I have a question, I have been reviewing some code and in one script, the authors use:
if(0){
#do something
}
Any help in what if(0) means?
The author (most likely) put the block of code in an if statement so that they could easily remove it if necessary without having to comment it out (or remove it). Similar to if(true) or if(false), you just need to change one value and it would skip that code.
Upon reviewing the code, developers should remove these kinds of statements once they've finalized all their source code not to confuse others.
Looks like something that will never be executed, since 0 = FALSE. Most probably this is a manual switch to test some code in parenthesis.
Related
zsh version 5.2
I'm attempting an array assignment using filename generation like so:
files=(/some/path/*/dir/myfile)
Indeed this is the way the zshoptions manual recommends to achieve what I want.
When no matches exist I want the array to be empty. Instead it's producing
no matches found: /some/path/*/dir/file
and the script terminates.
I've tried setting NULL_GLOB, CSH_NULL_GLOB and ensured NOMATCH is not set.
When matches do exist it works as expected.
Any help is appreciated.
Thank you in advance,
Wayne
Well of course I found the solution after posting my question.
For this to work EXTENDED_GLOB needs to be set as well as NULL_GLOB. Or a glob qualifier can be used so that NULL_GLOB only effects this particular expansion.
This is how to set NULL_GLOB for a single operation:
files=(/some/path/*/dir/myfile(N))
Hope that can help someone else who encounters this.
Wayne
I wonder if there is a way to display the current time in the R command line, like in MS DOS, we can use
Prompt $T $P$G
to include the time clock in every prompt line.
Something like
options(prompt=paste(format(Sys.time(), "%H:%M:%S"),"> "))
will do it, but then it is fixed at the time it was set. I'm not sure how to make it update automatically.
Chase points the right way as options("prompt"=...) can be used for this. But his solutions adds a constant time expression which is not what we want.
The documentation for the function taskCallbackManager has the rest:
R> h <- taskCallbackManager()
R> h$add(function(expr, value, ok, visible) {
+ options("prompt"=format(Sys.time(), "%H:%M:%S> "));
+ return(TRUE) },
+ name = "simpleHandler")
[1] "simpleHandler"
07:25:42> a <- 2
07:25:48>
We register a callback that gets evaluated after each command completes. That does the trick. More fancy documentation is in this document from the R developer site.
None of the other methods, which are based on callbacks, will update the prompt unless a top-level command is executed. So, pressing return in the console will not create a change. Such is the nature of R's standard callback handling.
If you install the tcltk2 package, you can set up a task scheduler that changes the option() as follows:
library(tcltk2)
tclTaskSchedule(1000, {options(prompt=paste(Sys.time(),"> "))}, id = "ticktock", redo = TRUE)
Voila, something like the MS DOS prompt.
NB: Inspiration came from this answer.
Note 1: The wait time (1000 in this case) refers to the # of milliseconds, not seconds. You might adjust it downward when sub-second resolution is somehow useful.
Here is an alternative callback solution:
updatePrompt <- function(...) {options(prompt=paste(Sys.time(),"> ")); return(TRUE)}
addTaskCallback(updatePrompt)
This works the same as Dirk's method, but the syntax is a bit simpler to me.
You can change the default character that is displayed through the options() command. You may want to try something like this:
options(prompt = paste(Sys.time(), ">"))
Check out the help page for ?options for a full list of things you can set. It is a very useful thing to know about!
Assuming this is something you want to do for every R session, consider moving that to your .Rprofile. Several other good nuggets of programming happiness can be found hither on that topic.
I don't know of a native R function for doing this, but I know R has interfaces with other languages that do have system time commands. Maybe this is an option?
Thierry mentioned system.time() and there is also proc.time() depending on what you need it for, although neither of these give you the current time.
I have a problem with my code. It doesn't print out an error, it works but it doesn't do what I actually need it to do. I won't be posting all of the code because it is not needed, I will just post the line we need.
So I have this line of code:
TIME1=$( { time awkfun ; } 2>&1 >/dev/null);
Where awkfun is a function that should print 500 integers, and TIME1 is a variable to store the time that the function will need to run and print. Problem is that normally as I said the function would print around 500 integers in this occasion though that I am using this, it just runs and does the calculations but is not printing. So it actually runs the awkfun function but doesn't let it print, in the time output for this function I also need the time it needs to print everything.
How to do that?
I hope I explained it as good as possible, if any questions arise please don't hesitate to comment, thanks in advance!
P.S
Running in ksh in oracle solaris.
Here is the correct sequence. Adjust as needed:
TIME1=`time (awkfun > /dev/tty) 2>&1`
I am new to programming with no formal programming training, so please excuse this newbie question. I can't seem to find a definitive answer on it, and need to be sure.
Here's the scenario: sometimes I need to try different lines of code to see if they work, but still want to keep the original lines in there in case the new lines don't work (or for future reference). I thought by putting // in front of the line I want to "comment out" would mean the line of code would be ignored when compiling the program, but my app crashed and pointed to a line I thought was commented out. I am using Xcode 4.
Is there another symbol I should be using to comment out lines or blocks of code? For example, will /* at the beginning and */ at the end accomplish this?
For "commenting out" big blocks of code, I do conditionals like:
#if I_AM_TESTING_SO_DONT_COMPILE_THIS
...
...
...
...
#endif // I_AM_TESTING_SO_DONT_COMPILE_THIS
And yes, the traditional C-style /* and */ also work to "comment out" blocks of code.
The reason your compile crashed with // is because while a single line gets commented out, you might have had some extra code on a second line.
Such as:
//NSString * someContentFromSomewhere = [[NSString alloc] initWithContentsOfURL:
[NSURL URLWithString: #"http://www.stackoverflow.com"]];
The first line is commented out. The second line is a syntax error.
Also, make sure you do a clean build. It's possible that sometimes old object files are used and even though you commented out a line in the IDE, it may not have build with that updated file.
This question already has answers here:
Multiline Comment Workarounds?
(11 answers)
Closed 9 years ago.
I found this old thread (from over a year ago), which explains how come R doesn't support a multi-line comments (like /* comment */ of PHP, for example).
I am wondering if this has been resolved in the past year, or if there are other alternatives? (For example, in notepad++ with npptor, you can mark a bunch of lines and press ctrl+q to mark them all as comments, are there similar solutions for other IDE's ?)
R Studio (and Eclipse + StatET): Highlight the text and use CTRL+SHIFT+C to comment multiple lines in Windows.
For macOS, use command+SHIFT+C.
You can, if you want, use standalone strings for multi-line comments — I've always thought that prettier than if (FALSE) { } blocks. The string will get evaluated and then discarded, so as long as it's not the last line in a function nothing will happen.
"This function takes a value x, and does things and returns things that
take several lines to explain"
doEverythingOften <- function(x) {
# Non! Comment it out! We'll just do it once for now.
"if (x %in% 1:9) {
doTenEverythings()
}"
doEverythingOnce()
...
return(list(
everythingDone = TRUE,
howOftenDone = 1
))
}
The main limitation is that when you're commenting stuff out, you've got to watch your quotation marks: if you've got one kind inside, you'll have to use the other kind for the comment; and if you've got something like "strings with 'postrophes" inside that block, then there's no way this method is a good idea. But then there's still the if (FALSE) block.
The other limitation, one that both methods have, is that you can only use such blocks in places where an expression would be syntactically valid - no commenting out parts of lists, say.
Regarding what do in which IDE: I'm a Vim user, and I find
NERD Commenter an utterly excellent tool for quickly commenting or uncommenting multiple lines. Very user-friendly, very well-documented.
Lastly, at the R prompt (at least under Linux), there's the lovely Alt-Shift-# to comment the current line. Very nice to put a line 'on hold', if you're working on a one-liner and then realise you need a prep step first.
CTRL+SHIFT+C in Eclipse + StatET and Rstudio.
if(FALSE) {
...
}
precludes multiple lines from being executed. However, these lines still have to be syntactically correct, i.e., can't be comments in the proper sense. Still helpful for some cases though.
No multi-line comments in R as of version 2.12 and unlikely to change. In most environments, you can comment blocks by highlighting and toggle-comment. In emacs, this is 'M-x ;'.
Put the following into your ~/.Rprofile file:
exclude <- function(blah) {
"excluded block"
}
Now, you can exclude blocks like follows:
stuffiwant
exclude({
stuffidontwant
morestuffidontwant
})
Unfortunately, there is still no multi-line commenting in R.
If your text editor supports column-mode, then use it to add a bunch of #s at once. If you use UltraEdit, Alt+c will put you in column mode.