259 lines
7.8 KiB
Markdown
259 lines
7.8 KiB
Markdown
---
|
|
id: loops
|
|
title: Loops
|
|
---
|
|
|
|
## General Iteration
|
|
|
|
<!--DOCUSAURUS_CODE_TABS-->
|
|
|
|
<!--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
|
|
```
|
|
|
|
<!--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
|
|
```
|
|
|
|
<!--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*.
|
|
|
|
<!--END_DOCUSAURUS_CODE_TABS-->
|
|
|
|
## 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 )
|
|
```
|