ligo/gitlab-pages/docs/language-basics/loops.md

---
id: loops
title: Loops
---

import Syntax from '@theme/Syntax';

## General Iteration



<Syntax syntax="pascaligo">

General iteration in PascaLIGO takes the shape of general loops, which
should be familiar to programmers of imperative languages as "while
loops". Those loops are of the form `while <condition> <block>`. Their
associated block is repeatedly evaluated until the condition becomes
true, or never evaluated if the condition is false at the start. The
loop never terminates if the condition never becomes true. Because we
are writing smart contracts on Tezos, when the condition of a "while"
loops fails to become true, the execution will run out of gas and stop
with a failure anyway.

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

```pascaligo group=a
function gcd (var x : nat; var y : nat) : nat is
  block {
    if x < y then {
      const z : nat = x;
      x := y; y := z
    }
    else skip;
    var r : nat := 0n;
    while y =/= 0n block {
      r := x mod y;
      x := y;
      y := r
    }
  } with x
```

You can call the function `gcd` defined above using the LIGO compiler
like so:
```shell
ligo run-function
gitlab-pages/docs/language-basics/src/loops/gcd.ligo gcd '(2n*2n*3n*11n, 2n*2n*2n*3n*3n*5n*7n)'
# Outputs: +12
```

</Syntax>
<Syntax syntax="cameligo">

CameLIGO is a functional language where user-defined values are
constant, therefore it makes no sense in CameLIGO to feature loops,
which we understand as syntactic constructs where the state of a
stopping condition is mutated, as with "while" loops in PascaLIGO.

Instead, CameLIGO implements a *folded operation* by means of a
predefined function named `Loop.fold_while`. It takes an initial value
of a certain type, called an *accumulator*, and repeatedly calls a
given function, called *folded function*, that takes that
accumulator and returns the next value of the accumulator, until a
condition is met and the fold stops with the final value of the
accumulator. The iterated function needs to have a special type: if
the type of the accumulator is `t`, then it must have the type `bool *
t` (not simply `t`). It is the boolean value that denotes whether the
stopping condition has been reached.

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

```cameligo group=a
let iter (x,y : nat * nat) : bool * (nat * nat) =
  if y = 0n then false, (x,y) else true, (y, x mod y)

let gcd (x,y : nat * nat) : nat =
  let x,y = if x < y then y,x else x,y in
  let x,y = Loop.fold_while iter (x,y)
  in x
```

To ease the writing and reading of the iterated functions (here,
`iter`), two predefined functions are provided: `Loop.resume` and
`Loop.stop`:

```cameligo group=a
let iter (x,y : nat * nat) : bool * (nat * nat) =
  if y = 0n then Loop.stop (x,y) else Loop.resume (y, x mod y)

let gcd (x,y : nat * nat) : nat =
  let x,y = if x < y then y,x else x,y in
  let x,y = Loop.fold_while iter (x,y)
  in x
```

> Note that `stop` and `continue` (now `Loop.resume`) are
> *deprecated*.

You can call the function `gcd` defined above using the LIGO compiler
like so:
```shell
ligo run-function
gitlab-pages/docs/language-basics/src/loops/gcd.mligo gcd (2n*2n*3n*11n, 2n*2n*2n*3n*3n*5n*7n)'
# Outputs: +12
```

</Syntax>
<Syntax syntax="reasonligo">

ReasonLIGO is a functional language where user-defined values are
constant, therefore it makes no sense in ReasonLIGO to feature loops,
which we understand as syntactic constructs where the state of a
stopping condition is mutated, as with "while" loops in PascaLIGO.

Instead, ReasonLIGO features a *fold operation* as a predefined
function named `Loop.fold_while`. It takes an initial value of a
certain type, called an *accumulator*, and repeatedly calls a given
function, called *iterated function*, that takes that accumulator and
returns the next value of the accumulator, until a condition is met
and the fold stops with the final value of the accumulator. The
iterated function needs to have a special type: if the type of the
accumulator is `t`, then it must have the type `bool * t` (not simply
`t`). It is the boolean value that denotes whether the stopping
condition has been reached.

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

```reasonligo group=a
let iter = ((x,y) : (nat, nat)) : (bool, (nat, nat)) =>
  if (y == 0n) { (false, (x,y)); } else { (true, (y, x mod y)); };

let gcd = ((x,y) : (nat, nat)) : nat => {
  let (x,y) = if (x < y) { (y,x); } else { (x,y); };
  let (x,y) = Loop.fold_while (iter, (x,y));
  x
};
```

To ease the writing and reading of the iterated functions (here,
`iter`), two predefined functions are provided: `Loop.resume` and
`Loop.stop`:

```reasonligo group=b
let iter = ((x,y) : (nat, nat)) : (bool, (nat, nat)) =>
  if (y == 0n) { Loop.stop ((x,y)); } else { Loop.resume ((y, x mod y)); };

let gcd = ((x,y) : (nat, nat)) : nat => {
  let (x,y) = if (x < y) { (y,x); } else { (x,y); };
  let (x,y) = Loop.fold_while (iter, (x,y));
  x
};
```

> Note that `stop` and `continue` (now `Loop.resume`) are
> *deprecated*.

</Syntax>


## Bounded Loops

In addition to general loops, PascaLIGO features a specialised kind of
*loop to iterate over bounded intervals*. These loops are familiarly
known as "for loops" and they have the form `for <variable assignment>
to <upper bound> <block>`, as found in imperative languages.

Consider how to sum the natural numbers up to `n`:

```pascaligo group=c
function sum (var n : nat) : int is block {
  var acc : int := 0;
  for i := 1 to int (n) block {
    acc := acc + i
  }
} with acc
```

(Please do not use that function: there exists a closed form formula.)

You can call the function `sum` defined above using the LIGO compiler
like so:
```shell
ligo run-function
gitlab-pages/docs/language-basics/src/loops/sum.ligo sum 7n
# Outputs: 28
```

PascaLIGO "for" loops can also 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 <element var> in <collection type> <collection var> <block>`, 
where `<collection type>` is any of the following keywords:
`list`, `set` or `map`.

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

```pascaligo group=d
function sum_list (var l : list (int)) : int is block {
  var total : int := 0;
  for i in list l block {
    total := total + i
  }
} with total
```

You can call the function `sum_list` defined above using the LIGO compiler
like so:
```shell
ligo run-function
gitlab-pages/docs/language-basics/src/loops/collection.ligo sum_list
'list [1;2;3]'
# Outputs: 6
```

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

```pascaligo group=d
function sum_set (var s : set (int)) : int is block {
  var total : int := 0;
  for i in set s block {
    total := total + i
  }
} with total
```

You can call the function `sum_set` defined above using the LIGO compiler
like so:
```shell
ligo run-function
gitlab-pages/docs/language-basics/src/loops/collection.ligo sum_set
'set [1;2;3]'
# Outputs: 6
```

Loops over maps are actually loops over the bindings of the map, that
is, a pair key-value noted `key -> value` (or any other
variables). Given a map from strings to integers, here is how to sum
all the integers and concatenate all the strings.

Here is an example where the keys are concatenated and the values are
summed up.

```pascaligo group=d
function sum_map (var m : map (string, int)) : string * int is block {
  var string_total : string := "";
  var int_total : int := 0;
  for key -> value in map m block {
    string_total := string_total ^ key;
    int_total := int_total + value
  }
} with (string_total, int_total)
```

You can call the function `sum_map` defined above using the LIGO compiler
like so:
```shell
ligo run-function
gitlab-pages/docs/language-basics/src/loops/collection.ligo sum_map
'map ["1"->1; "2"->2; "3"->3]'
# Outputs: ( "123", 6 )
```