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====== XL operators ======

XL-Operators are designed to simplify the usage of XL queries and rewriting rules. Moreover these operators can improve the performance of a query due to their deep integration into the model.
In the following, the operators a separated into analytical and selection operators to either get a value for analysis or a node/ a collection of nodes for rewriting.

====== Analytical Operators ======

| Command | description | example |
| min((*//query//*)[//value//]) | returns the smallest value | min((*F*)[length]) |
| max((*//query//*)[//value//]) | returns the largest value | max((*F*)[length]) |
| sum((*//query//*)[//value//]) | calculate the sum of the values | sum((*F*)[length]) |
| mean((*//query//*)[//value//]) | calculate the mean of the values | mean((*F*)[length]) |
| prod((*//query//*)[//value//]) | calculates the product of the values | prod((*F*)[length]) |
| count((*//query//*) )| returns the number of pattern matching the query | count((*F*) ) |
| empty((*//query//*) ) | returns true if no pattern matches the query | empty((*F*) ) |
| forall((*//query//*)[//value//]//condition//) | checks if a condition is true for the all results of the query |forall((*f:F*)[length]==1) |  
| exist((*//query//*)[//value//]//condition//) | checks if a condition is true for at least one result of the query |exist((*f:F*)[length]==1) |


For the operators min,max,sum,mean,prod it is possible to give "omitnan" as a second parameter to ignore NAN values. eg. ''sum((*F*)[length],"omitnan")'' 
====== Selection Operators ======

| command | description| example|
| first((*//query//*) ) | get the first element found with the query | first((*F*) ) |
| last((*//query//*) ) | get the last element found with the query | last((*F*) ) |
| slice((*//query//*),//s//,//e//) | return the subset(from s to e) of the result | slice((*F*),2,4), slice((*F*)[length],2,4)  |
|selectWhere((*//query//*),//condition//)| Select a node of the query that fits the condition | selectWhere((*f:F*),f[length]==1) |
|selectWhereMin((*//query//*),//parameter//)| Select a node of the query where the parameter is the smallest | selectWhereMin((*f:F*),f[length])  |
|selectWhereMax((*//query//*),//parameter//)| Select a node of the query where the parameter is the largest | selectWhereMax((*f:F*),f[length]) |
|selectRandomly((*//query//*) )| select a Random node from the query | selectRandomly((*F*) ) |
|selectRandomly((*//query//*),prob)| ??? | ???  |


====== Other ======

|command | description|example|
| array((*//query//*) ) | creates an java array of the items or values  | array((*F*) ), array((*F*)[length]) | 
| string((*//query//*) ) | creates an comma separated string of the items or values  | string((*F*) ), string((*F*)[length]) | 



additional information can be found in the [[https://grogra.gitlab.io/groimp-javadoc/xl.core/de/grogra/xl/util/Operators.html|javadoc]]