Although it seems fairly simple, there is a small nuance to the obvious solution.
The following code would cover most situations:
arr_1=(1 2 3 4)
arr_2=($arr_1)
However, empty strings do not copy over. The following code:
arr_1=('' '' 3 4)
arr_2=($arr_1)
print -l \
"Array 1 size: $#arr_1" \
"Array 2 size: $#arr_2"
will yield:
Array 1 size: 4
Array 2 size: 2
How would I go about getting a true copy of an array?
It would be an "Array Subscripts" issue, so you could properly specify an array subscript form to select all elements of an array ($arr_1 for instance) within double quotes:
arr_1=('' '' 3 4)
arr_2=("${arr_1[#]}")
#=> arr_2=("" "" "3" "4")
Each elements of $arr_1 would be properly surrounded with double quotes even if it is empty.
A subscript of the form ‘[*]’ or ‘[#]’ evaluates to all elements of an array; there is no difference between the two except when they appear within double quotes.
‘"$foo[*]"’ evaluates to ‘"$foo[1] $foo[2] ..."’, whereas ‘"$foo[#]"’ evaluates to ‘"$foo[1]" "$foo[2]" ...’.
...
When an array parameter is referenced as ‘$name’ (with no subscript) it evaluates to ‘$name[*]’,
-- Array Subscripts, zshparam(1)
And empty elements of arrays will be removed according to "Empty argument removal", so
arr_2=($arr_1)
#=> arr_2=(${arr_1[*]})
#=> arr_2=(3 4)
Above behavior is not good in this case.
24. Empty argument removal
If the substitution does not appear in double quotes, any resulting zero-length argument, whether from a scalar or an element of an array, is elided from the list of arguments inserted into the command line.
-- Empty argument removal, Rules Expansion zshexpn(1)
Related
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"]
I want to write a function that returns 0 if the property exists and 1 if it does not.
for example:
typeset -A hashtable
hashtable[a]='this is a valid element'
testprop hashtable[a] # returns 0
testprop hashtable[b] # returns 1
is that possible?
The parameter expansion ${+name} almost does, what you want. If name is a set parameter 1 is substituted, otherwise 0 is substitued.
To get the desired interface, this can be wrapped into a function:
function testprop {
case ${(P)+${1}} in
0) return 1;;
1) return 0;;
esac
}
alias testprop='noglob testprop'
Explanation:
The parameter expansion flag P tells zsh to interprete the value ${1} as further parameter name.
${+name} is substituted by 1 if name is set, 0 otherwise.
The alias is necessary so that the parameter to testprob does not need to be quoted. Otherwise the square brackets around the index would be interpreted as globbing operator.
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
According to man zshexpn (5.0.2):
~ Force string arguments to any of the flags below that follow within the parentheses to be treated as
patterns.
For example, using the s flag to perform field splitting requires a string argument:
% print -l ${(s:9:):-"foo893bar923baz"}
foo8
3bar
23baz
My reading of the ~ flag suggests that I should be able to specify a pattern in place of a literal string to split on, so that the following
% print -l ${(~s:<->:):-"foo893bar923baz"}
should produce
foo
bar
baz
Instead, it behaves the same as if I omit the ~, performing no splitting at all.
% print -l ${(s:<->:):-"foo893bar923baz"}
foo893bar923baz
% print -l ${(~s:<->:):-"foo893bar923baz"}
foo893bar923baz
Ok, rereading the question, it's the difference between this:
$ val="foo???bar???baz"
$ print -l ${(s.?.)val}
foo
bar
baz
And this:
$ val="foo???bar???baz"
$ print -l ${(~s.?.)val}
foo???bar???baz
It operates on the variable, i.e. the "argument" to the split (from your documentation quote). In the first example, we substitute literal ?, and in the second, we treat the variable as a glob, and there are no literal ?, so nothing gets substituted.
Still, though, split works on characters and not globs in the substution itself, from the documentation:
s:string:
Force field splitting (see the option SH_WORD_SPLIT) at the separator string.
So, it doesn't look like you can split on a pattern. The ~ character modifies the interpretation of the string to be split.
Also, from the same pattern expansion documentation you reference, it continutes:
Compare with a ~ outside parentheses, which forces the entire
substituted string to be treated as a pattern. [[ "?" = ${(~j.|.)array} ]]
with the EXTENDED_GLOB option set succeeds if and
only if $array contains the string ‘?’ as an element. The argument may
be repeated to toggle the behaviour; its effect only lasts to the end
of the parenthesised group.
The difference between ${(~j.|.)array} and ${(j.|.)~array} is that the former treats the values inarray as global, and the latter treats the result as a glob.
See also:
${~spec} Turn on the GLOB_SUBST option for the evaluation of spec; if
the ‘~’ is doubled, turn it off. When this option is set, the string
resulting from the expansion will be interpreted as a pattern anywhere
that is possible, such as in filename expansion and filename
generation and pattern-matching contexts like the right hand side of
the ‘=’ and ‘!=’ operators in conditions.
Here is a demo that shows the differences:
$ array=("foo???bar???baz" "foo???bar???buz")
$ [[ "foo___bar___baz" = ${(~j.|.)array} ]] && echo true || echo false
false
$ [[ "foo___bar___baz" = ${(j.|.)~array} ]] && echo true || echo false
true
And for completeness:
$ [[ "foo___bar___baz" = ${(~j.|.)~array} ]] && echo true || echo false
true
I have an Ocaml function that is giving me errors.
What I am trying to do:
Recursively create a List of random numbers (0-2) of size "limit".
Here's what I have:
let rec carDoorNumbers = fun limit ->
match limit with
| [1] -> Random.int 3
| [] -> Random.int 3 :: carDoorNumbers (limit-1);;
I am getting this error:
Error: This expression has type 'a list
but an expression was expected of type int
Think about what your function has to do: given a limit, you have to create a list of numbers. So your type is something like carDoorNumbers : int -> int list.
Looking at that, it seems you have two errors. First, you're matching limit (which should be an int) against a list pattern. [1] -> ... matches a list containing only the element 1 and [] matches the empty list; you really want to match against the number 1 and any other number n.
The second error is that you return two different types in your match statement. Remember that you are supposed to be returning a list. In the first case, you are returning Random.int 3, which is an int rather than an int list. What you really want to return here is something like [Random.int 3].
The error you got is a little confusing. Since the first thing you returned was an int, it expects your second thing to also be an int. However, your second case was actually correct: you do return an int list! However, the compiler does not know what you meant, so its error is backwards; rather than changing the int list to an int, you need to change the int to an int list.
Your match expression treats limit like a list. Both [1] and [] are lists. That's what the compiler is telling you. But it seems limit should be an integer.
To match an integer, just use an integer constant. No square brackets.
(As a side comment, you might want to be sure the function works well when you pass it 0.)