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How do I convert between numeric types safely and idiomatically?
(1 answer)
Closed 4 months ago.
Is there a better way to use i and j for-in variables that avoids the use of std::convert::TryInto and return vec![i.try_into().unwrap(),j.try_into().unwrap()]; for dealing with the usize and i32 conversion problem between what is expected as a result and the actual value type of these variables?
The use of the module and the try_into() and unwrap() functions was because of the compiler error suggestion. But I want to know if there is another way to cast or convert numeric values.
use std::convert::TryInto;
impl Solution {
pub fn two_sum(nums: Vec<i32>, target: i32) -> Vec<i32> {
let mut current = 0;
for i in 0..nums.len() - 1 {
for j in 1..nums.len(){
if j != i {
current = nums[i] + nums[j];
if current == target {
return vec![i.try_into().unwrap(),j.try_into().unwrap()];
}
}
}
}
vec![]
}
}
There is the i as i32 syntax, but that can cause silent overflow if nums.len() > i32::MAX
where i32::MAX = 2_147_483_647 https://doc.rust-lang.org/std/i32/constant.MAX.html
impl Solution {
pub fn two_sum(nums: Vec<i32>, target: i32) -> Vec<i32> {
nums.iter()
.enumerate()
.find_map(|(i, &x)| {
nums.iter()
.enumerate()
.find(|(j, &y)| *j != i && (x + y) == target)
.map(|(j, _)| vec![i as i32, j as i32])
})
.unwrap_or_default()
}
}
Related
This is a very simple example, but how would I do something similar to:
let fact = |x: u32| {
match x {
0 => 1,
_ => x * fact(x - 1),
}
};
I know that this specific example can be easily done with iteration, but I'm wondering if it's possible to make a recursive function in Rust for more complicated things (such as traversing trees) or if I'm required to use my own stack instead.
There are a few ways to do this.
You can put closures into a struct and pass this struct to the closure. You can even define structs inline in a function:
fn main() {
struct Fact<'s> { f: &'s dyn Fn(&Fact, u32) -> u32 }
let fact = Fact {
f: &|fact, x| if x == 0 {1} else {x * (fact.f)(fact, x - 1)}
};
println!("{}", (fact.f)(&fact, 5));
}
This gets around the problem of having an infinite type (a function that takes itself as an argument) and the problem that fact isn't yet defined inside the closure itself when one writes let fact = |x| {...} and so one can't refer to it there.
Another option is to just write a recursive function as a fn item, which can also be defined inline in a function:
fn main() {
fn fact(x: u32) -> u32 { if x == 0 {1} else {x * fact(x - 1)} }
println!("{}", fact(5));
}
This works fine if you don't need to capture anything from the environment.
One more option is to use the fn item solution but explicitly pass the args/environment you want.
fn main() {
struct FactEnv { base_case: u32 }
fn fact(env: &FactEnv, x: u32) -> u32 {
if x == 0 {env.base_case} else {x * fact(env, x - 1)}
}
let env = FactEnv { base_case: 1 };
println!("{}", fact(&env, 5));
}
All of these work with Rust 1.17 and have probably worked since version 0.6. The fn's defined inside fns are no different to those defined at the top level, except they are only accessible within the fn they are defined inside.
As of Rust 1.62 (July 2022), there's still no direct way to recurse in a closure. As the other answers have pointed out, you need at least a bit of indirection, like passing the closure to itself as an argument, or moving it into a cell after creating it. These things can work, but in my opinion they're kind of gross, and they're definitely hard for Rust beginners to follow. If you want to use recursion but you have to have a closure, for example because you need something that implements FnOnce() to use with thread::spawn, then I think the cleanest approach is to use a regular fn function for the recursive part and to wrap it in a non-recursive closure that captures the environment. Here's an example:
let x = 5;
let fact = || {
fn helper(arg: u64) -> u64 {
match arg {
0 => 1,
_ => arg * helper(arg - 1),
}
}
helper(x)
};
assert_eq!(120, fact());
Here's a really ugly and verbose solution I came up with:
use std::{
cell::RefCell,
rc::{Rc, Weak},
};
fn main() {
let weak_holder: Rc<RefCell<Weak<dyn Fn(u32) -> u32>>> =
Rc::new(RefCell::new(Weak::<fn(u32) -> u32>::new()));
let weak_holder2 = weak_holder.clone();
let fact: Rc<dyn Fn(u32) -> u32> = Rc::new(move |x| {
let fact = weak_holder2.borrow().upgrade().unwrap();
if x == 0 {
1
} else {
x * fact(x - 1)
}
});
weak_holder.replace(Rc::downgrade(&fact));
println!("{}", fact(5)); // prints "120"
println!("{}", fact(6)); // prints "720"
}
The advantages of this are that you call the function with the expected signature (no extra arguments needed), it's a closure that can capture variables (by move), it doesn't require defining any new structs, and the closure can be returned from the function or otherwise stored in a place that outlives the scope where it was created (as an Rc<Fn...>) and it still works.
Closure is just a struct with additional contexts. Therefore, you can do this to achieve recursion (suppose you want to do factorial with recursive mutable sum):
#[derive(Default)]
struct Fact {
ans: i32,
}
impl Fact {
fn call(&mut self, n: i32) -> i32 {
if n == 0 {
self.ans = 1;
return 1;
}
self.call(n - 1);
self.ans *= n;
self.ans
}
}
To use this struct, just:
let mut fact = Fact::default();
let ans = fact.call(5);
I used the num::BigUInt type to avoid integer overflows when calculating the factorial of a number.
However, I had to resort to using .clone() to pass rustc's borrow checker.
How can I refactor the factorial function to avoid cloning what could be large numbers many times?
use num::{BigUint, FromPrimitive, One};
fn main() {
for n in -2..33 {
let bign: Option<BigUint> = FromPrimitive::from_isize(n);
match bign {
Some(n) => println!("{}! = {}", n, factorial(n.clone())),
None => println!("Number must be non-negative: {}", n),
}
}
}
fn factorial(number: BigUint) -> BigUint {
if number < FromPrimitive::from_usize(2).unwrap() {
number
} else {
number.clone() * factorial(number - BigUint::one())
}
}
I tried to use a reference to BigUInt in the function definition but got some errors saying that BigUInt did not support references.
The first clone is easy to remove. You are trying to use n twice in the same expression, so don't use just one expression:
print!("{}! = ", n);
println!("{}", factorial(n));
is equivalent to println!("{}! = {}", n, factorial(n.clone())) but does not try to move n and use a reference to it at the same time.
The second clone can be removed by changing factorial not to be recursive:
fn factorial(mut number: BigUint) -> BigUint {
let mut result = BigUint::one();
let one = BigUint::one();
while number > one {
result *= &number;
number -= &one;
}
result
}
This might seem unidiomatic however. There is a range function, that you could use with for, however, it uses clone internally, defeating the point.
I don't think take a BigUint as parameter make sense for a factorial. u32 should be enough:
use num::{BigUint, One};
fn main() {
for n in 0..42 {
println!("{}! = {}", n, factorial(n));
}
}
fn factorial_aux(accu: BigUint, i: u32) -> BigUint {
if i > 1 {
factorial_aux(accu * i, i - 1)
}
else {
accu
}
}
fn factorial(n: u32) -> BigUint {
factorial_aux(BigUint::one(), n)
}
Or if you really want to keep BigUint:
use num::{BigUint, FromPrimitive, One, Zero};
fn main() {
for i in (0..42).flat_map(|i| FromPrimitive::from_i32(i)) {
print!("{}! = ", i);
println!("{}", factorial(i));
}
}
fn factorial_aux(accu: BigUint, i: BigUint) -> BigUint {
if !i.is_one() {
factorial_aux(accu * &i, i - 1u32)
} else {
accu
}
}
fn factorial(n: BigUint) -> BigUint {
if !n.is_zero() {
factorial_aux(BigUint::one(), n)
} else {
BigUint::one()
}
}
Both version doesn't do any clone.
If you use ibig::UBig instead of BigUint, those clones will be free, because ibig is optimized not to allocate memory from the heap for numbers this small.
The num crate in Rust provides a way of representing zeros and ones via T::zero() and T::one(). Is there a way of representing other integers, such as two, three, etc.?
Consider the following (artificial) example:
extern crate num;
trait IsTwo {
fn is_two(self) -> bool;
}
impl<T: num::Integer> IsTwo for T {
fn is_two(self) -> bool {
self == (T::one() + T::one())
}
}
Is there a better way of representing T::one() + T::one() as 2?
One way of representing arbitrary integers in generic code is to use the num::NumCast trait:
impl<T: num::Integer + num::NumCast> IsTwo for T {
fn is_two(self) -> bool {
self == T::from(2).unwrap()
}
}
A related way is to use the num::FromPrimitive trait:
impl<T: num::Integer + num::FromPrimitive> IsTwo for T {
fn is_two(self) -> bool {
self == T::from_i32(2).unwrap()
}
}
Related questions and answers: [1, 2].
You can write a function:
fn two<T>() -> T
where T: num::Integer,
{
let mut v = T::zero();
for _ in 0..2 {
v = v + T::one();
}
v
}
I've chosen this form because it's easily made into a macro, which can be reused for any set of values:
num_constant!(two, 2);
num_constant!(forty_two, 42);
I hear the concerns now... "but that's a loop and inefficient!". That's what optimizing compilers are for. Here's the LLVM IR for two when compiled in release mode:
; Function Attrs: noinline readnone uwtable
define internal fastcc i32 #_ZN10playground3two17hbef99995c3606e93E() unnamed_addr #3 personality i32 (i32, i32, i64, %"unwind::libunwind::_Unwind_Exception"*, %"unwind::libunwind::_Unwind_Context"*)* #rust_eh_personality {
bb3:
br label %bb8
bb8: ; preds = %bb3
ret i32 2
}
That's right - it's been optimized to the value 2. No loops.
It's relatively simple to forge any number from 0 and 1:
you need to create 2, which is hardly difficult
you then proceed in converting your number to base 2, which takes O(log2(N)) operations
The algorithm is dead simple:
fn convert<T: Integer>(n: usize) -> T {
let two = T::one() + T::one();
let mut n = n;
let mut acc = T::one();
let mut result = T::zero();
while n > 0 {
if n % 2 != 0 {
result += acc;
}
acc *= two;
n /= 2;
}
result
}
And will be efficient both in Debug (O(log2(N)) iterations) and Release (the compiler optimizes it out completely).
For those who wish to see it in action, here on the playground we can see that convert::<i32>(12345) is optimized to 12345 as expected.
As an exercise to the reader, implement a generic version of convert which takes any Integer parameter, there's not much operations required on n after all.
How can I replace a value if it fails a predicate?
To illustrate:
assert_eq!((3-5).but_if(|v| v < 0).then(0), 0)
I thought there would be something on Option or Result to allow this, but I cannot find it.
I thought there would be something on Option or Result
But neither of these types appear here. Subtracting two numbers yields another number.
It appears you just want a traditional if-else statement:
fn main() {
let a = 3 - 5;
assert_eq!(if a < 0 { 0 } else { a }, 0);
}
Since you have two values that can be compared, you may also be interested in max:
use std::cmp::max;
fn main() {
assert_eq!(max(0, 3 - 5), 0);
}
You can make your proposed syntax work, but I'm not sure it's worth it. Presented without further comment...
fn main() {
assert_eq!((3 - 5).but_if(|&v| v < 0).then(0), 0)
}
trait ButIf: Sized {
fn but_if<F>(self, f: F) -> ButIfTail<Self>
where F: FnOnce(&Self) -> bool;
}
// or `impl<T> ButIf for T {` for maximum flexibility
impl ButIf for i32 {
fn but_if<F>(self, f: F) -> ButIfTail<Self>
where F: FnOnce(&Self) -> bool,
{
ButIfTail(f(&self), self)
}
}
struct ButIfTail<T>(bool, T);
impl<T> ButIfTail<T> {
fn then(self, alt: T) -> T {
if self.0 {
alt
} else {
self.1
}
}
}
Update: This got a bit nicer since Rust 1.27, when Option::filter was added:
assert_eq!(Some(3 - 5).filter(|&v| v >= 0).unwrap_or(0), 0);
Prior to Rust 1.27, you would have needed an iterator in order to write a single, chained expression without lots of additional custom machinery:
assert_eq!(Some(3 - 5).into_iter().filter(|&v| v >= 0).next().unwrap_or(0), 0);
parts.count() leads to ownership transfer, so parts can't be used any more.
fn split(slice: &[u8], splitter: &[u8]) -> Option<Vec<u8>> {
let mut parts = slice.split(|b| splitter.contains(b));
let len = parts.count(); //ownership transfer
if len >= 2 {
Some(parts.nth(1).unwrap().to_vec())
} else if len >= 1 {
Some(parts.nth(0).unwrap().to_vec())
} else {
None
}
}
fn main() {
split(&[1u8, 2u8, 3u8], &[2u8]);
}
It is also possible to avoid unnecessary allocations of Vec if you only need to use the first or the second part:
fn split<'a>(slice: &'a [u8], splitter: &[u8]) -> Option<&'a [u8]> {
let mut parts = slice.split(|b| splitter.contains(b)).fuse();
let first = parts.next();
let second = parts.next();
second.or(first)
}
Then if you actually need a Vec you can map on the result:
split(&[1u8, 2u8, 3u8], &[2u8]).map(|s| s.to_vec())
Of course, if you want, you can move to_vec() conversion to the function:
second.or(first).map(|s| s.to_vec())
I'm calling fuse() on the iterator in order to guarantee that it will always return None after the first None is returned (which is not guaranteed by the general iterator protocol).
The other answers are good suggestions to answer your problem, but I'd like to point out another general solution: create multiple iterators:
fn split(slice: &[u8], splitter: &[u8]) -> Option<Vec<u8>> {
let mut parts = slice.split(|b| splitter.contains(b));
let parts2 = slice.split(|b| splitter.contains(b));
let len = parts2.count();
if len >= 2 {
Some(parts.nth(1).unwrap().to_vec())
} else if len >= 1 {
Some(parts.nth(0).unwrap().to_vec())
} else {
None
}
}
fn main() {
split(&[1u8, 2u8, 3u8], &[2u8]);
}
You can usually create multiple read-only iterators. Some iterators even implement Clone, so you could just say iter.clone().count(). Unfortunately, Split isn't one of them because it owns the passed-in closure.
One thing you can do is collect the results of the split in a new owned Vec, like this:
fn split(slice: &[u8], splitter: &[u8]) -> Option<Vec<u8>> {
let parts: Vec<&[u8]> = slice.split(|b| splitter.contains(b)).collect();
let len = parts.len();
if len >= 2 {
Some(parts.iter().nth(1).unwrap().to_vec())
} else if len >= 1 {
Some(parts.iter().nth(0).unwrap().to_vec())
} else {
None
}
}