for

for(s in X){ 
     statements 
}

As in "if-else", the parentheses after "for" are required. If more than one statement are to be executed in case a condition is true, we must specify a block of statements with braces { }. Otherwise the braces are optional.

X is a pair/vector/matrix/table. It loops over a matrix column by column, and it loops a table row by row. When looping over a matrix, each column is represented as a vector; when looping over a table, each row is presented as a dictionary with column names as keys and cell values as values.

Looping over a pair:

for(s in 2:4){print s};
// output
2
3
for(s in 4:1) print s;
// output
3
2
1

Looping over a vector:

x=4 0 1 3;
for(s in x) print s;
// output
4
0
1
3

Looping over a matrix: column by column.

m=1..6$3:2;
m;

for(s in m){print s};
// output
[1,2,3]
[4,5,6]

Looping over a table: row by row.

x = 1 2 3
y = 4 5 6
t = table(x,y)
for(s in t) print s;
// output
y->4
x->1

y->5
x->2

y->6
x->3