# List

"List" is a native TADS 3 datatype. Lists are actually instances of the intrinsic class List, which means that you can call methods defined by the List intrinsic class on list objects.

## Value Semantics

Lists have "value semantics," which means that a given list value is immutable. Any operation that appears to modify a list value is actually creating a new list value, and leaving the original value unchanged. This behavior makes it very easy to work with lists, because you never have to worry about how many variables or properties refer to a list - even if several variables refer to a list value, each variable effectively has its own private copy of the list, so changes made to one variable's list won't affect any others. This also means that you can pass a list to a function without having to worry about whether the function will modify the list, and similarly that you can modify a list passed in as an argument to your functions without having to worry about how the changes will affect the caller.

This is discussed in more detail in the Vector class section.

## String conversions

A list can be converted to a string using the toString() function. A list can also be used in a context where a non-string value is implicitly converted to a string, such as on the right-hand side of a "+" operator where the left-hand side is a string.

The string conversion of a list consists of the list's elements, each itself first converted to a string if necessary, concatenated together, with commas separating elements. For example, toString([1, 2, 3]) yields the string '1,2,3'.

## List iterations

A List is a subclass of Collection, so you can use the createIterator() method to create an Iterator to iterate over the elements of the list. The Iterator that a List creates is called an IndexedIterator, and visits the elements of the list in index order (the first element visited is the element at index 1, the second is the element at index 2, and so forth).

## List methods

List is a subclass of Collection, and thus defines the Collection methods. In addition, List defines the methods shown below.

append(val)

Appends the value val to this list, returning the resulting list. This is almost the same as using the + operator to append val to the list, but differs in that this method always treats val as a single new element, even if val is a list. Consider the following:

    local x = [1, 2, 3];
    local y = x + [4, 5];
    local z = x.append([4, 5]);

In this example, the value of y is the list [1, 2, 3, 4, 5]: if the value on the right of a + operator is a list, the + operator appends each value of the list as a separate element. In contrast, the value of z is [1, 2, 3, [4, 5]], because the append() method always treats its argument as a single element of the new list.

Note that you can achieve the same effect using the + operator by constructing a new list for the right operand:

    local y2 = x + [[4, 5]];

This will ensure that the value of y2 is the same as the value of z: [1, 2, 3, [4, 5]]. However, the append() method is more efficient than constructing a new list for the right operand of +, because it's not necessary to construct the intermediate list using append().

appendUnique(lst)

Appends the elements of the list lst to this list, returning a new list consisting only of the unique elements of the combination. Each value appearing in the result list will appear in the result list only once. (The same effect could be obtained by concatenating the two lists with the + operator then applying getUnique() to the result, but this method is more efficient because it avoids constructing the fully concatenated list as an intermediate value.)

car()

Returns the first element of the list. If the list has no elements, returns nil.

(The names "car" and "cdr" are historical; they come from the Lisp programming language, which borrowed them from an old IBM mainframe instruction set.)

cdr()

Returns the "tail" of the list; that is, the rest of the list after removing the first element. If the list has no elements, returns nil. This function is almost the same as sublist(2), except that sublist() would return an empty list if given an empty list, whereas cdr() returns nil in this case.

(The names "car" and "cdr" are historical; they come from the Lisp programming language, which borrowed them from an old IBM mainframe instruction set.)

countOf(val)

Returns the number of elements whose values equal val.

countWhich(cond)

Returns the number of elements for which the callback function cond returns a non-false value (anything but nil or 0). For each element, this method calls the callback, passing the current element as the argument. If the callback returns anything but nil or 0, the method counts the element. After invoking the callback for each element, the method returns the resulting count.

forEach(func)

Invokes the callback function (func)(value) for each element, in order from first to last, passing the value of one element as value to the callback on each invocation. The callback function takes one argument, which is the current element, and returns no value. This method returns no value. This method is a convenient means of executing some code for each element of the list.

forEachAssoc(func)

Invokes the callback function (func)(index, value) for each element, in order from first to last, passing each element's index and value to the function func. The callback function returns no value. This method returns no value. This method is a convenient means of executing some code for each element of the list; unlike forEach(), this method provides its callback with the index as well as the value for each element it visits.

generate(func, n)

Creates a new list containing n elements by invoking the callback function func once for each element, and using the return value as the element value. This is a class method that you call on the List class directly, as in List.generate(f, 10).

func is a callback function, which can be a regular function or an anonymous function. func can take zero or one argument. The one-argument form is invoked with the index of the current element as the argument on each call.

generate() is convenient for creating a list of values based on a formula. For example, this creates a list of the first ten positive even integers:

    local e = List.generate({i: i*2}, 10);

That creates a list containing ten elements. Each element's value is determined by calling the anonymous function {i: i*2} with the index of that element. So the value of the first element, at index 1, is determined by calling the anonymous function with i=1. In this case the function returns i*2 == 1*2 == 2. The second element is determined by calling the function again with i=2, which returns 4, and so on for the remaining elements.

This creates a list of the first 20 Fibonacci numbers:

    local a = 0, b = 1;
    local fib = List.generate({: local f = a, a = b, b = f + a, f }, 20);

(For a simple way of generating a list consisting of a repeated fixed value, see the makeList() function.)

getUnique()

Returns a new list consisting of the unique elements of the original list. For each value in the original list, the value will appear in the new list only once. The order of the elements in the new list is that of the first appearances of the unique elements of the original list. For example, if the original list is [1, 5, 2, 5, 3, 5, 4, 5], this method will return [1, 5, 2, 3, 4].

indexOf(val)

Returns the index of the first element of the list equal to the given value. If val does not appear anywhere in the list, the method returns nil. The first element is at index 1.

indexOfMax(func?)

If func is omitted, returns the index of the element with the maximum value, comparing values to one another as though using the > operator.

If func is specified, it must be a function pointer. The method calls func() for each element in the list, passing the element's value as the function argument. The function must return a value. The result of indexOfMax in this case is the index of the element for which func() returned the maximum value.

For example, if lst is a list of string values, lst.indexOfMax({x: x.length()}) returns the index of the longest string in the list.

indexOfMin(func?)

If func is omitted, returns the index of the element with the minimum value, comparing values to one another as though using the < operator.

If func is specified, it must be a function pointer. The method calls func() for each element in the list, passing the element's value as the function argument. The function must return a value. The result of indexOfMin in this case is the index of the element for which func() returned the minimum value.

For example, if lst is a list of string values, lst.indexOfMin({x: x.length()}) returns the index of the shortest string in the list.

indexWhich(cond)

Finds the first element for which the given condition is true. The method iterates through the elements of the list, starting at the first element and proceeding in order, and applies the callback function cond to each element. The callback takes one argument, which is the value of the list element, and returns a condition result value. For each element, if the callback function returns a non-false value (i.e., any value except nil or 0), the method immediately stops the iteration and returns the index of that element. If the callback returns a false value (nil or 0) for every element of the list, the method returns nil.

insertAt(index, val1, val2, …)

Returns a new list which results from inserting the given values (val1, val2, and so on) into the existing list before the element at the position given by index. At least one value argument is required; if multiple value arguments are provided, the values are inserted into the list in the same order in which they appear in the arguments.

If index is 1, the values are inserted before the first element of the existing list (this means that insertAt(1, x) is equivalent to prepend(x)), and if index is equal to the number of elements in the original list plus 1, the values are inserted after the last element of the existing list (so lst.insertAt(lst.length() + 1, x) is equivalent to lst.append(x)). If index is negative, it counts backwards from the end of the list: -1 inserts before the last element, -2 inserts before the second to last, and so on. If index is zero, elements are inserted after the existing last element.

Because the new elements are inserted before any existing element at the insertion point, the index in the result list of the first element inserted always equals index. For example, consider this statement:

    lst = lst.insertAt(5, 'a', 'b', 'c');

After this statement is executed, lst[5] will be 'a'.

Note that, if any of the values to be inserted are themselves lists, each one is inserted as a single list-valued element, in the same manner as append(val) and prepend(val).

intersect(lst2)

Returns a new list consisting of the intersection of this list and lst2; that is, a list consisting of the elements common to both this list and lst2. lst2 must also be a list. If the two lists have no elements in common, the result is an empty list. If an element of the shorter list (or, if the lists are of equal length, this list) appears more than once in the shorter list, and that element value also appears in the longer list, then the element will be in the result list the same number of times that it is in the shorter list. An element repeated in the longer list will not be repeated in the result list.

join(sep?)

Returns a string made by concatenating the elements of the list together in index order. If sep is provided, it's a string that's interposed between elements as a separator. If sep is omitted, the elements are concatenated with no separation.

Each element is converted to a string using the usual automatic conversions before it's concatenated. If an element can't be converted to string, the method throws an error.

lastIndexOf(val)

Returns the index of the last element in the list whose value equals val. If no element in the list equals val, this method returns nil.

lastIndexWhich(cond)

Returns the index of the last element in the list for which the callback function cond returns a non-false value (anything except nil or 0). This method is similar to indexWhich(cond), but scans the list in reverse order, starting at the last element and working towards the first. Returns nil if the callback returns nil or 0 for every element.

lastValWhich(cond)

Returns the value of the last element in the list for which the callback function cond returns a non-false value. This method is similar to lastIndexWhich(cond), but returns the value of the matching element rather than its index. Returns nil if the callback returns nil or 0 for every element.

length()

Returns the number of elements in the list.

mapAll(func)

For each element of the list, this method invokes the callback function func, passing the current element as the single argument, then adds the callback's return value to a new list. The method returns the resulting new list, which has the same number of elements as the original list. Each element of the returned list contains the result returned by func for the corresponding element of the original list. This method does not modify the original list, but creates a new list.

Here's an example that creates a new list consisting of multiplying each element of an original list by 2 (naturally, for this to work, the original list must consist entirely of integer values):

    x = [1, 2, 3, 4];
    y = x.applyAll({x: x*2});

maxVal(func?)

If func is omitted, returns the maximum of the element values in the list, comparing values to one another as though using the > operator.

If func is specified, it must be a function pointer. The method calls func() for each element in the list, passing the element's value as the function argument. The function must return a value. The result of maxVal in this case is the value of the element which maximizes func. Note that the element value is returned, not the return value of func.

For example, if lst is a list of string values, lst.maxVal({x: x.length()}) returns the longest string in the list.

minVal(func?)

If func is omitted, returns minimum of the element values in the list, comparing values to one another as though using the < operator.

If func is specified, it must be a function pointer. The method calls func() for each element in the list, passing the element's value as the function argument. The function must return a value. The result of minVal in this case is the element that minimizes func(). Note that the element value is returned, not the return value of func.

For example, if lst is a list of string values, lst.minVal({x: x.length()}) returns the shortest string in the list.

prepend(val)

Returns a new list which results from inserting the value val before the first element of the existing list. This method is similar to append(val), but this method inserts the new element at the beginning of the list rather than at the end. Note that if val is a list, it is prepended as a single list-valued element.

removeElementAt(index)

Returns a new list which results from deleting the element at the given index. For example, [100, 200, 300].removeElementAt(2) yields [100, 300].

If index is negative, it counts from the end of the list: -1 is the last element, -2 is the second to last, and so on. Otherwise, index must be at least 1 and at most the number of elements in the list; if index is outside this range, the method throws an "index out of range" exception.

removeRange(startIndex, endIndex)

Returns a new list which results from deleting the elements starting at startIndex and ending with (and including) endIndex. If endIndex equals startIndex, only one value is deleted, so removeRange(x, x) is equivalent to removeElementAt(x).

If either index is negative, it counts from the end of the list: -1 is the last element, -2 is the second to last, and so on. Otherwise, startIndex and endIndex must be at least 1 and at most the number of elements in the list, and endIndex must be greater than or equal to startIndex.

sort(descending?, comparisonFunction?)

Returns a new list consisting of the elements of this list rearranged into a sorted order. By default, this method sorts the elements of the list into ascending order, but you can reverse this ordering by specifying true for the descending parameter.

The optional comparisonFunction can be used to specify the ordering of the result. If this argument is not specified (or is nil), the method will sort the elements according to the standard system ordering of values; hence, the elements must be of comparable types (such as all integers or all strings). By specifying a comparison function, you can provide your own special ordering, and you can also sort values that have no system-defined order, such as object values.

If provided, the comparisonFunction value is a pointer to a function taking two arguments, which are two values from the list to be compared. The function returns an integer less than zero if the first value is less than the second, zero if the two values are equal, or an integer greater than zero if the first value is greater than the second.

For example, if you wanted to sort a list of objects so that the objects are placed into descending order of their "priority" property values, where the "priority" property has an integer value, you could do this:

    lst = [obj1, obj2, obj3];
    sortedLst = lst.sort(true, { a, b: a.priority - b.priority });

splice(startIndex, deleteCount, ...)

Returns a new list which results from deleting the deleteCount elements starting at startIndex, then inserting any additional argument values as new list elements in their place. If deleteCount is zero, no elements are deleted. If there are no additional arguments, no new elements are inserted.

If startIndex is negative, it counts from the end of the list: -1 is the last element, -2 is the second to last, and so on. Use 0 to indicate one past the last element, to insert the new elements after the last existing element.

You can get the same effect as this method using a combination of removeRange() and insertAt(). splice() is clearer and more concise for cases where you want to replace a range with new values. It's also a little more efficient, since it doesn't have to construct an extra list to hold the intermediate result between the deletion and insertion.

sublist(start, length?)

Creates and returns a new list consisting of a sublist of this list starting at the element of this list at index start, and continuing for the number of elements given by length, if present, or to the end of this list if not. If start is negative, it counts from the end of the list: -1 is the last element, -2 is the second to last, and so on. If length is negative, it gives the number of items to omit from the end of the result list; -1 omits the last element of the source list, -2 omits the last two elements, and so on.

Examples:

    [1, 2, 3].sublist(2) yields [2, 3]
    [1, 2, 3].sublist(2, 1) yields [2]
    [1, 2, 3, 4, 5].sublist(1, -2) yields [1, 2, 3]
    [1, 2, 3, 4, 5].sublist(2, -2) yields [2, 3]

subset(func)

Creates and returns a new list containing the elements of this list for which the callback function func returns true (i.e., any value other than nil or 0). For each element of the source list, this method invokes the callback function, passing the value of the current element as the callback function's single argument. If the callback returns nil or 0, the method omits the element from the result; otherwise, the method includes the element in the result list. The new list's elements will be in the same order as the selected elements from the source list.

This method does not modify the original list.

This example uses a short-form anonymous function to create a new list that contains only the elements from an original list whose values are greater than 10.

    x = [5, 10, 15, 20];
    y = x.subset({x: x > 10});

valWhich(cond)

Returns the value of the first element for which the callback function cond returns non-false (i.e., anything except nil or 0). This method is similar to indexWhich(cond), but returns the value of the first matching element rather than its index. If cond returns nil or 0 for every element of the list, this method returns nil.

TADS 3 System Manual
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