I want to have a function within which several if conditions looking for specific integer values.
The function in some situations might be called several times in sequence in order to return the resulting values of several different if conditions.
The idea I had for this, in order to reduce the code's bulkiness, was to have the portion of the script executing the function as required run through an integer array whose each stored value is sent to the function.
But in order for this to reduce the amount of bulky code I need a way to add several values at once rather than having the values each added separately.
Is that possible in Pinescript? Or perhaps you have a different solution for executing such function?
To answer my own question, it seems a simple way to update an array to a set of new values is to use the built in function array.from.
var array<int> arr = array.new<int>(0, 0)
arr := array.from(1, 4, 10)
It doesn't even require clearing and appending the new values as it simply replaces the testArr array with the new input.
If one wants to append several additional values to an existing array rather than replacing all of the original values, it can be done like this:
arr := array.concat(arr, array.from(1, 4, 10))
If I understand your question correctly you would like a single mass update (addition) to array elements? No such function exists, each element must be individually updated, ideally in a loop if you have a consistent calculation.
From my understanding, an array is a simple table of values, such as local t = {"a","b","c"}, and a dictionary is a table of objects, such as local t = {a = 1, b = 2, c = 3} Of course, let me know if I'm wrong in either or both cases.
Anyways, my question lies in how we index the entries in either of these cases. For example, let's say I have the following code:
local t = {"TestEntry"}
print(t["TestEntry"])
Of course, this prints nil. However, when we use a dictionary the same way:
local t = {TestEntry = 1}
print(t["TestEntry"])
This, naturally, prints 1. My question is, why does it work this way for dictionaries, but not arrays?
Finally, I'd like to address the issue that led me to this question. Let's say, before I want to run a chunk of code, I need to see if a specific value is inside a table. It would be convenient if I could just check if it is in the table with table["GivenEntry"], but, as we have seen, this would only work if the entry in the table is actually an object. In my specific case, I am simply using an array, so it is not an object.
Thus, I had to resort to using a for loop to check the table:
local t = {"TestEntry1","TestEntry2"}
for i,v in pairs(t) do
if v == "TestEntry1" then
--do code
end
end
After doing this, it almost seemed as if it would be easier to create a silly dictionary, like:
local t = {TestEntry1 = "TestEntry1"}
because then, I could simply run t["TestEntry1"], and I wouldn't have to worry about having an empty table (because then the for loop would not run). Are there ramifications to creating a dictionary for such purposes? Is it less efficient in general?
Your input is appreciated,
Thank you.
In Lua both arrays an dictionaries are the same type (the table). local t = {"TestEntry"} is essentially short for local t = {[1] = "TestEntry"} (The brackets are needed by Lua for a number, you would access it with t[1]).
So the options for checking if "TestEntry1" is in the table are as you have written. A dictionary takes more memory and depending on how many values you have may take a while to create, but accessing a key should be constant time. Whereas to loop through the table will take longer and longer the more items you have so it is a tradeoff you have to decide on.
There are faster ways to search an array however (e.g. if it is sorted: https://en.wikipedia.org/wiki/Binary_search_algorithm)
a = 0:99
s = size(a)
disp(s(2))
Can the last two lines be written as one? In other languages I am able to do f(x)[i], but Matlab seems to complain.
In this specific case where you are using the size function, you can add an additional argument to specify the dimension you want the size of, allowing you to easily do this in one line:
disp(size(a, 2)); % Displays the size of the second dimension
In the more general case of accessing an array element without having to store it in a local variable first, things get a little more complicated, since MATLAB doesn't have the same kind of indexing shorthand you would find in other languages. Octave, for example, would allow you to do disp(size(a)(2)).
It is possible to collapse those two lines in one single statement and achieve a sort of f(x)[i] thanks to the functional form of the indexing operator: subsref.
disp(subsref(size(a), struct('type', '()', 'subs', {{2}})))
I have a vector array that contains Time values in an asceding order. With relational expressions I can obtain subset values from that array, after that I need the first value of that subset without creating new variables.
For example.
Time is an column vector, then I can use Time(something==X) to get a subset values of Time, but then I need the first value of Time(something==X), I can't use Time(something==X)(1) like some programming languages u.u
Unfortunately with MATLAB you need to use temporary variables. It doesn't support this kind of indexing, though it is quite natural and I would love if they supported it.
You would have to do this:
x = Time(something==X);
y = x(1);
Octave does have the ability of doing this kind of indexing though. The only way I can think of you escaping this is if you use cell arrays. However, if you want to use a normal vector, then you're SOL.
EDIT: May 13th, 2014 - Referencing David's comment, it is possible to do this without a temporary variable, but readability is very poor. In the end, a temporary variable is still the better way for readability and reproducibility. Check the following SO post that he has referenced:
How can I index a MATLAB array returned by a function without first assigning it to a local variable?
Within C code, I have an array and a zero-based index used to lookup within it, for example:
char * names[] = {"Apple", "Banana", "Carrot"};
char * name = names[index];
From an embedded Lua script, I have access to index via a getIndex() function and would like to replicate the array lookup. Is there an agreed on "best" method for doing this, given Lua's one-based arrays?
For example, I could create a Lua array with the same contents as my C array, but this would require adding 1 when indexing:
names = {"Apple", "Banana", "Carrot"}
name = names[getIndex() + 1]
Or, I could avoid the need to add 1 by using a more complex table, but this would break things like #names:
names = {[0] = "Apple", "Banana", "Carrot"}
name = names[getIndex()]
What approach is recommended?
Edit: Thank you for the answers so far. Unfortunately the solution of adding 1 to the index within the getIndex function is not always applicable. This is because in some cases indices are "well-known" - that is, it may be documented that an index of 0 means "Apple" and so on. In that situation, should one or the other of the above solutions be preferred, or is there a better alternative?
Edit 2: Thanks again for the answers and comments, they have really helped me think about this issue. I have realized that there may be two different scenarios in which the problem occurs, and the ideal solution may be different for each.
In the first case consider, for example, an array which may differ from time to time and an index which is simply relative to the current array. Indices have no meaning outside the code. Doug Currie and RBerteig are absolutely correct: the array should be 1-based and getIndex should contain a +1. As was mentioned, this allows the code on both the C and Lua sides to be idiomatic.
The second case involves indices which have meaning, and probably an array which is always the same. An extreme example would be where names contains "Zero", "One", "Two". In this case, the expected value for each index is well-known, and I feel that making the index on the Lua side one-based is unintuitive. I believe one of the other approaches should be preferred.
Use 1-based Lua tables, and bury the + 1 inside the getIndex function.
I prefer
names = {[0] = "Apple", "Banana", "Carrot"}
name = names[getIndex()]
Some of table-manipulation features - #, insert, remove, sort - are broken.
Others - concat(t, sep, 0), unpack(t, 0) - require explicit starting index to run correctly:
print(table.concat(names, ',', 0)) --> Apple,Banana,Carrot
print(unpack(names, 0)) --> Apple Banana Carrot
I hate constantly remembering of that +1 to cater Lua's default 1-based indices style.
You code should reflect your domain specific indices to be more readable.
If 0-based indices are fit well for your task, you should use 0-based indices in Lua.
I like how array indices are implemented in Pascal: you are absolutely free to choose any range you want, e.g., array[-10..-5]of byte is absolutely OK for an array of 6 elements.
This is where Lua metemethods and metatables come in handy. Using a table proxy and a couple metamethods, you can modify access to the table in a way that would fit your need.
local names = {"Apple", "Banana", "Carrot"} -- Original Table
local _names = names -- Keep private access to the table
local names = {} -- Proxy table, used to capture all accesses to the original table
local mt = {
__index = function (t,k)
return _names[k+1] -- Access the original table
end,
__newindex = function (t,k,v)
_names[k+1] = v -- Update original table
end
}
setmetatable(names, mt)
So what's going on here, is that the original table has a proxy for itself, then the proxy catches every access attempt at the table. When the table is accessed, it increment the value it was accessed by, simulating a 0-based array. Here are the print result:
print(names[0]) --> Apple
print(names[1]) --> Banana
print(names[2]) --> Carrot
print(names[3]) --> nil
names[3] = "Orange" --Add a new field to the table
print(names[3]) --> Orange
All table operations act just as they would normally. With this method you don't have to worry about messing with any unordinary access to the table.
EDIT: I'd like to point out that the new "names" table is merely a proxy to access the original names table. So if you queried for #names the result would be nil because that table itself has no values. You'd need to query for #_names to access the size of the original table.
EDIT 2: As Charles Stewart pointed out in the comment below, you can add a __len metamethod to the mt table to ensure the #names call gives you the correct results.
First of all, this situation is not unique to applications that mix Lua and C; you can face the same question even when using Lua only apps. To provide an example, I'm using an editor component that indexes lines starting from 0 (yes, it's C-based, but I only use its Lua interface), but the lines in the script that I edit in the editor are 1-based. So, if the user sets a breakpoint on line 3 (starting from 0 in the editor), I need to send a command to the debugger to set it on line 4 in the script (and convert back when the breakpoint is hit).
Now the suggestions.
(1) I personally dislike using [0] hack for arrays as it breaks too many things. You and Egor already listed many of them; most importantly for me it breaks # and ipairs.
(2) When using 1-based arrays I try to avoid indexing them and to use iterators as much as possible: for i, v in ipairs(...) do instead of for i = 1, #array do).
(3) I also try to isolate my code that deals with these conversions; for example, if you are converting between lines in the editor to manage markers and lines in the script, then have marker2script and script2marker functions that do the conversion (even if it's simple +1 and -1 operations). You'd have something like this anyway even without +1/-1 adjustments, it would just be implicit.
(4) If you can't hide the conversion (and I agree, +1 may look ugly), then make it even more noticeable: use c2l and l2c calls that do the conversion. In my opinion it's not as ugly as +1/-1, but has the advantage of communicating the intent and also gives you an easy way to search for all the places where the conversion happens. It's very useful when you are looking for off-one bugs or when API changes cause updates to this logic.
Overall, I wouldn't worry about these aspects too much. I'm working on a fairly complex Lua app that wraps several 0-based C components and don't remember any issues caused by different indexing...
Why not just turn the C-array into a 1-based array as well?
char * names[] = {NULL, "Apple", "Banana", "Carrot"};
char * name = names[index];
Frankly, this will lead to some unintuitive code on the C-side, but if you insist that there must be 'well-known' indices that work in both sides, this seems to be the best option.
A cleaner solution is of course not to make those 'well-known' indices part of the interface. For example, you could use named identifiers instead of plain numbers. Enums are a nice match for this on the C side, while in Lua you could even use strings as table keys.
Another possibility is to encapsulate the table behind an interface so that the user never accesses the array directly but only via a C-function call, which can then perform arbitrarily complex index transformations. Then you only need to expose that C function in Lua and you have a clean and maintainable solution.
Why not present your C array to Lua as userdata? The technique is described with code in PiL, section 'Userdata'; you can set the __index, __newindex, and __len metatable methods, and you can inherit from a class to provide other sequence manipulation functions as regular methods (e.g., define an array with array.remove, array.sort, array.pairs functions, which can be defined as object methods by a further tweak to __index). Doing things this way means you have no "synchronisation" issues between Lua and C, and it avoids risks that "array" tables get treated as ordinary tables resulting in off-by-one errors.
You can fix this lua-flaw by using an iterator that is aware of different index bases:
function iarray(a)
local n = 0
local s = #a
if a[0] ~= nil then
n = -1
end
return function()
n = n + 1
if n <= s then return n,a[n] end
end
end
However, you still have to add the zeroth element manually:
Usage example:
myArray = {1,2,3,4,5}
myArray[0] = 0
for _,e in iarray(myArray) do
-- do something with element e
end