Can someone explain this switch-case problem

[SlenderMan]

What is the output of the following code fragment if the input value is 4?

int num;
int alpha = 10;
cin >> num;
switch (num)
{
case 3: 
    alpha++;
    break;
case 4: 
case 6: 
    alpha = alpha + 3;
case 8: 
    alpha = alpha + 4;
    break;
default: 
    alpha = alpha + 5;
}
cout << alpha << endl;
A) 13
B) 14
C) 17
D) 22

A) 13
B) 14
C) 17
D) 22


I thought it was 13 but the answer is 17, can someone explain because I don't see where the answer comes from.

[o1vk4iN6]

case 4: 
case 6: 
    alpha = alpha + 3;
    //break;
case 8: 
    alpha = alpha + 4;
    break;

Case 8 is also run for 4 and 6.

[SlenderMan]

Is that because there's no break after case 6?

[tipaye]

Because you don't have a "break" statement between case 6 and case 8, execution falls through to case 8 and 4 gets added to 13.

int num;
int alpha = 10;
cin >> num;
switch (num)
{
case 3: 
    alpha++;
    break;
case 4: 
case 6: 
    alpha = alpha + 3;
  break;
case 8: 
    alpha = alpha + 4;
    break;
default: 
    alpha = alpha + 5;
}
cout << alpha << endl;