I have an if statement in a for loop, and I want it to create a variable with the lifetime of that iteration of the for loop.
for condition_raw in conditions_arr {
println!("{}", condition_raw);
let matching = !condition_raw.contains('!');
if matching {
let index = condition_raw.find('=').unwrap_or_default();
} else {
let index = condition_raw.find('!').unwrap_or_default();
}
let key = &condition_raw[..index];
}
let key = &condition_raw[..index]; currently throws cannot find value index in this scope
not found in this scope rustc E0425
I'll ignore the condition variable which does not seem to be used at all in your example.
A let statement creates a binding that holds at most for the current scope. For this reason, when you create the index variable inside the if, you are not making it accessible anywhere else. There are two ways to solve this issue.
The first way is to explicitly declare index as being part of the outer scope, and only define it inside the if statement.
for condition_raw in conditions_arr {
let matching = !condition_raw.contains('!');
let index;
if matching {
index = condition_raw.find('=').unwrap_or_default();
} else {
index = condition_raw.find('!').unwrap_or_default();
}
let key = &condition_arr[..index];
}
There is no risk of accidentally not defining index, since Rust will make sure that index is defined (exactly once) in all possible branching of your code before it is used. Yet, it's not a pretty solution because it violates a "locality" principle, that is that pieces of code should have effects on or pieces of code that are sufficiently close. In this case, the let index; is not too far from its definition, but it could be arbitrarily far, which makes it painful for someone who reads your code to remember that there is a declared but not yet defined.
Alternatively, you could use the fact that most things in Rust are expressions:
for condition_raw in conditions_arr {
let matching = !condition_raw.contains('!');
let index = if matching {
condition_raw.find('=').unwrap_or_default();
} else {
condition_raw.find('!').unwrap_or_default();
}
let key = &condition_arr[..index];
}
But, in fact, you could factorize your code even more, which is usually better:
for condition_raw in conditions_arr {
let matching = !condition_raw.contains('!');
let index = condition_raw.find(if matching {
'='
} else {
'!'
}).unwrap_or_default();
let key = &condition_arr[..index];
Or, even more
for condition_raw in conditions_arr {
let index = condition_raw
.find('!')
.or_else(|| condition_raw.find('='))
.unwrap_or_default();
let key = &condition_arr[..index];
}
An idiomatic way to assign variables from an if else statement is as follows:
let index: usize = if matching {
condition_raw.find('=').unwrap_or_default()
} else {
condition_raw.find('!').unwrap_or_default()
};
Idiomatic way of assigning a value from an if else condition in Rust
In Rust, an if/else block is an expression. That is to say, the block itself has a value, equivalent to the last expression in whatever section was executed. With that in mind, I would structure your code like this:
Related
I have this line in my program:
let date = file.metadata().unwrap().modified().unwrap();
Can it be changed into form of if let Ok(date) = file.metadata().something.... and still be one liner?
Forgot to add: can't use ? operator, bc this is in a closure in for_each().
Using Result::and_then:
if let Ok(date) = file.metadata().and_then(|md| md.modified()) {
// stuff
}
Using the "try" operator (?):
// containing function returns `Result<T, E>` where `E: From<io::Error>`
let date = file.metadata()?.modified()?;
If you're inside a closure which must return (), and you want to ignore the error, I'd actually recommend using let else as such:
let Ok(metadata) = file.metadata() else { return };
let Ok(date) = metadata.modified() else { return };
// ...
This has the advantage that it doesn't increase the indentation level.
Just like vec![2,3,4], can we define a similar macro vecvec to initialize vector of vector. Eg.
let vv0 = vecvec![[2,3,4],[5,6,7]]; // vec of 2 vecs
let vv1 = vecvec![[1,2,3]];
let vv2 = vecvec![[1,2,3], []];
let vv3 = vecvec![[1,3,2]; 2];
You just need to think through the problem. You really only have 2 main cases. The first case being if elements are listed (Ex: a, b, c) and the second where a single value and length are given (Ex: a; b). We can even check our work by reading the documentation for vec!. In the documentation we can see vec! is defined as follows:
macro_rules! vec {
() => { ... };
($elem:expr; $n:expr) => { ... };
($($x:expr),+ $(,)?) => { ... };
}
As you can see, they have 3 cases. We didn't specify the the case were no items are included, but that does not really matter since your macro can call vec! and have it handle that case for you.
We can just copy the cases in their macro and add the functionality inside. The only other issue that might stop you is that [a, b, c] is an expression in of itself. Luckily we can just skip that by specifying items as requiring brackets and pick out the items ourselves before passing them off to vec!.
macro_rules! vecvec {
([$($elem:expr),*]; $n:expr) => {{
let mut vec = Vec::new();
vec.resize_with($n, || vec![$($elem),*]);
vec
}};
($([$($x:expr),*]),* $(,)?) => {
vec![$(vec![$($x),*]),*]
};
}
Instead of defining a new macro. You can initialize the vector of the vector.
In the example below, I'm explicitly setting type. It's not necessary but a good practice.
let vv0:Vec<Vec<u32>> = vec![vec![2,3,4],vec![5,6,7]];
let vv1:Vec<Vec<u32>> = vec![vec![2,3,4],vec![5]];
let vv2:Vec<Vec<u32>> = vec![vec![],vec![5,6,7]];
let vv3:Vec<Vec<u32>> = vec![vec![2,3,4],vec![]];
I was trying to change some of vector elements, while itering over the vector.
for operator in operators {
// add left side
let node = nodes[operator.index-1].clone();
nodes[operator.index].children.push(node);
// add right side
let node = nodes[operator.index+1].clone();
nodes[operator.index].children.push(node);
// remove used nodes
nodes.remove(operator.index+1);
nodes.remove(operator.index-1);
// two items were removed, so every index higher than the current one needs to be lowered by 2
for operator2 in &mut operators {
if operator2.index > operator.index {
operator2.index -= 2;
}
}
}
Sadly this isn't possible in rust, because error says that 'operators' was moved. I tried to change for to look like this:
for operator in &operators, but then it has problem getting 'operators' as mutable and immutable at the same time. What can I do to make it work?
Use simple loops with indices:
for operator_idx in 0..operators.len() {
let operator = &operators[operator_idx];
// add left side
let node = nodes[operator.index-1].clone();
nodes[operator.index].children.push(node);
// add right side
let node = nodes[operator.index+1].clone();
nodes[operator.index].children.push(node);
// remove used nodes
nodes.remove(operator.index+1);
nodes.remove(operator.index-1);
// two items were removed, so every index higher than the current one needs to be lowered by 2
for operator2 in &mut operators {
if operator2.index > operator.index {
operator2.index -= 2;
}
}
}
I'm learning Rust and have been going through leetcode problems. One of them includes merging two linked lists, whose nodes are optional. I want to write a while loop that would go on until at least 1 node becomes None, and I was trying to use the while let loop for that.
However, it looks like the while let syntax supports only one optional, e.g.:
while let Some(n) = node {
// do stuff
}
but I can't write
while let Some(n1) = node1 && Some(n2) = node2 {
}
Am I misunderstanding the syntax? I know I can rewrite it with a while true loop, but is there a more elegant way of doing it?
Also, can one do multiple checks with if let? Like if let None=node1 && None=node2 {return}
You can pattern match with Option::zip:
while let Some((n1, n2)) = node1.zip(node2) {
...
}
In addition to what #Netwave said, on nightly you can use the unstable let_chains feature:
#![feature(let_chains)]
while let Some(n1) = node1 && let Some(n2) = node2 {
// ...
}
Is there a more idiomatic way to express something like the following?
fn main() {
let mut foo: Option<u8> = None;
match foo {
Some(foo_val) if ! (foo_val < 5) /* i.e. the negation of my acceptance condition */ => {}
_ => { foo.replace(5); }
}
}
It seems like most of the time there's an alternative to having an arm that doesn't do anything, but I've been unable to find one for this particular case.
What I'd like to say is the more direct if foo.is_none() || /* some way to extract and test the inner value */ { ... }, or perhaps some chaining trick that's eluding me.
// in None case
// │ in Some(_) case
// ┌┴─┐ ┌───────────────────┐
if foo.map_or(true, |foo_val| foo_val < 5) {
// ...
}
For more information see Option::map_or.
There are many ways to do it. One of the simplest (and arguably most readable) is something like this:
if foo.unwrap_or(0) < 5 {
...
}
The above will be true in both cases:
when foo is Some with a value smaller than 5;
when foo is None.
In some more complex scenarios, where the "default" value needs to be calculated and performance is critical, you might want to consider unwrap_or_else.
As Lukas suggested, the map_or method can also be used. Note that arguments passed to map_or are eagerly evaluated, so if performance is critical, you might want to consider map_or_else as an alternative.
You can do it with filter (using the negation of your condition) and is_none:
if foo.filter(|&x| !(x < 5)).is_none() {
// Here either foo was None or it contained a value less than 5
}
I'm not sure I completely understand your question but you can try something like that:
fn main() {
let foo: Option<u8> = None;
let result = foo.filter(|foo_val| !(*foo_val < 5) ).unwrap_or(5);
println!("Result: {result}");
}
More example on Playground
The matches! macro seems like a good fit:
if !matches!(foo, Some(a) if a>=5) { foo.replace(5) }
Rust Playground
I'll throw in another solution just for fun....
foo = foo.
or(Some(5)). // if None return Some(5)
map(|x| if x<5 { 5 } else { x });
or (for this specific example)
foo = foo.
or(Some(5)). // if None return Some(5)
map(|x| u8::max(x, 5));
With filter and or,
foo = foo.filter(|a| *a >= 5)
.or(Some(5));
There is the unstable method Option::is_some_and that has been built for exactly this purpose:
if foo.is_some_and(|foo_val| foo_val < 5) {
// ...
}
As it's unstable, it's currently only usable on nightly. See the tracking issue for up to date information.