Switch statement within while loop in C

问题:

There are several postings concerning switch statements within while loops, except for the fact that none of them are done in C, at least from what I've seen. C++ can create boolean expressions, which I'm aware of, but not in C. I have a while loop that contains a switch control. However, when I write break statements within my switch, it goes back to the beginning of the loop and makes my program run forever. Ignore the functions I use, for they work for sure. I just need some clarification on my handling of the nesting. Thanks!

Here is my main.c:

while(1)
{
    printf("0) Exit\n1) List Tasks\n2) Add Task\n");                                            
    printf("3)Delete Task\n4) Add Task From File\n");                                           
    printf("What would you like to do?\n");
    fgets(buf1, 50, stdin);                                                                     
    p = atoi(buf1);
    switch(p)
    {
          case 0: 
            break;
          case 1: 
            printTaskList(pTaskList);
            break;
          case 2:
            printf("Enter task name: ");
            fgets(buf2,100,stdin);
            printf("Enter task priority: ");
            fgets(buf3,100,stdin);
            printf("Enter task start date: ");
            fgets(buf4,50,stdin);
            pTask = makeTask(buf2,buf4,buf3);
            addTaskToEnd(pTaskList,pTask);
            break;
          case 3:
            printTaskList(pTaskList);
            printf("What task would you like to delete? ");
            fgets(buf6,50,stdin);
            taskNum = atoi(buf6);
            removeTask(pTaskList,taskNum);
            break;
          case 4:
            printf("Enter the filename ");
            fgets(buf7,50,stdin);
            break;
          default:
            printf("ERROR: %d: Incorrect menu option\n", p);
     }
}

回答1:

break; will exit out of the nearest enclosing switch or loop. To jump two levels, you'll have to use the dreaded goto, or reorganize so a return will accomplish the desired semantics.

while(1) {
    switch(x) {
    case 0: goto EndWhile;
    }
}
EndWhile: ;

Or

void loop() {
    while(1) {
        switch(x) {
        case 0: return;
        }
    }
}
//...
loop();
//...

You can totally use a Boolean expression in C. Just use an int.

int quit = 0;
while(!quit) {
    switch(x) {
    case 0: quit = 1; break;
    }
}

C also has a boolean data type, if you really want it.

#include <stdbool.h>

bool quit = false;
while(!quit) {
    switch(x) {
    case 0: quit = true; break;
    }
}

And one more option, for the totally insane.

#include <setjmp.h>

jmp_buf jbuf;
if (!setjmp(jbuf))
    while(1) {
        switch(x) {
        case 0: longjmp(jbuf, 1);
        }
    }

回答2:

C++ can create boolean expressions, which I'm aware of, but not in C.

Oh? So what is 0 == 0, if not a boolean expression? If you're referring to a variable of the type bool, make sure you #include <stdbool.h>.

However, when I write break statements within my switch, it goes back to the beginning of the loop and makes my program run forever. Ignore the functions I use, for they work for sure. I just need some clarification on my handling of the nesting. Thanks!

No. It doesn't go back to the beginning of the loop. Execution continues from the end of the switch statement (immediately after the switches closing brace, }). In your circumstance, this is functionally identical to going back to the beginning of the loop.

There are a few of options available to you. Others are suggesting using a sole variable to control your loop. This would be a good idea, but they're suggesting using new variables, despite the fact that the variable already exists in your logic. If you want to solve your problem this way, consider:

do {
    /* ... */
} while (p != 0);

Alternatively, return from main when you want to exit, or call exit(0); from other functions. This isn't all that helpful if you want to run extra cleanup/output code.

In this scenario, the switch control structure isn't really any better than a chain of if/else if/else branches. If you were to use the if/else if/else branches, you'd be able to break; out of the loop more easily. I'd use a switch control structure when I want execution to flow through into other cases.

As another option, you could use a goto expression to break out of the loop to a label outside of main. This would be best when there are multiple possible exit points from the loop, such as handling and cleanup following allocation errors.

回答3:

You can have boolean expressions in C too. The C standard from 1999 has a stdbool.h header and a data type bool. In older C dialects, such as the one in Visual Studio 2012 (!), there is no boolean data type, so you need to use plain ints instead:

int keep_looping = 1;
while (keep_looping) {
    ....
    if (....)
        keep_looping = 0;
}

回答4:

You never exit the loop. The break will just break your switch. You should replace

while(1)

with something else that implies that the while-loop should be exited

回答5:

Your break only leaves the switch statement, then continues on in the

while(1)

loop forever.

回答6:

use exit(0);
wherever you want to exit.

In this partcular case you have to put the condition in the while loop parameter

other ways to come out from the while loop might be to use: return statement. But this will also exit the current function.

Appropriate solution might be to use goto label; inside the switch and then use label: statement; outside the while loop.

Eg:

while(1){
  switch(x){

   case 0:goto label;

   default:break;
   }
}
label:
printf("out of while loop");