M Language Tutorial - for and Array

for command

GTM>for i=1:1:5 write i,!
1
2
3
4
5

The above for loop a variable i from 1 to 5, increasing by 1 in each iteration.
This one will loop over a list of arguments

GTM>for i="hello","world","bye","moon" write i,!
hello
world
bye
moon

array
In M array is stored as sparse B-tree structure, index or subscript can be anything and can be in any number

GTM> 
GTM>set a(1)="hello",a(2)="world" 
GTM>write a(1)," ",a(2)
hello world
GTM>
GTM>write a(3)         
%GTM-E-UNDEF, Undefined local variable: a(3)

the index of an Array can be any thing so it is like hash/dictionary in other language.

GTM>set a("hello")=1

GTM>set a("world")=2

GTM>write a("hello")
1
GTM>write a("world")
2
GTM>write a("moon") 
%GTM-E-UNDEF, Undefined local variable: a(moon)

We can use any number of subscripts

GTM>set a(1,2)="moon"

GTM>set a(1,1)="world"

GTM>write a(1,1)
world
GTM>write a(1,2)
moon
GTM>write a(1,0)
%GTM-E-UNDEF, Undefined local variable: a(1,0)

built-in function $order is used to get index/subscript of an element of an Array, that is particularly useful for traversal over an Array.

GTM>set b(3)="hello",b(5)="world",b("hello")=1,b("world")=2

GTM>write $order(b("")); with empty string we get a subscript of the first element of an Array
3
GTM>write $order(b(3)); passing one element we get a subscript of next element  
5
GTM>write $order(b(5))
hello
GTM>write $order(b("hello"))
world
GTM>write $order(b("world")); passing the last element we get empty string 

If array is multi dimensional, $order will return subscript of an element in one dimension

GTM>set a("h",1)="hello",a("w",1)="world"

GTM>write $order(a("")); return subscript in the first dimension of first element 
h
GTM>write $order(a("h")); return subscript in the first dimension of second element 
w
GTM>write $order(a("w"))

GTM>write $order(a("h","")); return subscript in the second dimension of first element  
1
GTM>write $order(a("h",1)) 

GTM>write $order(a("w","")); return subscript in the second dimension of second element 
1
GTM>write $order(a("w",1)) 

traversal over a array
for is typical command used to traversal an array

GTM>kill a for i=1:1:10 set a(i)=i*i

We use kill to erase content of the variable a if exists and a for to create 10 elements.

GTM>set i="" for  set i=$order(a(i)) quit:i=""  write i," ",a(i),!; two SPACE after for and two SPACE after quit:i=""
1 1
2 4
3 9
4 16
5 25
6 36
7 49
8 64
9 81
10 100

Here, the argument-less For repeats until stopped by a terminating quit. This line prints a table of i and a(i)
block of multi line of codes under for
M support execute multi line of code under for in the routine

GTM>zedit "nested"
finish
 for name="ivan","john","janes" do
 . if name]"jo"; the operator ] test if name follows string "jo"
 . else  write name,!
 quit

GTM>zlink "nested"

GTM>do finish^nested
ivan
janes
GTM>