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# 2. Data Structure Primitives

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## 2.1 Constructors

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### word

    WORD word1 word2
    (WORD word1 word2 word3 ...)

outputs a word formed by concatenating its inputs.

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### list

    LIST thing1 thing2
    (LIST thing1 thing2 thing3 ...)

outputs a list whose members are its inputs, which can be any Logo datum (word, list, or array).

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### sentence

    SENTENCE thing1 thing2
    SE thing1 thing2
    (SENTENCE thing1 thing2 thing3 ...)
    (SE thing1 thing2 thing3 ...)

outputs a list whose members are its inputs, if those inputs are not lists, or the members of its inputs, if those inputs are lists.

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### fput

    FPUT thing list

outputs a list equal to its second input with one extra member, the first input, at the beginning. If the second input is a word, then the first input must be a one-letter word, and FPUT is equivalent to WORD.

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### lput

    LPUT thing list

outputs a list equal to its second input with one extra member, the first input, at the end. If the second input is a word, then the first input must be a one-letter word, and LPUT is equivalent to WORD with its inputs in the other order.

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### array

    ARRAY size
    (ARRAY size origin)

outputs an array of size members (must be a positive integer), each of which initially is an empty list. Array members can be selected with `ITEM` and changed with `SETITEM`. The first member of the array is member number 1 unless an origin input (must be an integer) is given, in which case the first member of the array has that number as its index. (Typically 0 is used as the origin if anything.) Arrays are printed by `PRINT` and friends, and can be typed in, inside curly braces; indicate an origin with `{a b c}@0`.

See section item , setitem , print .

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### mdarray

    MDARRAY sizelist				(library procedure)
    (MDARRAY sizelist origin)

outputs a multi-dimensional array. The first input must be a list of one or more positive integers. The second input, if present, must be a single integer that applies to every dimension of the array.

Ex: `(MDARRAY [3 5] 0)` outputs a two-dimensional array whose members range from `[0 0]` to `[2 4]`.

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### listtoarray

    LISTTOARRAY list
    (LISTTOARRAY list origin)

outputs an array of the same size as the input list, whose members are the members of the input list.

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### arraytolist

    ARRAYTOLIST array

outputs a list whose members are the members of the input array. The first member of the output is the first member of the array, regardless of the array's origin.

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### combine

    COMBINE thing1 thing2				(library procedure)

if thing2 is a word, outputs `WORD thing1 thing2`. If thing2 is a list, outputs `FPUT thing1 thing2`.

See section word , fput

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### reverse

    REVERSE list					(library procedure)

outputs a list whose members are the members of the input list, in reverse order.

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### gensym

    GENSYM						(library procedure)

outputs a unique word each time it's invoked. The words are of the form `G1`, `G2`, etc.

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## 2.2 Data Selectors

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### first

    FIRST thing

if the input is a word, outputs the first character of the word. If the input is a list, outputs the first member of the list. If the input is an array, outputs the origin of the array (that is, the index of the first member of the array).

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### firsts

    FIRSTS list

outputs a list containing the `FIRST` of each member of the input list. It is an error if any member of the input list is empty. (The input itself may be empty, in which case the output is also empty.) This could be written as

    to firsts :list
    output map "first :list
    end

but is provided as a primitive in order to speed up the iteration tools `MAP`, `MAP.SE`, and `FOREACH`.

    to transpose :matrix
    if emptyp first :matrix [op []]
    op fput firsts :matrix transpose bfs :matrix
    end

See section map , map.se , foreach

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### last

    LAST wordorlist

if the input is a word, outputs the last character of the word. If the input is a list, outputs the last member of the list.

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### butfirst

    BUTFIRST wordorlist
    BF wordorlist

if the input is a word, outputs a word containing all but the first character of the input. If the input is a list, outputs a list containing all but the first member of the input.

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### butfirsts

    BUTFIRSTS list
    BFS list

outputs a list containing the `BUTFIRST` of each member of the input list. It is an error if any member of the input list is empty or an array. (The input itself may be empty, in which case the output is also empty.) This could be written as

    to butfirsts :list
    output map "butfirst :list
    end

but is provided as a primitive in order to speed up the iteration tools `MAP`, `MAP.SE`, and `FOREACH`.

See section map , map.se , foreach

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### butlast

    BUTLAST wordorlist
    BL wordorlist

if the input is a word, outputs a word containing all but the last character of the input. If the input is a list, outputs a list containing all but the last member of the input.

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### item

    ITEM index thing

if the thing is a word, outputs the indexth character of the word. If the thing is a list, outputs the indexth member of the list. If the thing is an array, outputs the indexth member of the array. Index starts at 1 for words and lists; the starting index of an array is specified when the array is created.

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### mditem

    MDITEM indexlist array				(library procedure)

outputs the member of the multidimensional array selected by the list of numbers indexlist.

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### pick

    PICK list					(library procedure)

outputs a randomly chosen member of the input list.

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### remove

    REMOVE thing list				(library procedure)

outputs a copy of list with every member equal to thing removed.

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### remdup

    REMDUP list					(library procedure)

outputs a copy of list with duplicate members removed. If two or more members of the input are equal, the rightmost of those members is the one that remains in the output.

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### quoted

    QUOTED thing					(library procedure)

outputs its input, if a list; outputs its input with a quotation mark prepended, if a word.

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## 2.3 Data Mutators

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### setitem

    SETITEM index array value

command. Replaces the indexth member of array with the new value. Ensures that the resulting array is not circular, i.e., value may not be a list or array that contains array.

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### mdsetitem

    MDSETITEM indexlist array value			(library procedure)

command. Replaces the member of array chosen by indexlist with the new value.

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### .setfirst

    .SETFIRST list value

command. Changes the first member of list to be value.

WARNING: Primitives whose names start with a period are dangerous. Their use by non-experts is not recommended. The use of `.SETFIRST` can lead to circular list structures, which will get some Logo primitives into infinite loops, and to unexpected changes to other data structures that share storage with the list being modified.

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### .setbf

    .SETBF list value

command. Changes the butfirst of list to be value.

WARNING: Primitives whose names start with a period are dangerous. Their use by non-experts is not recommended. The use of `.SETBF` can lead to circular list structures, which will get some Logo primitives into infinite loops; unexpected changes to other data structures that share storage with the list being modified; or to Logo crashes and coredumps if the butfirst of a list is not itself a list.

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### .setitem

    .SETITEM index array value

command. Changes the indexth member of array to be value, like `SETITEM`, but without checking for circularity.

WARNING: Primitives whose names start with a period are dangerous. Their use by non-experts is not recommended. The use of `.SETITEM` can lead to circular arrays, which will get some Logo primitives into infinite loops.

See section setitem.

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### push

    PUSH stackname thing				(library procedure)

command. Adds the thing to the stack that is the value of the variable whose name is stackname. This variable must have a list as its value; the initial value should be the empty list. New members are added at the front of the list.

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### pop

    POP stackname					(library procedure)

outputs the most recently `PUSH`ed member of the stack that is the value of the variable whose name is stackname and removes that member from the stack.

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### queue

    QUEUE queuename thing				(library procedure)

command. Adds the thing to the queue that is the value of the variable whose name is queuename. This variable must have a list as its value; the initial value should be the empty list. New members are added at the back of the list.

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### dequeue

    DEQUEUE queuename				(library procedure)

outputs the least recently `QUEUE`d member of the queue that is the value of the variable whose name is queuename and removes that member from the queue.

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## 2.4 Predicates

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### wordp

    WORDP thing
    WORD? thing

outputs `TRUE` if the input is a word, `FALSE` otherwise.

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### listp

    LISTP thing
    LIST? thing

outputs `TRUE` if the input is a list, `FALSE` otherwise.

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### arrayp

    ARRAYP thing
    ARRAY? thing

outputs `TRUE` if the input is an array, `FALSE` otherwise.

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### emptyp

    EMPTYP thing
    EMPTY? thing

outputs `TRUE` if the input is the empty word or the empty list, `FALSE` otherwise.

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### equalp

    EQUALP thing1 thing2
    EQUAL? thing1 thing2
    thing1 = thing2

outputs `TRUE` if the inputs are equal, `FALSE` otherwise. Two numbers are equal if they have the same numeric value. Two non-numeric words are equal if they contain the same characters in the same order. If there is a variable named `CASEIGNOREDP` whose value is `TRUE`, then an upper case letter is considered the same as the corresponding lower case letter. (This is the case by default.) Two lists are equal if their members are equal. An array is only equal to itself; two separately created arrays are never equal even if their members are equal. (It is important to be able to know if two expressions have the same array as their value because arrays are mutable; if, for example, two variables have the same array as their values then performing `SETITEM` on one of them will also change the other.)

See section caseignoredp , setitem

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### notequalp

    NOTEQUALP thing1 thing2
    NOTEQUAL? thing1 thing2
    thing1 <> thing2

outputs `FALSE` if the inputs are equal, `TRUE` otherwise. See `EQUALP` for the meaning of equality for different data types.

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### beforep

    BEFOREP word1 word2
    BEFORE? word1 word2

outputs `TRUE` if word1 comes before word2 in ASCII collating sequence (for words of letters, in alphabetical order). Case-sensitivity is determined by the value of `CASEIGNOREDP`. Note that if the inputs are numbers, the result may not be the same as with `LESSP`; for example, `BEFOREP 3 12` is false because 3 collates after 1.

See section caseignoredp , lessp

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### .eq

    .EQ thing1 thing2

outputs `TRUE` if its two inputs are the same datum, so that applying a mutator to one will change the other as well. Outputs `FALSE` otherwise, even if the inputs are equal in value.

WARNING: Primitives whose names start with a period are dangerous. Their use by non-experts is not recommended. The use of mutators can lead to circular data structures, infinite loops, or Logo crashes.

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### memberp

    MEMBERP thing1 thing2
    MEMBER? thing1 thing2

if thing2 is a list or an array, outputs `TRUE` if thing1 is `EQUALP` to a member of thing2, `FALSE` otherwise. If thing2 is a word, outputs `TRUE` if thing1 is a one-character word `EQUALP` to a character of thing2, `FALSE` otherwise.

See section equalp .

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### substringp

    SUBSTRINGP thing1 thing2
    SUBSTRING? thing1 thing2

if thing1 or thing2 is a list or an array, outputs `FALSE`. If thing2 is a word, outputs `TRUE` if thing1 is `EQUALP` to a substring of thing2, `FALSE` otherwise.

See section equalp .

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### numberp

    NUMBERP thing
    NUMBER? thing

outputs `TRUE` if the input is a number, `FALSE` otherwise.

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### vbarredp

    VBARREDP char
    VBARRED? char
    BACKSLASHEDP char                               (library procedure)
    BACKSLASHED? char                               (library procedure)

outputs `TRUE` if the input character was originally entered into Logo within vertical bars (|) to prevent its usual special syntactic meaning, `FALSE` otherwise. (Outputs `TRUE` only if the character is a backslashed space, tab, newline, or one of `()[]+-*/=<>":;\~?|` )

The names `BACKSLASHEDP` and `BACKSLASHED?` are included in the Logo library for backward compatibility with the former names of this primitive, although it does not output `TRUE` for characters originally entered with backslashes.

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## 2.5 Queries

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### count

    COUNT thing

outputs the number of characters in the input, if the input is a word; outputs the number of members in the input, if it is a list or an array. (For an array, this may or may not be the index of the last member, depending on the array's origin.)

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### ascii

    ASCII char

outputs the integer (between 0 and 255) that represents the input character in the ASCII code. Interprets control characters as representing vbarred punctuation, and returns the character code for the corresponding punctuation character without vertical bars. (Compare `RAWASCII`.)

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### rawascii

    RAWASCII char

outputs the integer (between 0 and 255) that represents the input character in the ASCII code. Interprets control characters as representing themselves. To find out the ASCII code of an arbitrary keystroke, use `RAWASCII RC`.

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### char

    CHAR int

outputs the character represented in the ASCII code by the input, which must be an integer between 0 and 255.

See section ascii .

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### member

    MEMBER thing1 thing2

if thing2 is a word or list and if `MEMBERP` with these inputs would output `TRUE`, outputs the portion of thing2 from the first instance of thing1 to the end. If `MEMBERP` would output `FALSE`, outputs the empty word or list according to the type of thing2. It is an error for thing2 to be an array.

See section memberp .

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### lowercase

    LOWERCASE word

outputs a copy of the input word, but with all uppercase letters changed to the corresponding lowercase letter.

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### uppercase

    UPPERCASE word

outputs a copy of the input word, but with all lowercase letters changed to the corresponding uppercase letter.

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### standout

    STANDOUT thing

outputs a word that, when printed, will appear like the input but displayed in standout mode (boldface, reverse video, or whatever your version does for standout). The word contains machine-specific magic characters at the beginning and end; in between is the printed form (as if displayed using `TYPE`) of the input. The output is always a word, even if the input is of some other type, but it may include spaces and other formatting characters. Note: a word output by `STANDOUT` while Logo is running on one machine will probably not have the desired effect if printed on another type of machine.

In the Macintosh classic version, the way that standout works is incompatible with the use of characters whose ASCII code is greater than 127. Therefore, you have a choice to make: The instruction

    CANINVERSE 0

disables standout, but enables the display of ASCII codes above 127, and the instruction

    CANINVERSE 1

restores the default situation in which standout is enabled and the extra graphic characters cannot be printed.

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### parse

    PARSE word

outputs the list that would result if the input word were entered in response to a `READLIST` operation. That is, `PARSE READWORD` has the same value as `READLIST` for the same characters read.

See section readlist , readword

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### runparse

    RUNPARSE wordorlist

outputs the list that would result if the input word or list were entered as an instruction line; characters such as infix operators and parentheses are separate members of the output. Note that sublists of a runparsed list are not themselves runparsed.

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