Thread: could someone please explain switch cases?

switch is like if, but when u have to check for more possibilities, it would be hard, so u use switch

syntax:

switch(a)
{
a_1 : do_1
a_2 : do_2
a_3 : do_3
...
a_n : do_n
}

example, a simple prog (u have to input a number between 1 and 5)

Code:

#include <stdio.h>

int main()
{
 int a;
 printf("Please input a number (1-5): ");
 scanf("%d", &a);

 switch(a){
     case 1: printf("one\n");
              break;
     case 2: printf("two\n");
              break;
     case 3: printf("three\n");
              break;
     case 4: printf("four\n");
              break;
     case 5: printf("five\n");
              break;
 }
 return 0;
}

u use break to stop the prog going on checking the other cases in the switch



another example:

Code:

#include <stdio.h>

int main()
{
 int a;
 printf("Please input a number (1-5): ");
 scanf("%d", &a);

 switch(a)
   {
     case 1: printf("one or...");
     case 2: printf("...two \n");
             break;
     case 3: printf("three\n");
              break;
     case 4: printf("four...");
     case 5: printf("...or five \n");
              break;
   }
return 0;
}

What happens, if someone inputs a number: n<1 or n>5

here another example:

Code:

#include <stdio.h>

int main()
{
 int a,b;
 char opera;
 printf("Calculating \n");
 printf(" (number)(operator)(number) no spaces\n");

 printf("Input your numbers: ");
 scanf("%d%c%d", &a, &opera, &b);

 switch(opera){
     case '+': printf("%d + %d = %d \n",a ,b ,a+b);
                break;
     case '-': printf("%d - %d = %d \n", a, b, a-b);
                break;
     case '*': printf("%d * %d = %d \n", a, b, a*b);
                break;
     case '/': printf("%d / %d = %d \n", a, b, a/b);
                break;
     default: printf("%c : no operator\n", opera);
   }
 return 0;
}

When somebody inputs nothing or something wrong insteads of the operators (+,-,/,*) the prog uses the default case, and outputs an error.

When ur c++ programmer, u can also use cout instead of printf and cin instead of scanf

here the source (c++) using cout and cin

Code:

#include <iostream>
using namespace std;

int main()
{
 int a;
 cout << "Please input a number (1-5): ";
 cin >> a;

 switch(a){
     case 1: cout << "one\n";
              break;
     case 2: cout << "two\n";
              break;
     case 3: cout << "three\n";
              break;
     case 4: cout << "four\n";
              break;
     case 5: cout << "five\n";
              break;
 }
 return 0;
}

#######################

Code:

#include <iostream>
using namespace std;

int main()
{
 int a;
 cout << "Please input a number (1-5): ";
 cin >> a;

 switch(a)
   {
     case 1: cout << "one or...";
     case 2: cout << "...two \n";
             break;
     case 3: cout << "three\n";
              break;
     case 4: cout << "four...";
     case 5: cout << "...or five \n";
              break;
   }
return 0;
}

######################

Code:

#include <iostream>
using namespace std;

int main()
{
 int a,b;
 char opera;
 cout << "Calculating \n";
 cout << " (number)(operator)(number) no spaces\n";

 cout << "Input your numbers: ";
 cin >> a >> opera >> b;

 switch(opera){
     case '+': cout << a << "+" << b << "=" << (a+b);
                break;
     case '-': cout << a << "-" << b << "=" << (a-b);
                break;
     case '*': cout << a << "*" << b << "=" << (a*b);
                break;
     case '/': cout << a << "/" << b << "=" << (a/b);
                break;
     default: cout << opera << ": no operator\n";
   }
 return 0;
}

as an interesting tidbit, I believe the target value of the a switch statement must resolve to an integer. That is in :

switch(target)

target must be evaluated as an int.

That means basing the switch on something like a string, as below, won't work:

Code:

cout << "who is the most famous person of the twentieth century" << endl;
char name[80];
cin.getline(name, 79);

switch(name)
{
  case "Albert Einstein":
    cout << "Yup" << endl;
    break;
  case "Klem Kaddidlehopper":
    cout << "Not really" << endl;
    break;
  default:
    cout << "it's all a matter of opinion anyway" << endl;
    break;
}

The above code would need to be done with series of if/else statements, or create a menu to select from, or some other intermediary type code.

a switch statement is not exactly

Code:

if (x == 'c') do_something();

Instead, it looks like this:

Code:

if (x == 'c') goto label_c;
else if (x == 'd') goto label_d;
else goto default;
{
label_c:
  do_something();
  break;
label_d:
  something_Else();
  break;
default:
  break;
}

This means that goto's are used, which can be a bad thing when declaring local variables.

Code:

label_a:
  int a;
label_b:
  ++a;

What would happen if there was a goto label_b command? The compiler would generate an error. A solution to this is to surround all switch case labels with brackets.

Code:

label_a:
{
  int a;

}
label_b:
  ++a;  //this is still an error, but at least now it's obvious: it's out of scope

Regardless of how you use them, inherently they jump around a lot which is not good. In assembly they would be quite a mess and they can get messy even in C but that is not to say that you should never use them. Just be aware of the overhead you might be adding to your code.

Code:

switch(x)
{
  case 1:  DoSomething(); break;
  case 2:  DoSomethingElse();break;
}

A terrible example in assembly - not at all optimized so no complaints please - I just coded it off the top of my head.

Code:

  mov       ax,[x]

COMPARE:
  cmp       ax,1
  jz        FIRSTRESULT
  jmp       COMPAREAGAIN

FIRSTRESULT:
  call      DoSomething
  jmp       ENDOFCOMPARE

COMPAREAGAIN:
  mov       ax,[x]
  cmp       ax,2
  jz        SECONDRESULT
  jmp       ENDOFCOMPARE

SECONDRESULT:
  call      DoSomethingElse

ENDOFCOMPARE:

Again, a crude example but I'm not programming a compiler here.