Breaking up from while loop in lex - c

Question

so I have using lex tool in linux and fell into an embarrassing position position , I couldn't break from a while loop.
e.g : I wrote ;
while(1)
{
int x = yylex();
switch(x):
case(ID):printf("ID");
case(NUM):printf("NUM");
}
Now I am reading from a file using yyin ; the problem is that the while loop does not fail after reading the whole file and keeps asking for more inputs at the terminal . Therefore I could not invoke other functions after calling the while loop . I know I am missing something basic , it will be great if somebody can provide an insight into this problem .
P.S# Thanks all for your answers ; tension and lack of sleep before homework submission date ; btw - I figured my answer already .

Answer 1

I see some issues with the C code, independent of what's going on inside the yylex() function. You quote the code as:
while(1)
{
int x = yylex();
switch(x):
case(ID):printf("ID");
case(NUM):printf("NUM");
}
This clearly isn't the source you compiled because it is not acceptable as C. You need to replace the first colon (after switch(x)) with an open brace {, and you need another close brace } at the end, to yield:
while(1)
{
int x = yylex();
switch(x)
{
case(ID):printf("ID");
case(NUM):printf("NUM");
}
}
This is syntactically valid C code, but there are still problems:
There should probably be a break; after each case.
There should probably be a default: clause, which might be used to terminate the loop (but you can't use break to do that inside the scope of a switch).
The printf() statements should print a newline so that the data appears.
Since yylex() returns 0 when it reaches the end, you should probably revise the code so it is more like:
void function(void)
{
int x;
while ((x = yylex()) != 0)
{
switch (x)
{
case ID:
printf("ID\n");
break;
case NUM:
printf("NUM\n");
break;
default:
printf("Other: %d\n", x);
break;
}
}
}
This will at least show you what's going on, and does not have an infinite loop unless you've written your lex analyzer such that it does not terminate properly. I opted not to have the default case terminate the loop since the loop is bounded by yylex() returning 0 instead.
When you write while (1), you are writing an infinite loop. It is incumbent on you to consider whether the loop is really infinite or not. If it is not, then you should aim to be able to make a test at the top of the loop that controls whether there is another cycle to the loop or not.

Answer 2

yylex() returns 0 at the logical end of input. Try this:
int x = -1;
while(!(x == 0))
{
x = yylex();
switch(x):
{
case(0): printf("that's all, folks!"); break;
case(ID): printf("ID"); break;
case(NUM): printf("NUM"); break;
}
}

Answer 3

Well, while I'll admit I'm not familiar with what lex is, I do know quite a bit about the linux environment and the terminal, so...
Try typing 'Ctrl' and 'c' at the same time while in the terminal. If that doesn't work, use 'Ctrl' + 'z'. If you don't mind having the process stopped in the background until the computer turns off and it goes away entirely, after Ctrl + Z you don't need to do anything else. If Ctrl + C worked, then the program's ended and you're fine.
If, for whatever reason, neither work, if you know the name of the program you wrote, you can type into the terminal:
pkill -9 [insert name of program here]
...which will send an unignorable kill signal to the process, and return all its memory (RAM- it won't undo changes made to disk) to the parent process.
If you DON'T know the name of the program, but you think you'll probably be able to recognize it, you can try:
top
which will bring up all the currently running processes in the terminal. Once you locate the name of your still-running program, press q to exit top, and then type in the kill command above with the name you recognized in top. If you don't know the program's name, and Ctrl + Z worked, but you don't want the stopped job to keep hogging its memory until you've turned off the computer, you can type:
ps -a
while in the terminal session from which you ran (and stopped) the program, and the job which shows up labeled 'stopped' is, in all likelihood, yours. Again, using the pkill command above will kill it. If, for whatever reason, you know the process ID but not the name (I don't know why you would, but there you go...), you could use the command 'kill' instead of 'pkill' (instructions for how to use these commands are all over the internet so I won't mention them here. They also have helpful help manual pages which can be accessed via:
man [the name of the program you want help with; 'pkill' or 'kill', in this case.]

Answer 4

To add to David's answer, here's a different version:
do
{
int x = yylex();
if (!x)
break;
switch(x):
{
case(ID): printf("ID"); break;
case(NUM): printf("NUM"); break;
}
} while(1);
This was only to show that you can break out of a while(1). You keep the x in local scope, and you don't evaluate the first pass or switch unnecessarily. Of course someone will most likely pull out the trump card of "it isn't as readable"...