I'm getting a weird behavior in Rego and I wonder why does it happen.
Link to Rego Playground
When I create an empty array, and than assign to it new array, the count of the first array is still zero:
package play
x[{"msg": msg}]{
c := []
a := [1,2]
b := [3,4]
c = array.concat(a,b)
count(c) > 0
msg := "Length of c is greater than zero"
}
And the output is:
{
"x": []
}
So, I have 2 questions:
Why do I get false in the line count(c)> 0?
How can I assign array to existing one? ( I need it because I have function that returns array and I'm trying to return the concatenation of 2 arrays. e.g.:
func[{"msg": msg}] = c{
a := [1,2]
b := [3,4]
c = array.concat(a,b)
}
Thanks!
Rego values and variables are immutable, so there's no way to assign a new value to an already existing variable. Your example compiles due to using the unification operator (=) rather than the assignment operator (:=).
In the example you provided, simply remove the first assignment:
package play
x[{"msg": msg}]{
a := [1,2]
b := [3,4]
c := array.concat(a,b)
count(c) > 0
msg := "Length of c is greater than zero"
}
Related
I have a dataset without column names defined. How can I assign these programmatically using gota? For comparison, Pandas has df.rename.
Say my data looks like this:
1,2,3
4,5,6
7,8,9
I basically want this in my dataframe:
A,B,C
1,2,3
4,5,6
7,8,9
You can call ReadCSV with load options. In go, this method is called functional options.
This is the example in gota:
func main() {
f, err := os.Open("sample2.csv")
if err != nil {
panic(err.Error())
}
defer f.Close()
names := dataframe.Names("A", "B", "C")
noHeader := dataframe.HasHeader(false)
df := dataframe.ReadCSV(f, names, noHeader)
r := df.Records()
fmt.Println(r)
}
Functions Load* and Read* in gota all have LoadOption:
https://pkg.go.dev/github.com/go-gota/gota/dataframe
How can I create a fixed multidimensional array in Specman/e using varibles?
And then access individual elements or whole rows?
For example in SystemVerilog I would have:
module top;
function automatic my_func();
bit [7:0] arr [4][8]; // matrix: 4 rows, 8 columns of bytes
bit [7:0] row [8]; // array : 8 elements of bytes
row = '{1, 2, 3, 4, 5, 6, 7, 8};
$display("Array:");
foreach (arr[i]) begin
arr[i] = row;
$display("row[%0d] = %p", i, row);
end
$display("\narr[2][3] = %0d", arr[2][3]);
endfunction : my_func
initial begin
my_func();
end
endmodule : top
This will produce this output:
Array:
row[0] = '{'h1, 'h2, 'h3, 'h4, 'h5, 'h6, 'h7, 'h8}
row[1] = '{'h1, 'h2, 'h3, 'h4, 'h5, 'h6, 'h7, 'h8}
row[2] = '{'h1, 'h2, 'h3, 'h4, 'h5, 'h6, 'h7, 'h8}
row[3] = '{'h1, 'h2, 'h3, 'h4, 'h5, 'h6, 'h7, 'h8}
arr[2][3] = 4
Can someone rewrite my_func() in Specman/e?
There are no fixed arrays in e. But you can define a variable of a list type, including a multi-dimensional list, such as:
var my_md_list: list of list of my_type;
It is not the same as a multi-dimensional array in other languages, in the sense that in general each inner list (being an element of the outer list) may be of a different size. But you still can achieve your purpose using it. For example, your code might be rewritten in e more or less like this:
var arr: list of list of byte;
var row: list of byte = {1;2;3;4;5;6;7;8};
for i from 0 to 3 do {
arr.add(row.copy());
print arr[i];
};
print arr[2][3];
Notice the usage of row.copy() - it ensures that each outer list element will be a copy of the original list.
If we don't use copy(), we will get a list of many pointers to the same list. This may also be legitimate, depending on the purpose of your code.
In case of a field (as opposed to a local variable), it is also possible to declare it with a given size. This size is, again, not "fixed" and can be modified at run time (by adding or removing items), but it determines the original size of the list upon creation, for example:
struct foo {
my_list[4][8]: list of list of int;
};
What is the best way to iterate over the elements of a finite set object in Dafny? An example of working code would be delightful.
This answer explains how to do it using a while loop, rather than by defining an iterator. The trick is to use the "assign such that" operator,
:|, to obtain a value y such that that y is in the set, and then repeat on that set with the y removing, continuing until there are not more elements. The decreases clause is necessary here. With it, Dafny proves termination of the while loop, but without it, not.
method Main()
{
var x: set<int> := {1, 2, 3};
var c := x;
while ( c != {} )
decreases c;
{
var y :| y in c;
print y, ", ";
c := c - { y };
}
}
I am a newbie to golang and want to find a way to define a single byte variable.
It's a demo program in Effective Go reference.
package main
import (
"fmt"
)
func unhex(c byte) byte{
switch {
case '0' <= c && c <= '9':
return c - '0'
case 'a' <= c && c <= 'f':
return c - 'a' + 10
case 'A' <= c && c <= 'F':
return c - 'A' + 10
}
return 0
}
func main(){
// It works fine here, as I wrap things with array.
c := []byte{'A'}
fmt.Println(unhex(c[0]))
//c := byte{'A'} **Error** invalid type for composite literal: byte
//fmt.Println(unhex(c))
}
As you see I can wrap a byte with array, things goes fine, but How can I define a single byte without using array? thanks.
In your example, this would work, using the conversion syntax T(x):
c := byte('A')
Conversions are expressions of the form T(x) where T is a type and x is an expression that can be converted to type T.
See this playground example.
cb := byte('A')
fmt.Println(unhex(cb))
Output:
10
If you don't want to use the := syntax, you can still use a var statement, which lets you explicitly specify the type. e.g:
var c byte = 'A'
I am really new to SML and I can't figure out how to get answer for the same;
It goes something like: 3^4 < 32 but 3^5 > 32 so my answer is 4 (power of 3), similarly if I have numbers 4 and 63 then 4^2<63 but 4^3>63 so my answer is 2(power of 4).
I have come up with the following code
val log (b, n) =
let
val counter = ref b
val value = 0
in
while !counter > n do
( counter := !counter*b
value := !value + 1)
end;
So here value is what I need as my answer but I get a lot of errors. I know I am wrong at many places. Any help would be appreciated.
I can perhaps do this the normal ML way but I want to learnt impure ML also...
fun loghelper(x,n,b) = if x>n then 0 else (1+loghelper((x*b),n,b));
fun log(b,n) = loghelper(b,n,b);
ok so finally here is the correct code for the while loop and it works as well;
fun log (b, n) =
let
val counter = ref b
val value = ref 0
in
while (!counter <= n) do
(counter := !counter*b;
value := !value + 1);
!value
end;
You have several problems in your code:
Errors:
Instead of val log (b, n) = it should be fun log (b, n) =. fun is a convenience syntax that lets you define functions easily. If you wanted to write this with val you would write: val log = fn (b, n) => (it gets more complicated in the cases of recursive functions or functions with multiple curried arguments)
You need a semicolon to separate two imperative statements: ( counter := !counter*b;
value := !value + 1)
value needs to be a ref: val value = ref 0
Logic:
Your function doesn't return anything. A while loop has the unit type, so your function returns () (unit). You probably want to return !value. To do this, you need to add a semicolon after the whole while loop thing, and then write !value
Your while loop condition doesn't really make sense. It seems reversed. You probably want while !counter <= n do
Your base case is not right. Either value should start at 1 (since counter starts at b, and b is b to the first power); or counter should start at 1 (since b to the zeroth power is 1). The same issue exists with your functional version.