Thread: problem on switch

Originally Posted by MacGyver

Code:

scanf("%d",&options);

....

case 1: /* NOT NOT NOT NOT case '1' */

As I said, 1 is 1. '1' is 49.

oh is it like this?
i typed 1,it really ouput u choose 1
however, i found that if i type in a character
it output " u choose 2"
why?

Code:

int options;
 	  printf("\n\n\nOptions:");
 	  scanf("%d",&options);


switch (options) {
	   case 1:
	   		printf("u choose 1");
	   		break;

however, i found that if i type in a character
it output " u choose 2"
why?

This is what we call "undefined behaviour". The switch statement branch to "case 2:" because the value of option was 2 at the time of the test. Why it was 2 ? Well it could have been anything else. If you try reading a number with scanf but the user enter something else, then nothing happens to your "storing variable".

If you want the user to be able only to enter numbers, then this little piece of code will help you:

Code:

int i;
int option;

// ...

do
{
    printf("Options: ");
    i = scanf("%d", &option);
    while (getchar() != '\n');             // Empty the input buffer
} while (i != 1);

Originally Posted by foxman

This is what we call "undefined behaviour". The switch statement branch to "case 2:" because the value of option was 2 at the time of the test. Why it was 2 ? Well it could have been anything else. If you try reading a number with scanf but the user enter something else, then nothing happens to your "storing variable".

If you want the user to be able only to enter numbers, then this little piece of code will help you:

Code:

int i;
int option;

// ...

do
{
    printf("Options: ");
    i = scanf("%d", &option);
    while (getchar() != '\n');             // Empty the input buffer
} while (i != 1);

yes, i can really prevent the characters with your code
however if i type 7 (which is out of 1-6), it shows please enter again,
then no matter what i input, the exe. window close itself
it there something wrong with my switch ?

thank you very much!

Like iMalc said, you should look at the FAQ, there's useful stuff in there.

no matter what i input, the exe. window close itself

It does so because your program is structured that way. If you want it to ask for user input until the user gives a certain input, then you need to use a loop.

however if i type 7 (which is out of 1-6), it shows please enter again

Ok. Let's say you want the user to enter a number between 1 and 6 inclusively. Well it's quite simple:

Code:

int i;
int option;

// ...

do
{
    printf("Options: ");
    i = scanf("%d", &option);
    while (getchar() != '\n');             // Empty the input buffer
} while (i != 1 || option < 1 || option > 6);

That said, here is how i write a typical menu function:

Code:

int continu = 1;

// ...

do
{
    // Print menu option here
    puts("1. Do something");
    puts("2. Exit");

    do
    {
        // Read user input here (as previously shown)
    } while (i != 1 || choix < 1 || choix > 2);

    switch(choix)
    {
    case 1:
        // Do something here
        break;
    default:
        continu = 0;
        break;
    }

} while (continu);

Also, note that you can output the "spade character" this way: printf("\6");.

iMalc: You changed your avatar, right ? I think the last one was showing heapsort; is this one something like shaker sort ?