How does one iterate over a sequence in xquery by twos? - xquery

I want to iterate over a sequence in xquery and grab 2 elements at a time. What is the easiest way to do this?

XQuery 3.0 Solution
For this and more complex paging use cases the Window Clause has been created in XQuery 3.0. But, it is not yet supported by many XQuery processors.
Windowing example
Here is a working example that you could execute for example on try.zorba :
for tumbling window $pair in (2, 4, 6, 8, 10, 12, 14)
start at $s when fn:true()
end at $e when $e - $s eq 1
return <window>{ $pair }</window>
Result
<window>2 4</window><window>6 8</window><window>10 12</window><window>14</window>

One option is to iterate over all items and just take the items once the items reach the divisor, in this case 2. The one downside is that you won't reach the last group of items if the items aren't even multiples of the divisor. For instance, the last element of a sequence with an odd number of elements will not be returned with this approach.
for $item at $index in $items
return
if ($item mod 2 = 0) then
($items[$index - 1], $items[$index])
else
()
Another option is to use mod and the index of the item. Using this approach you can make certain to include all elements in the $items sequence by adding one less than the number of items in your group to the count.
let $group-size := 2
return
for $index in (1 to fn:count($items)+($group-size - 1))[. mod $group-size = 0]
return
($items[$index - 1] , $items[$index])

let $s := ("a","b","c","d","e","f")
for $i in 1 to xs:integer(count($s) div 2)
return
<pair>
{($s[$i*2 - 1],$s[$i*2])}
</pair>
returns
<pair>a b</pair>
<pair>c d</pair>
<pair>e f</pair>

for $item at $index in $items
return
(
if ($index mod 2 eq 0) then
(
$items[xs:integer(xs:integer($index) - 1)], $items[xs:integer($index)]
)
else
()
)

Related

Unable to understand this xquery statement

I am new to xquery and unable to understand what does it means :
$bottles=getallBottles()
$cups=getallCups()
<containers>
{
($bottles,$cups) //this line i am unable to get
}
<containers>
The comma forms a sequence. Presumably $bottles is a sequence of zero-to-many items and $cups is a sequence of zero-to-many items. The comma forms a sequence of all of the items in $bottles and all of the items in $cups.
For example:
let $x := (1, 2, 3)
let $y := ('a', 'b', 'c')
return ($x,$y)
yields:
1 2 3 a b c
In the above example, the parentheses are necessary so that forming the sequence of $x, $y takes precedence over return and the entire constructed sequence is returned.
In an example similar to the original question, parentheses are unnecessary because precedence is not ambiguous:
let $x := <a><x>5</x><x>6</x></a>
let $y := <b><y>1</y><y>2</y></b>
return <container>{$x, $y}</container>
yields:
<container><a><x>5</x><x>6</x></a><b><y>1</y><y>2</y></b></container>

Round time in Xpath

What is the simplest and correct way to round the time and dateTime in XPath?
For example, how to define a function local:round-time-to-minutes in such way that the following test-case:
let $t1 := xs:time( "12:58:37" )
let $t2 := local:round-time-to-minutes( $t1 )
return format-time( $t2, '[H01]:[m01]:[s01]' )
will return "12:59:00".
Don't sure what is better in case of "23:59:31" — to return "00:00:00" or to raise a dynamic error.
And similar function local:round-datetime-to-minutes to handle dateTime?
(it doesn't have such edge case as above)
Let these functions use "round half towards positive infinity" rule, where half is 30.0 seconds.
This is how the solution proposed by #michael.hor257k would look in XQuery:
declare variable $ONE_MIN := xs:dayTimeDuration("PT1M");
declare variable $MIDNIGHT := xs:time("00:00:00");
declare function local:round-time-to-minutes($time) {
$MIDNIGHT + round(($time - $MIDNIGHT) div $ONE_MIN) * $ONE_MIN
};
Another solution is to subtract the number of second from the given dateTime and add one minute (60 seconds) if the number of seconds is not less than 30.
To convert a number of seconds into duration we multiple it on 1S duration (actually, this operation can be eliminated by a compiler).
declare function local:round-time-to-minutes ( $time as xs:time ) {
let $s := seconds-from-time( $time )
return $time - xs:dayTimeDuration('PT1S') * ( $s - 60 * ($s idiv 30) )
};
declare function local:round-dateTime-to-minutes ( $dt as xs:dateTime ) {
let $s := seconds-from-dateTime( $dt )
return $dt - xs:dayTimeDuration('PT1S') * ( $s - 60 * ($s idiv 30) )
};
This solution is uniform for the cases of xs:time and xs:dateTime types.

How to use predicate exactly in MiniZinc

New MiniZinc user here ... I'm having a problem understanding the syntax of the counting constraint:
predicate exactly(int: n, array[int] of var int: x, int: v)
"Requires exactly n variables in x to take the value v."
I want to make sure each column in my 10r x 30c array has at least one each of 1,2 and 3, with the remaining 7 rows equal to zero.
If i declare my array as
array[1..10,1..30] of var 0..3: s;
how can I use predicate exactly to populate it as I need? Thanks!
Well, the "exactly" constraint is not so useful here since you want at least one occurrence of 1, 2, and 3. It's better to use for example the count function:
include "globals.mzn";
array[1..10,1..30] of var 0..3: s;
solve satisfy;
constraint
forall(j in 1..30) (
forall(c in 1..3) (
count([s[i,j] | i in 1..10],c) >= 1
)
)
;
output [
if j = 1 then "\n" else " " endif ++
show(s[i,j])
| i in 1..10, j in 1..30
];
You don't have do to anything about 0 since the domain is 0..3 and all values that are not 1, 2, or 3 must be 0.
Another constraint is "at_least", see https://www.minizinc.org/2.0/doc-lib/doc-globals-counting.html .
If you don't have read the MiniZinc tutorial (https://www.minizinc.org/downloads/doc-latest/minizinc-tute.pdf), I strongly advice you to. The tutorial teaches you how to think Constraint Programming and - of course - MiniZinc.

XQuery difference between same function different implementation

Return the number of cycles:
let $bd := doc("document")
return count ( for $c in $bd//cycle
where $c[#id]
return $c
)
Every cycle has an ID, not important here but it is a must to specify it.
What is the difference between the above use of count and the below use of count?
let $bd := doc("document")
let $c := $bd//cycle[#id]
return count($c)
I dont know the difference between these 2 XQueries return same result but following the same pattern the next 2 queries should work but the 2nd one doesnt... Here they are:
The total of hours of modules which is above 100.
*Working query*
let $bd:=doc("document")
return sum (
for $m in $bd//module[#id]
where $m/hours>100
return $m/hours
)
*Not working query*
let $bd := doc("document")
for $c in $bd//module[#id]
where $c/hours>100
return sum($c/hours)
Id like to know why following the same "pattern" the second query is not working.
The output of the not working query is this one:
160 160 256 224 192 160
Its not the result i need, I want the sum of all them.
The first two expressions are functionally equivalent. The difference is the use of FLWOR vs. XPath to select your sequence.
In the second example, you are calling sum() on each item of the sequence ($c/hours), instead of on the sequence itself:
let $bd := doc("document")
return sum(
for $c in $bd//module[#id]
where $c/hours>100
return $c/hours)
You could also use XPath:
let $bd := doc("document")
let $c := $bd//module[#id][hours>100]
return sum($c/hours)
Or similarly assign the result of the FLWOR to a variable and sum that:
let $bd := doc("document")
let $c :=
for $m in $bd//module[#id]
where $m/hours>100
return $m/hours
return sum($c)

Max to 0 to Max algorithm

First off, let me warn you that my Maths knowledge is fairly limited (hence my coming here to ask about this).
It's a silly example, but what I essentially want is to have a variable number of rows, and be able to set a maximum value, then for each progressive row decrease that value until half way at which point start increasing the value again back up to the maximum by the final row.
To illustrate this... Given that: maxValue = 20, and rows = 5, I need to be able to get to the following values for the row values (yes, even the 0):
row 1: 20
row 2: 10
row 3: 0
row 4: 10
row 5: 20
There are limitations though because I'm trying to do this in Compass which uses SASS. See here for the available operations, but to give you the gist of it, only basic operations are available.
I'm able to loop through the rows, so just need the calculation that will work for each individual row in the series. This is the kind of loop I'm able to use:
$maxValue:20;
$rows:5;
#for $i from 1 through $rows {
// calculation here.
}
I haven't really worked with SASS before, but try something like this using a basic if and the floor function, not sure if will work
// set various variables
$maxValue:20;
$rows:5;
// get row number before middle row, this instance it will be 2
$middleRow = floor( $maxValue / $rows )
// get increment amount, this instance should be 10
$decreaseValue = ( max / floor( rows / 2) )
#for $i from 0 through $rows - 1 {
#if $i <= $middleRow {
( $maxValue- ( $decreaseValue * $i ) )
}
#else{
// times by -1 to make positive value
( $maxValue - ( $decreaseValue * -$i ) ) * -1
}
}
I have tested the above in js ( with relative syntax ) and it yielded the required results ( jsBin example ). Hope this helps

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