I hope I get someone who understand this. I have been trying to concatenate Julia string for quit a while now but I still have an issue. I have this loop where I am trying to concatenate the string and a number from the loop then add the new value to an array, everything is fine when I print the value in the loop but printing the arrays then all the elements of the array are split again to individual characters.
my code is as bellow
a = 1
for i in nums_loop
i_val = i[a]
append!(const_names, (string(x, string(a))))
println(string(x, string(a)))
a += 1
end
print(const_names)
the output is as bellow
X1
X2
Any['X', '1', 'X', '2']
This seems the easiest way: first initiliaze your array_names with an empty string, later removing it with popfirst! (bad practise to call the array constant if you are actually changing its content)
array_names=[" "]
num_loops=2
for i=1:num_loops
push!(array_names, "X$i")
end
popfirst!(array_names)
println(array_names)
This gives me the result:
julia> println(array_names)
["X1", "X2"]
This question is only for curiosity. My colleague and I were trying to write a function which returns NULL, but doesn't print it.
Before we found return(invisible(NULL)), I tried return({dummy<-NULL}) which works, but only once. After the first evaluation, the functions starts printing again:
test <- function() {
return({x<-NULL})
}
# no printout
test()
# with printout
test()
# with printout
test()
How does this come about?
I think this is due to some older return handling built into R. There are many return functions, withVisible, invisible, etc. When you return an assignment x<-null inside the return function it will not automatically print. If you want an assignment to print...
test <- function() {
withAutoprint(x<-NULL)
}
# with printout this time
test()
# with printout
test()
# with printout
test()
I think this just may be hard coded into the return function, maybe pulling something from this logic below, just a shot in the dark though.
Source: R Documentation
x <- 1
withVisible(x <- 1) # *$visible is FALSE
x
withVisible(x) # *$visible is TRUE
Again if we do not use an expression and simply return a variable or value inside our return function we get automatic printing. The reason I am guessing it returns on a second call has to do with the fact x was already assigned previously.
EDIT: I found this deep into the documentation on auto printing. "Whether the returned value of a top-level R expression is printed is controlled by the global boolean variable R_Visible. This is set (to true or false) on entry to all primitive and internal functions based on the eval column of the table in file src/main/names.c: the appropriate setting can be extracted by the macro PRIMPRINT."(Source)
In Julia v1.01 I would like to create a function from a string.
Background: In a numerical solver, a testcase is defined via a JSON file. It would be great if the user could specify the initial condition in string form.
This results in the following situation: Assume we have (from the JSON file)
fcn_as_string = "sin.(2*pi*x)"
Is there a way to convert this into a function fcn such that I can call
fcn(1.0) # = sin.(2*pi*1.0)
Performance is not really an issue, as the initial condition is evaluated once and then the actual computation consumes most of the time.
Can't get my code displayed correctly in a comment so here's a quick fix for crstnbr's solution
function fcnFromString(s)
f = eval(Meta.parse("x -> " * s))
return x -> Base.invokelatest(f, x)
end
function main()
s = "sin.(2*pi*x)"
f = fcnFromString(s)
f(1.)
end
julia> main()
-2.4492935982947064e-16
The functions Meta.parse and eval allow you to do this:
fcn_as_string = "sin.(2*pi*x)"
fcn = eval(Meta.parse("x -> " * fcn_as_string))
#show fcn(1.0)
This return -2.4492935982947064e-16 (due to rounding errors).
Someone please help me understand this. I have the following code below. I am trying to append index[i]-1 to an empty array. However I am getting this error: "BoundsError: attempt to access 0-element Array{Any,1} at index [1]" :
sample_size_array = [9,5,6,9,2,6,9]
n_minus_1 = []
array_length = length(sample_size_array)
for i in 1:array_length
n_minus_1[i].append(sample_size_array[i] -1)
end
println(n_minus_1)
If Julia does not understand array[0] then why is i starting at 0 and not at 1?
Your code has two problems:
in the first iteration you are trying to access n_minus_1 array at index 1 while this array is still empty (has 0 length) - this throws you an error;
in Julia you do not invoke methods using a . (this symbol is used for different purposes - in this case it is parsed as field access and also would throw an error later)
To solve both those problems use push! function which appends an element at the end of an array. The code could look like this:
sample_size_array = [9,5,6,9,2,6,9]
n_minus_1 = []
array_length = length(sample_size_array)
for i in 1:array_length
push!(n_minus_1, sample_size_array[i]-1)
end
println(n_minus_1)
However in this case the whole operation can be written even simpler as:
n_minus_1 = sample_size_array .- 1
and you do not need any loop (and here you see another use of . in Julia - in this case we use it to signal that we want to subtract 1 from every element of sample_size_array).
I am trying to learn Erlang and I am working on the practice problems Erlang has on the site. One of them is:
Write the function time:swedish_date() which returns a string containing the date in swedish YYMMDD format:
time:swedish_date()
"080901"
My function:
-module(demo).
-export([swedish_date/0]).
swedish_date() ->
[YYYY,MM,DD] = tuple_to_list(date()),
string:substr((integer_to_list(YYYY, 3,4)++pad_string(integer_to_list(MM))++pad_string(integer_to_list(DD)).
pad_string(String) ->
if
length(String) == 1 -> '0' ++ String;
true -> String
end.
I'm getting the following errors when compiled.
demo.erl:6: syntax error before: '.'
demo.erl:2: function swedish_date/0 undefined
demo.erl:9: Warning: function pad_string/1 is unused
error
How do I fix this?
After fixing your compilation errors, you're still facing runtime errors. Since you're trying to learn Erlang, it's instructive to look at your approach and see if it can be improved, and fix those runtime errors along the way.
First let's look at swedish_date/0:
swedish_date() ->
[YYYY,MM,DD] = tuple_to_list(date()),
Why convert the list to a tuple? Since you use the list elements individually and never use the list as a whole, the conversion serves no purpose. You can instead just pattern-match the returned tuple:
{YYYY,MM,DD} = date(),
Next, you're calling string:substr/1, which doesn't exist:
string:substr((integer_to_list(YYYY,3,4) ++
pad_string(integer_to_list(MM)) ++
pad_string(integer_to_list(DD))).
The string:substr/2,3 functions both take a starting position, and the 3-arity version also takes a length. You don't need either, and can avoid string:substr entirely and instead just return the assembled string:
integer_to_list(YYYY,3,4) ++
pad_string(integer_to_list(MM)) ++
pad_string(integer_to_list(DD)).
Whoops, this is still not right: there is no such function integer_to_list/3, so just replace that first call with integer_to_list/1:
integer_to_list(YYYY) ++
pad_string(integer_to_list(MM)) ++
pad_string(integer_to_list(DD)).
Next, let's look at pad_string/1:
pad_string(String) ->
if
length(String) == 1 -> '0' ++ String;
true -> String
end.
There's a runtime error here because '0' is an atom and you're attempting to append String, which is a list, to it. The error looks like this:
** exception error: bad argument
in operator ++/2
called as '0' ++ "8"
Instead of just fixing that directly, let's consider what pad_string/1 does: it adds a leading 0 character if the string is a single digit. Instead of using if to check for this condition — if isn't used that often in Erlang code — use pattern matching:
pad_string([D]) ->
[$0,D];
pad_string(S) ->
S.
The first clause matches a single-element list, and returns a new list with the element D preceded with $0, which is the character constant for the character 0. The second clause matches all other arguments and just returns whatever is passed in.
Here's the full version with all changes:
-module(demo).
-export([swedish_date/0]).
swedish_date() ->
{YYYY,MM,DD} = date(),
integer_to_list(YYYY) ++
pad_string(integer_to_list(MM)) ++
pad_string(integer_to_list(DD)).
pad_string([D]) ->
[$0,D];
pad_string(S) ->
S.
But a simpler approach would be to use the io_lib:format/2 function to just format the desired string directly:
swedish_date() ->
io_lib:format("~w~2..0w~2..0w", tuple_to_list(date())).
First, note that we're back to calling tuple_to_list(date()). This is because the second argument for io_lib:format/2 must be a list. Its first argument is a format string, which in our case says to expect three arguments, formatting each as an Erlang term, and formatting the 2nd and 3rd arguments with a width of 2 and 0-padded.
But there's still one more step to address, because if we run the io_lib:format/2 version we get:
1> demo:swedish_date().
["2015",["0",56],"29"]
Whoa, what's that? It's simply a deep list, where each element of the list is itself a list. To get the format we want, we can flatten that list:
swedish_date() ->
lists:flatten(io_lib:format("~w~2..0w~2..0w", tuple_to_list(date()))).
Executing this version gives us what we want:
2> demo:swedish_date().
"20150829"
Find the final full version of the code below.
-module(demo).
-export([swedish_date/0]).
swedish_date() ->
lists:flatten(io_lib:format("~w~2..0w~2..0w", tuple_to_list(date()))).
UPDATE: #Pascal comments that the year should be printed as 2 digits rather than 4. We can achieve this by passing the date list through a list comprehension:
swedish_date() ->
DateVals = [D rem 100 || D <- tuple_to_list(date())],
lists:flatten(io_lib:format("~w~2..0w~2..0w", DateVals)).
This applies the rem remainder operator to each of the list elements returned by tuple_to_list(date()). The operation is needless for month and day but I think it's cleaner than extracting the year and processing it individually. The result:
3> demo:swedish_date().
"150829"
There are a few issues here:
You are missing a parenthesis at the end of line 6.
You are trying to call integer_to_list/3 when Erlang only defines integer_to_list/1,2.
This will work:
-module(demo).
-export([swedish_date/0]).
swedish_date() ->
[YYYY,MM,DD] = tuple_to_list(date()),
string:substr(
integer_to_list(YYYY) ++
pad_string(integer_to_list(MM)) ++
pad_string(integer_to_list(DD))
).
pad_string(String) ->
if
length(String) == 1 -> '0' ++ String;
true -> String
end.
In addition to the parenthesis error on line 6, you also have an error on line 10 where yo use the form '0' instead of "0", so you define an atom rather than a string.
I understand you are doing this for educational purpose, but I encourage you to dig into erlang libraries, it is something you will have to do. For a common problem like this, it already exists function that help you:
swedish_date() ->
{YYYY,MM,DD} = date(), % not useful to transform into list
lists:flatten(io_lib:format("~2.10.0B~2.10.0B~2.10.0B",[YYYY rem 100,MM,DD])).
% ~X.Y.ZB means: uses format integer in base Y, print X characters, uses Z for padding