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[M (MUMPS)/en] Add M/MUMPS language
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---
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language: M (MUMPS)
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contributors:
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- ["Fred Turkington", "http://z3ugma.github.io"]
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filename: LEARNM.m
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---
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M, or MUMPS (Massachusetts General Hospital Utility Multi-Programming System) is
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a procedural language with a built-in NoSQL database. Or, it’s a database with
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an integrated language optimized for accessing and manipulating that database.
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A key feature of M is that accessing local variables in memory and persistent
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storage use the same basic syntax, so there's no separate query
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language to remember. This makes it fast to program with, especially for
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beginners. M's syntax was designed to be concise in an era where
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computer memory was expensive and limited. This concise style means that a lot
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more fits on one screen without scrolling.
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The M database is a hierarchical key-value store designed for high-throughput
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transaction processing. The database is organized into tree structures called
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"globals", which are sparse data structures with parallels to modern formats
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like JSON.
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Originally designed in 1966 for the healthcare applications, M continues to be
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used widely by healthcare systems and financial institutions for high-throughput
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real-time applications.
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### Example
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Here's an example M program to calculate the Fibonacci series:
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```
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fib ; compute the first few Fibonacci terms
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new i,a,b,sum
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set (a,b)=1 ; Initial conditions
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for i=1:1 do quit:sum>1000
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. set sum=a+b
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. write !,sum
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. set a=b,b=sum
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```
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### Comments
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```
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; Comments start with a semicolon (;)
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```
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### Data Types
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M has two data types:
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```
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; Numbers - no commas, leading and trailing 0 removed.
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; Scientific notation with 'E'.
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; Floats with IEEE 754 double-precision values (15 digits of precision)
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; Examples: 20, 1e3 (stored as 1000), 0500.20 (stored as 500.2)
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; Strings - Characters enclosed in double quotes.
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; "" is the null string. Use "" within a string for "
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; Examples: "hello", "Scrooge said, ""Bah, Humbug!"""
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```
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### Commands
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Commands are case insensitive, and have a shortened abbreviation, often the first letter. Commands have zero or more arguments,depending on the command. M is whitespace-aware. Spaces are treated as a delimiter between commands and arguments. Each command is separated from its arguments by 1 space. Commands with zero arguments are followed by 2 spaces.
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#### W(rite)
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Print data to the current device.
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```
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WRITE !,"hello world"
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```
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! is syntax for a new line. Multiple statements can be provided as additional arguments:
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```
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w !,"foo bar"," ","baz"
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```
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#### R(ead)
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Retrieve input from the user
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```
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READ var
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r !,"Wherefore art thou Romeo? ",why
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```
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Multiple arguments can be passed to a read command. Constants are outputted. Variables are retrieved from the user. The terminal waits for the user to enter the first variable before displaying the second prompt.
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```
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r !,"Better one, or two? ",lorem," Better two, or three? ",ipsum
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```
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#### S(et)
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Assign a value to a variable
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```
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SET name="Benjamin Franklin"
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s centi=0.01,micro=10E-6
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w !,centi,!,micro
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;.01
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;.00001
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```
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#### K(ill)
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Remove a variable from memory or remove a database entry from disk.
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```
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KILL centi
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k micro
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```
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### Globals and Arrays
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In addition to local variables, M has persistent variables stored to disk called _globals_. Global names must start with a __caret__ (__^__). Globals are the built-in database of M.
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Any variable can be an array with the assignment of a _subscript_. Arrays are sparse and do not have a predefined size. Arrays should be visualized like trees, where subscripts are branches and assigned values are leaves. Not all nodes in an array need to have a value.
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```
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s ^cars=20
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s ^cars("Tesla",1,"Name")="Model 3"
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s ^cars("Tesla",2,"Name")="Model X"
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s ^cars("Tesla",2,"Doors")=5
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w !,^cars
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; 20
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w !,^cars("Tesla")
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; null value - there's no value assigned to this node but it has children
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w !,^cars("Tesla",1,"Name")
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; Model X
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```
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Arrays are automatically sorted in order. Take advantage of the built-in sorting by setting your value of interest as the last child subscript of an array rather than its value.
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```
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; A log of temperatures by date and time
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s ^TEMPS("11/12","0600",32)=""
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s ^TEMPS("11/12","1030",48)=""
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s ^TEMPS("11/12","1400",49)=""
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s ^TEMPS("11/12","1700",43)=""
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```
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### Operators
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```jinja
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; Assignment: =
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; Unary: + Convert a string value into a numeric value.
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; Arthmetic:
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; + addition
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; - subtraction
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; * multiplication
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; / floating-point division
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; \ integer division
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; # modulo
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; ** exponentiation
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; Logical:
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; & and
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; ! or
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; ' not
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; Comparison:
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; = equal
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; '= not equal
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; > greater than
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; < less than
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; '> not greater / less than or equal to
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; '< not less / greater than or equal to
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; String operators:
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; _ concatenate
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; [ contains a contains b
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; ]] sorts after a comes after b
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; '[ does not contain
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; ']] does not sort after
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```
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#### Order of operations
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Operations in M are _strictly_ evaluated left to right. No operator has precedence over any other.
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You should use parentheses to group expressions.
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```
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w 5+3*20
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;160
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;You probably wanted 65
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w 5+(3*20)
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```
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### Flow Control, Blocks, & Code Structure
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A single M file is called a _routine_. Within a given routine, you can break your code up into smaller chunks with _tags_. The tag starts in column 1 and the commands pertaining to that tag are indented.
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A tag can accept parameters and return a value, this is a function. A function is called with '$$':
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```
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; Execute the 'tag' function, which has two parameters, and write the result.
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w !,$$tag^routine(a,b)
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```
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M has an execution stack. When all levels of the stack have returned, the program ends. Levels are added to the stack with _do_ commands and removed with _quit_ commands.
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#### D(o)
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With an argument: execute a block of code & add a level to the stack.
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```
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d ^routine ;run a routine from the begining.
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; ;routines are identified by a caret.
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d tag ;run a tag in the current routine
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d tag^routine ;run a tag in different routine
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```
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Argumentless do: used to create blocks of code. The block is indented with a period for each level of the block:
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```
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set a=1
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if a=1 do
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. write !,a
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. read b
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. if b > 10 d
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. . w !, b
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w "hello"
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```
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#### Q(uit)
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Stop executing this block and return to the previous stack level.
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Quit can return a value.
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#### N(ew)
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Clear a given variable's value _for just this stack level_. Useful for preventing side effects.
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Putting all this together, we can create a full example of an M routine:
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```
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; RECTANGLE - a routine to deal with rectangle math
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q ; quit if a specific tag is not called
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main
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n length,width ; New length and width so any previous value doesn't persist
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w !,"Welcome to RECTANGLE. Enter the dimensions of your rectangle."
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r !,"Length? ",length,!,"Width? ",width
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d area(length,width) ;Do a tag
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s per=$$perimeter(length,width) ;Get the value of a function
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w !,"Perimeter: ",per
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q
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area(length,width) ; This is a tag that accepts parameters.
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; It's not a function since it quits with no value.
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w !, "Area: ",length*width
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q ; Quit: return to the previous level of the stack.
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perimeter(length,width)
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q 2*(length+width) ; Quits with a value; thus a function
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```
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### Conditionals, Looping and $Order()
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F(or) loops can follow a few different patterns:
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```jinja
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;Finite loop with counter
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;f var=start:increment:stop
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f i=0:5:25 w i," " ;0 5 10 15 20 25
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; Infinite loop with counter
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; The counter will keep incrementing forever. Use a conditional with Quit to get out of the loop.
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;f var=start:increment
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f j=1:1 w j," " i j>1E3 q ; Print 1-1000 separated by a space
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;Argumentless for - infinite loop. Use a conditional with Quit.
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; Also read as "forever" - f or for followed by two spaces.
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s var=""
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f s var=var_"%" w !,var i var="%%%%%%%%%%" q
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; %
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; %%
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; %%%
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; %%%%
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; %%%%%
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; %%%%%%
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; %%%%%%%
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; %%%%%%%%
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; %%%%%%%%%
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; %%%%%%%%%%
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```
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#### I(f), E(lse), Postconditionals
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M has an if/else construct for conditional evaluation, but any command can be conditionally executed without an extra if statement using a _postconditional_. This is a condition that occurs immediately after the command, separated with a colon (:).
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```jinja
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; Conditional using traditional if/else
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r "Enter a number: ",num
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i num>100 w !,"huge"
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e i num>10 w !,"big"
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e w !,"small"
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; Postconditionals are especially useful in a for loop.
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; This is the dominant for loop construct:
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; a 'for' statement
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; that tests for a 'quit' condition with a postconditional
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; then 'do'es an indented block for each iteration
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s var=""
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f s var=var_"%" q:var="%%%%%%%%%%" d ;Read as "Quit if var equals "%%%%%%%%%%"
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. w !,var
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;Bonus points - the $L(ength) built-in function makes this even terser
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s var=""
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f s var=var_"%" q:$L(var)>10 d ;
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. w !,var
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```
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#### Array Looping - $Order
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As we saw in the previous example, M has built-in functions called with a single $, compared to user-defined functions called with $$. These functions have shortened abbreviations, like commands.
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One of the most useful is __$Order()__ / $O(). When given an array subscript, $O returns the next subscript in that array. When it reaches the last subscript, it returns "".
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```jinja
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;Let's call back to our ^TEMPS global from earlier:
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; A log of temperatures by date and time
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s ^TEMPS("11/12","0600",32)=""
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s ^TEMPS("11/12","0600",48)=""
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s ^TEMPS("11/12","1400",49)=""
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s ^TEMPS("11/12","1700",43)=""
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; Some more
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s ^TEMPS("11/16","0300",27)=""
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s ^TEMPS("11/16","1130",32)=""
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s ^TEMPS("11/16","1300",47)=""
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;Here's a loop to print out all the dates we have temperatures for:
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n date,time ; Initialize these variables with ""
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; This line reads: forever; set date as the next date in ^TEMPS.
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; If date was set to "", it means we're at the end, so quit.
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; Do the block below
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f s date=$ORDER(^TEMPS(date)) q:date="" d
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. w !,date
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; Add in times too:
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f s date=$ORDER(^TEMPS(date)) q:date="" d
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. w !,"Date: ",date
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. f s time=$O(^TEMPS(date,time)) q:time="" d
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. . w !,"Time: ",time
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; Build an index that sorts first by temperature -
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; what dates and times had a given temperature?
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n date,time,temp
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f s date=$ORDER(^TEMPS(date)) q:date="" d
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. f s time=$O(^TEMPS(date,time)) q:time="" d
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. . f s temp=$O(^TEMPS(date,time,temp)) q:temp="" d
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. . . s ^TEMPINDEX(temp,date,time)=""
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;This will produce a global like
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^TEMPINDEX(27,"11/16","0300")
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^TEMPINDEX(32,"11/12","0600")
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^TEMPINDEX(32,"11/16","1130")
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```
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## Further Reading
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There's lots more to learn about M. A great short tutorial comes from the University of Northern Iowa and Professor Kevin O'Kane's [Introduction to the MUMPS Language][1] presentation.
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To install an M interpreter / database on your computer, try a [YottaDB Docker image][2].
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YottaDB and its precursor, GT.M, have thorough documentation on all the language features including database transactions, locking, and replication:
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* [YottaDB Programmer's Guide][3]
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* [GT.M Programmer's Guide][4]
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[1]: https://www.cs.uni.edu/~okane/source/MUMPS-MDH/MumpsTutorial.pdf
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[2]: https://yottadb.com/product/get-started/
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[3]: https://docs.yottadb.com/ProgrammersGuide/langfeat.html
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[4]: http://tinco.pair.com/bhaskar/gtm/doc/books/pg/UNIX_manual/index.html
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