What is the difference between putting $DB::single=1 and $DB::single=2 in your code? Both seem to have the identical effect of halting execution at the statement following the assignment when I do a 'c' on the perl debugger command line.
perldebug says that a value of 1 is equivalent to having just pressed 's' to get to the next statement, and 2 is the same as 'n', but what difference does it make how you got to the statement?
From perldebug:
If you set $DB::single to 2, it's equivalent to having just typed the n command (which executes over subroutine calls), whereas a value of 1 means the s command (which enters subroutine calls).
That much you know already.
From a user point of view, I'm pretty sure there is no difference. I base this on an examination of the actual DB.pm source code.
Let's follow this logically. You may want to refer to the source code. I've simplified some of the code to remove unnecessary detail but you should be able to get the idea from my descriptions.
When you are executing code in the debugger, there are (at least) two important variables, running and single. The combination of these is what decides whether code is run:
running single description
------- ------ -----------
0 ? not running
1 0 running flat-out
1 1 single stepping, execute into function
1 2 single stepping, execute over function
The DB() function is executed for every single line, and it contains the following snippet which will stop the running if single has been set (it always executes the current line regardless):
if ($DB::single) {
$DB::single = 0;
$running = 0;
}
That's why, if you set the variable in your Perl code, it will break (by break, I mean "stop running the code", not "damage somehow") the debugger at the next line.
When running is 0, the DB() function enters this little loop:
# Now sit in an event loop until something sets $running
do {
$c->idle; # call client event loop; must not block
} until $running;
In other words, it waits on a user command which sets running back to 1. This can be done by one of the following three functions:
sub next {
$DB::single = 2;
$running = 1;
}
sub step {
$DB::single = 1;
$running = 1;
}
sub cont {
$DB::single = 0;
$running = 1;
}
You can see that these three commands set up a different combination of single and running which will be used while executing the next Perl line (see the earlier table to see what these combinations mean).
The ability to use either 1 or 2 in your Perl code is a direct result of the fact that you're using a sneaky but clever trick to break execution from your Perl code itself, by setting a variable that would normally be set by a debugger command.
That's why it's not the value that matters so much as the fact you're forcing the debugger into a particular state.
Related
In the fish shell the syntax for conditionals is hard to find. Does anyone have a link to an explanation of how to write an if, with ands and ors?
In particular I would like to write
if not $status
do a command
end
To do a command when the previous command returned a non-success. How do I do that?
See http://fishshell.com/docs/current/commands.html#if and http://fishshell.com/docs/current/tutorial.html#tut_conditionals.
Fish's if structure looks like this:
if COMMAND
# do something if it succeeded
else
# do something if it failed ($status != 0)
end
Then there are also the not, and and or commands, that you can use like
if not COMMAND1; or COMMAND2
# and so on
If you really want to test a variable (e.g. $status), you need to use test as the command, like
if test $status -eq 0
Do keep in mind that $status changes after every command, so if you need to use the status of an earlier command (common in prompts) you need to do what Joachim Pileborg said, save it to another variable.
Also, test has some issues with quotes (because it's one of the few parts of fish to adhere to POSIX) - if in test $foo -eq 0 $foo is undefined, test will error out, and if it is undefined in test -n $foo, the test will be true (because POSIX demands that test with one argument be true).
As a sidenote, in fish versions before 2.3.0, you need to add begin and end around a condition with and or or because it was interpreted weirdly.
So you'd have to do
if begin COMMAND; or COMMAND2; end
# do something for status = 0
The shortest short form would be
the_previous_command; or do_a_command
# ..................^^^^^
Assuming you get your $status from "the_previous_command"
I use the status variable to display it on the prompt if it's non-zero. For that I use the following function:
function __pileon_status_prompt
set -l __status $status
if test $__status != 0
printf '[%s%s%s]' (set_color red) $__status (set_color normal)
end
end
As you can see I set a local variable to the value of $status and the check that variable in the condition.
I have made two different programs in QBasic and they both are saved in different .bas files, i.e one is 1.bas and the other 2.bas.
How to open program 1.bas while I am in program 2.bas, without closing it?
Program 1 should run inside program 2 for some time, and when it ends I should again be in program 2. Is there any way to do that?
I would like to know if there is a syntax for this that works in QBasic and/or QB64.
In Qbasic you can use CHAIN command to pass control to another .BAS file and when it is finished it will return to the first .BAS file. You can combine it with COMMON to also share variables between the two programs.
You could also use RUN but in QBasic you can't pass variables (not sure but I think the control will not return). And in QB64 it is possible to pass variables using RUN
See the standard COM1_EX.BAS and COM2_EX.BAS as an example, contents of COM1_EX.BAS:
' == COM1_EX.BAS - COMMON statement programming example ==
DIM Values(1 TO 50)
COMMON Values(), NumValues
PRINT "Enter values one per line. Type 'END' to quit."
NumValues = 0
DO
INPUT "-> ", N$
IF I >= 50 OR UCASE$(N$) = "END" THEN EXIT DO
NumValues = NumValues + 1
Values(NumValues) = VAL(N$)
LOOP
PRINT "Leaving COM1_EX.BAS to chain to COM2_EX.BAS"
PRINT "Press any key to chain... "
DO WHILE INKEY$ = ""
LOOP
CHAIN "com2_ex"
contents of COM2_EX.BAS:
' == COM2_EX.BAS - COMMON statement programming example ==
' Notice that the variables Values() and NumValues from COM1_EX
' will be called X() and N here in COM2_EX
DIM X(1 TO 50)
COMMON X(), N
PRINT
PRINT "Now executing file com2_ex.bas, reached through a CHAIN command"
IF N > 0 THEN
Sum = 0
FOR I = 1 TO N
Sum = Sum + X(I)
NEXT I
PRINT "The average of the values is"; Sum / N
END IF
I get zero for average sum. It does not pass the array values
Formerly, I used the Qbasic "Join" command successfully for my lengthy Structural programs but it never returned to the original program. In Structural Designing, you have to create a Member Matrix,Property Matrix,Do some Matrix Multiplications, and Load Matrix. Using another program to Inverse for the resulting Matrix. The "Chain" command in any Basic software does not work at all. There ought to be a command to run one program and go to execute another program and return the values/Text it created. As I am running Win 10, 64 bit and I'm not a Comp. expert,
I am currently writing a QBASIC program that runs an indefinite loop (while loop). However, if a certain condition is met, I want to exit the program. What command do I use, and also what is the syntax.
Thanks
ENDexits program, and clears all variables, which frees up memory.
STOPexits program, but retains the value of all variables, which makes it possible (in certain versions of QB) to continue execution at another point, by choosing Set next statement from the Debugmenu, and then Startfrom the Runmenu. END has the same effect as STOP + choosing Restart from the Runmenu once the program has terminated.
If you have a loop, and want to exit the program from inside it, you may use either
DO
IF condition THEN EXIT DO
LOOP
END
or
DO
IF condition THEN END
LOOP
You're looking for the END or SYSTEM statement. For example:
PRINT "Hello World!"
END
PRINT "This won't be printed."
If you're using regular old QBASIC/QuickBASIC, then you can ignore all of the QB64 details on the linked pages and just use either SYSTEM or END. Both will do the same thing for the most part.1
If you're using FreeBASIC, it's recommended to use END instead of SYSTEM since some things won't get cleaned up properly when you use SYSTEM. See SYSTEM for more information pertaining to FreeBASIC if that's what you're using.
1 The END statement when running the program using QB.EXE /RUN PROGRAM.BAS will print "Press any key to continue" before exiting to the QB/QBASIC environment. The SYSTEM statement when run the same way will simply return you to the DOS shell without any need for a key press. Also, typing SYSTEM in the "Immediate Window" of the QB/QBASIC environment will exit the environment and return to the DOS shell. Otherwise the two statements behave exactly the same in QB/QBASIC, whether for standalone (compiled) programs or .BAS modules.
You can keep any condition according to the need of your program. For eg:
CLS
LET a = 5
WHILE a > 0
PRINT a;
a = a - 1
WEND
END
Here, in the program while wends executes itself until a = 0. This will not run an infinite loop.
The answer is
exit();
to exit the program.
If I include a continue as a parameter of an eval() instruction, it does not work as expected. For example, when executing the following code:
listParam = {'a','b','c'};
a = 17;
b = NaN;
c = 4;
for ii=1:numel(listParam),
eval(['if isnan(',listParam{ii},'), continue; end']);
disp(['The parameter ', listParam{ii}, ' is not a NaN.']);
end
It will prompt the error:
Error: A CONTINUE may only be used within a FOR or WHILE loop.
Anybody knows why?
Note: I know that I should avoid eval() and the previous example could be refactored in a much better coding; but I found a weird behaviour and I am curious what is happening.
The expression you pass to eval must be a valid matlab expression on it's own, any surrounding code is not considered. This means each continue must be surrounded by a loop. Either put the surrounding for within your eval or put the continue outside.
As #Daniel has pointed out, eval is called by the script, while it is not directly controlled by the for loop. You can think it as: the continue in eval would move the program counter to the head of the code inside eval, but not that of the for loop; this of course would fail, as Matlab does not allow jumping between lines.
A continue can only appear directly inside a for or while loop. However, you can hack the code like this:
for ii=1:numel(listParam),
eval(['if isnan(',listParam{ii},'), x=1; else, x=0; end']);
if x
continue;
end
disp(['The parameter ', listParam{ii}, ' is not a NaN.']);
end
Strangely, x happens to appear in the script's stack. However, this is far from a good piece of code. Never use this method.
Edit: about the "control" scope of eval.
I'm not talking about the variable scope / workspace of eval. Hints can be found in several documentations like this and this. In short, eval uses the "current" workspace.
However, I found the following things interesting:
Running continue directly in eval
for ii = 1:2
eval('continue')
ii+10
end
This simply fails, as shown in the question. The error is Error: A CONTINUE may only be used within a FOR or WHILE loop. which means the continue inside eval cannot find any loop (the for loop).
Calling a separate script in a for loop
for ii = 1:2
foo
ii+10
end
while script foo.m is
ii
continue
First of all, in foo.m Mlint gives a red-line warning, which usually indicates an error that can stop the code from running, saying that CONTINUE is only valid in a FOR or WHILE loop. But pressing F5 on foo.m does not have any problem - in fact running a single continue anywhere does not crash the code.
Running the main script gives output
ii =
1
ii =
2
>>
....so foo.m catches the for loop?
eval('foo') - I really don't understand Matlab
for ii = 1:2
eval('foo')
ii+10
end
The result surprised me a bit.
ii =
1
ans =
11
ii =
2
ans =
12
>>
Thought: eval runs the code in an independent control flow, but shares its workspace with the current one. It is (something but not exactly) like an isolated world model, in which (here in Matlab) different pieces of code can interact with the same set of variables, but cannot interact with each other in the sense of control flow.
Unfortunately, I was unable to find any existing resources as reference to prove this idea.
This does not change my solution: in any case, try to avoid using eval.
I am working on some old qbasic code. It's a mess with all the Goto statements. Am I correct that the following line will always return?
IF FLAG = 0 THEN TARGET = X: GOSUB 55000: TEMP = XI - TEMP2: RETURN
So if I understand this correctly the colon separates statements on the same line. The if only pertains to TARGET = X. The GOSUB, TEMP =, and RETURN will always execute. Correct?
Part of my confusion is because the very next line reads
IF FLAG = 1 THEN STEP = X: GOSUB 115000
And since the label to the second statement is never used in a GOTO I can't see that it would ever get executed.
Yes, I believe your assessment is correct. The colon is a statement separator that lets you have multiple statements on the same line. Assuming your subroutine at 55000 returns, this line should return as well.
I was wrong. Running this program:
if 1=2 then print "Never printed" : print "how about this?"
print "End of program"
on qb64.net prints only End of program. I assume that its grammar details are the same as Qbasic's, although it is a reverse-engineered effort.
As an aside, this code is written in a pre-QBasic style (e.g. using GOSUB and line numbers). There is a script that often came with QBasic (remline.bas, I believe it was called) that is supposed to help translate these kinds of programs to a newer style. I have never used it myself, though.