Unicon Book

1.5. STRINGS AND CSETS 15
local i, j, old_r, r
i := read()
j := read()
old_r := r := min(i, j)
while r > 0 do {
old_r := r
if i > j then
i := r := i % j
else
j := r := j % i
}
write(old_r)
end
This example illustrates assignment; values are assigned to (or ”stored in”) variables
with the := operator. As you saw in the previous section, the function read() reads a line
from the input and returns its value. The modulo operator (%) is an important part of
this program: i % j is the remainder when i is divided by j.
While loops can use a reserved word do followed by an expression (often a compound
expression in curly braces). The expression following the do is executed once each time the
expression that controls the while succeeds. Inside the while loop, a conditional if-then-else
expression is used to select from two possible actions.
The names of the variables in this example are obscure, and there are no comments in
it other than the one at the top. Can you guess what this program does, without running
it? If you give up, try running it with a few pairs of positive numbers.
In addition to arithmetic operators, there are augmented assignment operators. To
increment the value in a variable by 2, these two statements are equivalent:
i +:= 2
i := i + 2
Augmented assignment works for most binary operators, not just arithmetic. The expression
i op:= expr means the same as i := i op expr.
1.5 Strings and Csets
The non-numeric atomic types in Unicon are character sequences (strings) and character
sets (csets). Icon came from the domain of string processing, and from it Unicon inherits
many sophisticated features for manipulating strings and performing pattern matching.
This section presents the simple and most common operations. More advanced operations
and examples using strings and csets are given in Chapter 4.