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Version: 1.3.0

Looping

JsLIGO currently supports iteration through loops, which we understand as syntactic constructs where the state of a stopping condition is mutated until it becomes true and the loop is exited. There are two kinds of loops: for loops and the more general while loops.

Here is how to compute the greatest common divisors of two natural numbers by means of Euclid's algorithm and a while loop:

function gcd (a: nat, b: nat) {
let [x,y] = [a,b]; // We will modify x and y
if (x < y) {
const z = x;
x = y; y = z; // Swapping x and y
}
let r: nat = 0n
while (y != 0n) {
r = x % y;
x = y;
y = r;
}
return x;
};

Note the use of a conditional statement to swap x and y.

Conditional logic enables forking the control flow depending on the state, that is, the values available at a given point in the code. Put in a less technical manner, conditionals enable decision making.

A conditional statement is made of three parts:

  1. a condition, that is, a boolean expression;
  2. a statement evaluated if, and only if, the condition is true;
  3. a statement evaluated if, and only if, the condition is false.

The syntax uses the keyword if to introduce the condition, a statement or block of statements between { and } for the second part, and the keyword else introduces the last statement or block of statements. The last part can be omitted, as in the example above.

Note: Currently JsLIGO does not support the keywords break & continue in the context of loops.

By comparison, here is how to compute the greatest common divisors of two natural numbers by means of Euclid's algorithm using tail recursion (no loops):

function iter (x: nat,y: nat): nat {
if (y == 0n) return x else return iter (y, x % y)
};
function gcd2 (x: nat,y: nat) : nat {
if (x < y) return iter (y, x) else return iter (x, y)
};

Note: The conditional statements are complete: they both feature an else statement.

Finally, here is how to check if a string is a palindrome using a for loop:

const getChar = (s: string, idx: nat) : string => String.sub(idx, 1n, s);
function isPalindrome (s: string): bool {
let p = "";
let length = String.length(s);
for (let i = length - 1 ; i >= 0 ; i--)
p += getChar(s, abs(i));
return p == s;
};

Note: The return statement ("early exit") is not valid in loops.

for-of loops

JsLIGO for-of loops can iterate through the contents of a collection, that is, a list, a set or a map. This is done with a loop of the form for (const <element var> of <collection var>) <block>.

Here is an example where the integers in a list are summed up.

function sum_list (l : list<int>) {
let acc = 0;
for (const i of l) acc += i;
return acc; // total
};

See the relevant sections on maps and sets for their loops.