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# XVal List comprehensions

List comprehensions can be used for mapping, filtering, folding, and scanning.<br>

\[ a + 5 | a <- \[1,2,3] ]

Returns: \[ 6,7,8 ]

Explanation: “a+5 where a drawn from \[1,2,3]”.  Loops over the list \[1,2,3], assigns each value to the variable a, calculates a + 5, and emits to the output list.<br>

\[ a | a <- \[1,2,3,4,5,6 ], a >= 4 ]

Returns: \[ 4,5,6 ]

Explanation: “a where a drawn from \[1,2,3,4,5,6] and a >= 4”.  Loops over the list \[1,2,3], assigns each value to the variable a, and emits to the output list if a is larger or equal to 4.<br>

Syntax:&#x20;

* \[ (state-variable: initial-state ->) expression | binding <- list-expression (, test-expression) ]
* ( (state-variable: initial-state ->) expression | binding <- list-expression (, test-expression) )

Using state-variable one can transfer the result of each evaluation of expression over to the next evaluation. state-variable is visible during the evaluation of expression.<br>

Using () brackets instead of \[] returns only the last computed expression, instead of a list of every computation. This results in list folding instead of list scanning.

### Examples

#### Sum a list of numbers

(s:0 -> x+s | x <- \[1,2,3,4])

Result: 10

#### Return a running sum of numbers

\[s:0 -> x+s | x <- \[1,2,3,4]]

Result: \[1,3,6,10]\ <br>

Return a running sum of numbers larger than 1

\[s:0 -> x+s | x <- \[1,2,3,4], x > 1]

Result: \[2,5,9]

#### Pick the last element that matches a condition

Or return the default state if none are found.

(s:-1 -> x < 3 && x || s | x <- \[1,2,3,4])

Result: 2

#### Pick elements that match a condition

\[x | x <- data, x.Name = ‘A’ || x.Name = ‘C’]

where data =  \[ { Name:”A”, Value: 1}, { Name: “B”, Value: 2}, { Name: “C”, Value: 3} ]Result: \[ { Name:”A”, Value: 1}, { Name: “C”, Value: 3} ]