In all other programming languages I've encountered, if statements always require a boolean to work, however in Lua the following code does not contain any errors. What is being checked in this if statement if both true and false statements can't be made? How can you only check "if (variable) then"? I am new to programming and currently working with Roblox Studio, any help would be very appreciated.
function onTouched(Obj)
local h = Obj.Parent:FindFirstChild("Humanoid")
if h then
h.Health = 0
end
end
script.Parent.Touched:Connect(onTouched)
Most languages have their own rules for how values are interpreted in if statements.
In Lua, false and nil are treated as false. All other values are treated as true.
if h == nil (null)
So if it couldn't find a humanoid in the object that touched the script's parent, it will be false (null), otherwise true (not null).
So
if [ObjectName] then
equals to if [ObjectName] != null then
*Only valid for objects (non primitive values)
It's like that in script languages.
if h then end
is basically equivalent to
if h ~= nil and h ~= false then end
In Lua all values that are not nil or false are considered to be logically true.
if h then end is usually used to check wether h is not nil. So if code depends on wether h has been defined you put it in a condition like that.
In your example h is being index. indexing nil values is not allowed. So befor you index it you should make sure it isn't nil to avoid errors.
Checking return values of functions is good practice.
How can I assert that 2012/08 (instead of Aug 2012) is returned from this function?
Thus I can start learning lisp on the job/with the function itself until the output satisfies.
I know some python unit testing (pytest) and am looking for something similar for lisp. However, my first attempt[0] C-c eval-buffer fails with Invalid function: "<2012-08-12 Mon>"
(assert (= (org-cv-utils-org-timestamp-to-shortdate ("<2012-08-12 Mon>")) "Aug 2012"))
(defun org-cv-utils-org-timestamp-to-shortdate (date_str)
"Format orgmode timestamp DATE_STR into a short form date.
Other strings are just returned unmodified
e.g. <2012-08-12 Mon> => Aug 2012
today => today"
(if (string-match (org-re-timestamp 'active) date_str)
(let* ((abbreviate 't)
(dte (org-parse-time-string date_str))
(month (nth 4 dte))
(year (nth 5 dte))) ;;'(02 07 2015)))
(concat
(calendar-month-name month abbreviate) " " (number-to-string year)))
date_str))
[0] https://www.emacswiki.org/emacs/UnitTesting
(assert (= (org-cv-utils-org-timestamp-to-shortdate ("<2012-08-12 Mon>"))
"Aug 2012"))
(defun org-cv-utils-org-timestamp-to-shortdate (...) ...)
statements are executed in sequence, which means your function will only be defined after the assertion is evaluated. This is a problem because the assertion calls that function. You should reorder your code to test it after the function is defined.
you cannot compare strings with =, if you call describe-function (C-h f) for =, you'll see that = is a numerical comparison (in fact, numbers or markers). For strings you need to use string=.
In a normal evaluation context, ie. not in a macro or a special form, the following is read as a function call:
("<2012-08-12 Mon>")
Parentheses are meaningful, the above form says: call function "<2012-08-12 Mon>" with zero arguments. This is not what you want, there is no need to add parentheses here around the string.
my #g = (1,2,3,4);
say reduce {is-prime}, #g; # ==> gives error
say reduce {is-prime *}, #g; #==> gives error
say reduce {is-prime}, (1,2,3,4); # ==> gives error
say so is-prime #g.all; # ==> gives error
How to check if all elements of list are prime in Raku?
The answers above are all helpful, but they fail to explain why your solution does not work. Basically reduce is not going to apply a function (in your case, is-prime) to every member of a list. You want map for that. The error says
Calling is-prime() will never work with signature of the proto ($, *%)
Because reduce expects an infix, thus binary, function, or a function with two arguments; what it does is to apply them to the first pair of elements, then to the result and the third element, and so on. Last statement does not work for a similar reason: you are calling is-prime with a list argument, not a single argument.
You're basically asking: are there any elements in this list which are not prime? I would write that as:
say "not all prime" if #g.first: !*.is-prime;
Please note though, that apparently 1 is not considered prime according to the is-prime function:
say 1.is-prime; # False
so the first would trigger on the 1 in your example, not on the 4.
There are of course may ways to do this. A very explicit way is using a for loop:
for #g -> $g {
if $g.is-prime {
say $g;
}
}
Or with a grep (you could leave the $_ implicit):
#g.grep({ $_.is-prime }).say
Both above are assuming you really want to filter the primes out. Of course you can also really check each number and get a boolean:
#g.map({ .is-prime }).say
There is a big problem with this:
say reduce {is-prime}, #g;
You created a lambda:
{ }
The only thing it does is calls a function:
is-prime
You didn't give the function any arguments though.
Is it just supposed to guess what the arguments should be?
If you meant to pass in is-prime as a reference, you should have used &is-prime rather than {is-prime}.
Of course that still wouldn't have worked.
The other problem is that reduce operates by recursively combining values.
It can't do that if it operates on one argument at a time.
The bare block lambda {}, takes zero or one argument, not two or more.
reduce is often combined with map.
It happens so often that there is a Wikipedia page about MapReduce.
say ( map &is-prime, #g ==> reduce { $^a and $^b } );
# False
say ( map &is-prime, 2,3,5 ==> reduce { $^a and $^b } );
# True
I wrote it that way so that map would be in the line before reduce, but perhaps it would be more clear this way:
say reduce {$^a and $^b}, map &is-prime, 2,3,5;
# True
reduce with an infix operator is so common that there is a shorter way to write it.
say [and] map &is-prime, 2,3,5;
# True
Of course it would be better to just find the first value that isn't prime, and say the inverse.
Since if there is even a single value that isn't prime that would mean they can't all be primes.
You have to be careful though, as you may think something like this would always work:
not #g.first: !*.is-prime;
It does happen to work for the values you gave it, but may not always.
first returns Nil if it can't find the value.
not (2,3,5).first: !*.is-prime;
# not Nil === True
not (2,3,4).first: !*.is-prime;
# not 4 === False
not (2,3,0,4).first: !*.is-prime;
# not 0 === True
That last one returned 0 which when combined with not returns True.
You could fix this with defined.
not defined (2,3,0,4).first: !*.is-prime;
# False
This only works if first wouldn't return an undefined element that happens to be in the list.
(Int,Any).first: Real
# Int
defined (Int,Any).first: Real
# False
You could fix that by asking for the index instead of the value.
You of course still need defined.
(Int,Any).first: :k, Real
# 0
defined (Int,Any).first: :k, Real
# True
The other way to fix it is to just use grep.
not (2,3,0,4).grep: !*.is-prime;
# not (0,4) === False
Since grep always returns a List, you don't have to worry about checking for 0 or undefined elements.
(A List is True if it contains any elements, no matter what the values.)
grep is smart enough to know that if you coerce to Bool that it can stop upon finding the first value.
So it short-circuits the same as if you had used first.
This results in some fairly funky code, with those two negating operators. So it should be put into a function.
sub all-prime ( +#_ ) {
# return False if we find any non-prime
not #_.grep: !*.is-prime
# grep short-circuits in Bool context, so this will stop early
}
This could still fail if you give it something weird
all-prime 2,3,5, Date.today;
# ERROR: No such method 'is-prime' for invocant of type 'Date'
If you care, add some error handling.
sub all-prime ( +#_ ) {
# return Nil if there was an error
CATCH { default { return Nil }}
# return False if we find any non-prime
not #_.grep: !*.is-prime
}
all-prime 2,3,5, Date.today;
# Nil
use the all junction:
say so all #g».is-prime; # False
I am currently in the course of learning Python 2.7 and have come across the Equality and Boolean operators
My question is:
Why False and 1 is False but True and 1 is 1
Likewise, False or 1 is 1 but True or 1 is True
Can someone kindly explain why this is happening
Many thanks
and returns the first 'falsy' (False, zero, empty string or list, etc.) value it sees, or the final value if none were falsy. Further values are not even evaluated, since they can't change the result.
or likewise returns the first 'truthy' (True, non-zero, non-empty string or list, etc.) value it sees (or the final one if there were none), and doesn't evaluate the rest.
This behavior is sometimes more convenient than strictly returning only True or False.
user=> ((symbol "or") true false)
false
user=> (or true false)
true
Why does the first form evaluate to false? I'd imagine the two input forms being equivalent.
Strangely, reversing the order of the arguments works:
user=> ((symbol "or") false true)
true
user => (or false true)
true
When you evaluate the list (or true false), Clojure first evaluates the first item. In this case, the first item is a symbol that names a macro, so Clojure expands the macro and evaluates the resulting data structure (which would be a let form in this case).
On the other hand, when you evaluate the list ((symbol "or") true false), Clojure again first evaluates the first item, but in this case, the first item is another list! The first element of that list is the symbol symbol, which names a function, so Clojure calls that function with the parameter "or", again producing the symbol or. In other words, you are basically evaluating ('or true false).
Here's the catch: Clojure does not take that thing that it just evaluated and then go and evaluate it again. Rather, since it wasn't a symbol in the first place, Clojure evaluates it and then assumes that it must be a function. Is it a function? Indeed, the answer is yes!
user> (ifn? 'do)
;=> true
Here's where it gets a little tricky. The implementation of invoke on the Symbol class is the same as the implementation of invoke on the Keyword class: it assumes that the first argument is a map and tries to look itself up in that map. If you provide a second argument, it will use that as a default value.
Now, obviously, if you try to treat true as a map and look up the symbol or in that map, you'll find nothing. So Clojure helpfully returns the default value that you provided: false. You could put any value you'd like in that second parameter spot, and as long as your symbol doesn't exist as a key in the first argument, you'll always get back your default value.