2;
if (age >= 18) label2 = "adult";
else           label2 = "child";

Do not nest ternaries. Past two outcomes, if/else if reads far better.

//COMMON MISTAKES

✗ wrong

switch (score) {
  case score > 90:

✓ right

if (score > 90) { … }

A case cannot hold a condition, only an exact value. Use if for ranges.

✗ wrong

case 1:
  cout << "one";
case 2:

✓ right

case 1:
  cout << "one";
  break;
case 2:

Without break the cases below run too, and you get two answers.

//KEY WORDS

case

One exact value to match. It must be a constant.

break

Leaves the switch. You almost always need it.

//QUICK CHECK

What happens if you forget break inside a switch?

//NOW YOU TRY

Read a number 1–7 and print the day's name using switch.

Open editor

Combining Conditions for Loops

All lessons

LESSON 13/53Making Decisions

switch and ? :

GOALWrite a clean choice when one value is compared against many.

When one variable is compared against many exact values, a chain of else if gets long. switch is made for exactly that.

ON THIS PAGE

The example
Line by line
Watch it run
When to reach for switch
The ternary operator ? :
Common mistakes
Key words
Quick check
Now you try

SHOW EXAMPLE IN

//THE EXAMPLE

main.cpp

#include <iostream>
using namespace std;

int main() {
    int day = 7;

    switch (day) {
        case 6:
        case 7:
            cout << "Weekend" << endl;
            break;
        case 1:
            cout << "Monday again" << endl;
            break;
        default:
            cout << "School day" << endl;
    }

    int age = 20;
    cout << (age >= 18 ? "adult" : "child") << endl;

    return 0;
}

> what you see

Weekend
adult

Try this code

//LINE BY LINE

switch (day) {

Compares the value of day against each case below.

case 6:case 7:

case 6 has no break, so 6 and 7 both fall through to the same result.

break;

Leaves the switch. Forget it and every case below runs as well.

default:

Runs when no case matched. It is the else of a switch.

age >= 18 ? "adult" : "child"

The ternary operator: "condition ? if-true : if-false". A one-line if/else.

Watch it run

n = 1. The switch matches n against each case, looking for the one that fits.

1 / 6

When to reach for `switch`

Good fit: one variable compared against many exact values (a menu, a day number, a letter grade).
Poor fit: ranges (score > 90), decimals, or comparing strings.

The ternary operator `? :`

When there are only two outcomes and both are short, they fit on one line.

string label = (age >= 18) ? "adult" : "child";

// ижил утгатай:
string label2;
if (age >= 18) label2 = "adult";
else           label2 = "child";

Do not nest ternaries. Past two outcomes, if/else if reads far better.

//COMMON MISTAKES

✗ wrong

switch (score) {
  case score > 90:

✓ right

if (score > 90) { … }

A case cannot hold a condition, only an exact value. Use if for ranges.

✗ wrong

case 1:
  cout << "one";
case 2:

✓ right

case 1:
  cout << "one";
  break;
case 2:

Without break the cases below run too, and you get two answers.

//KEY WORDS

case

One exact value to match. It must be a constant.

break

Leaves the switch. You almost always need it.

//QUICK CHECK

What happens if you forget break inside a switch?

//NOW YOU TRY

Read a number 1–7 and print the day's name using switch.

Open editor

Combining Conditions for Loops

switch and ? :

//THE EXAMPLE

//LINE BY LINE

Watch it run

When to reach for switch

The ternary operator ? :

//COMMON MISTAKES

//KEY WORDS

//QUICK CHECK

//NOW YOU TRY

switch and ? :

//THE EXAMPLE

//LINE BY LINE

Watch it run

When to reach for switch

The ternary operator ? :

//COMMON MISTAKES

//KEY WORDS

//QUICK CHECK

//NOW YOU TRY

When to reach for `switch`

The ternary operator `? :`

When to reach for `switch`

The ternary operator `? :`