I have the following code to read user input keys from terminal using termion
use std::io::{stdin, stdout};
use termion::event::Key;
fn main() {
let mut stdout = stdout().into_raw_mode().unwrap();
let stdin = stdin();
let char_from_config = 'e';
for c in stdin.keys() {
match c.unwrap() {
Key::Char('q') => {
break;
}
Key::Char('h') => {
// do something
}
Key::Char('l') => {
// do something else
}
_ => {}
}
stdout.flush().unwrap();
}
}
What I would want to do is to read not just Key::Char('q') but some other dynamic character value, which I collect from somewhere else, like Key::Char(char_from_config), but it doesn't work for some reason.
Is there a way to paste a variable containing char instead of an actual 'char' to match arms ?
When you write a match arm Key(c) => ..., c becomes part of a pattern that match will match against and if value matched this enum variant c will be equal to whatever this variant holds. You however want to say "match only if it's Key variant with this given value". You have two options how to do it.
You can either have const value (you probably don't want to do that):
const CHAR_FROM_CONFIG: char = 'e';
match ... {
Key(CHAR_FROM_CONFIG) => (),
_ => ()
}
Or use a match guard (you probably do want to do that):
let char_from_config = 'e';
match ... {
// c will match here any character, but this arm will succeed only
// when it will be equal to char_from_config
Key(c) if c == char_from_config => (),
_ => ()
}
Related
I modified the code from the Rust Book's Guessing Game Tutorial to make it a little shorter; for a slide. Alas, I've introduced a bug, and it no longer executes correctly: the first input works as expected, but subsequent entries now yield no feedback.
What is the best way to guard against this situation?
use std::io;
use std::cmp::Ordering;
use rand::Rng;
fn main() {
println!("Guess the number!");
let secret_number = rand::thread_rng().gen_range(1, 101);
let mut guess = String::new();
loop {
io::stdin().read_line(&mut guess)
.expect("Failed to read line");
let guess: u32 = match guess.trim().parse() {
Ok(num) => num,
Err(_) => continue,
};
println!("You guessed: {}", guess);
match guess.cmp(&secret_number) {
Ordering::Less => println!("Too small!"),
Ordering::Greater => println!("Too big!"),
Ordering::Equal => {
println!("You win!");
break;
}
}
}
}
read_line will append the line to buffer, so your guess will accumulate all the inputs include newline characters! Moving let mut guess = String::new(); inside the loop solves the problem:
fn main() {
...
loop {
let mut guess = String::new();
...
}
}
I'm trying to get information (true/false) from AsyncStorage in a function and create a string which is importent to fetch data in the next step. My problem is, the function is not finished until the string is required.
I tried many solutions from the internet like async function and await getItem or .done() or .then(), but none worked out for me.
//_getFetchData()
AsyncStorage.getAllKeys().then((result) => { //get all stored Keys
valuelength = result.length;
if (valuelength !== 0) {
for (let i = 0; i < valuelength; i++) {
if (result[i].includes("not") == false) { //get Keys without not
AsyncStorage.getItem(result[i]).then((resultvalue) => {
if (resultvalue === 'true') {
if (this.state.firstValue) {
this.state.channels = this.state.channels + "channel_id" + result[i];
console.log("channel: " + this.state.channels);
}
else {
this.state.channels = this.state.channels + "channel" + result[i];
}
}
});
}
return this.state.channels;
_fetchData() {
var channel = this._getFetchData();
console.log("channel required: " + channel);
}
The current behaviour is that the console displays first "channel required: " than "channel: channel_id0".
Aspects in your question are unclear:
You don't say when this.state.firstValue is set, and how that relates to what you are trying to accomplish.
You have a for-loop where you could be setting the same value multiple times.
You mutate the state rather than set it. This is not good, see this SO question for more on that.
There are somethings we can do to make your code easier to understand. Below I will show a possible refactor. Explaining what I am doing at each step. I am using async/await because it can lead to much tidier and easier to read code, rather than using promises where you can get lost in callbacks.
Get all the keys from AsyncStorage
Make sure that there is a value for all the keys.
Filter the keys so that we only include the ones that do not contain the string 'not'.
Use a Promise.all, this part is important as it basically gets all the values for each of the keys that we just found and puts them into an array called items
Each object in the items array has a key and a value property.
We then filter the items so that only the ones with a item.value === 'true' remain.
We then filter the items so that only the ones with a item.value !== 'true' remain. (this may be optional it is really dependent on what you want to do)
What do we return? You need to add that part.
Here is the refactor:
_getFetchData = async () => {
let allKeys = await AsyncStorage.getAllKeys(); // 1
if (allKeys.length) { // 2
let filteredKeys = allKeys.filter(key => !key.includes('not')); // 3
let items = await Promise.all(filteredKeys.map(async key => { // 4
let value = await AsyncStorage.getItem(key);
return { key, value }; // 5
}))
let filteredTrueItems = items.filter(item => items.value === 'true'); // 6
let filteredFalseItems = items.filter(item => items.value !== 'true'); // 7
// now you have two arrays one with the items that have the true values
// and one with the items that have the false values
// at this points you can decide what to return as it is not
// that clear from your question
// return the value that your want // 8
} else {
// return your default value if there are no keys // 8
}
}
You would call this function as follows:
_fetchData = async () => {
let channel = await this._getFetchData();
console.log("channel required: " + channel);
}
Although the above will work, it will not currently return a value as you haven't made it clear which value you wish to return. I would suggest you build upon the code that I have written here and update it so that it returns the values that you want.
Further reading
For further reading I would suggest these awesome articles by Michael Chan that discuss state
https://medium.learnreact.com/setstate-is-asynchronous-52ead919a3f0
https://medium.learnreact.com/setstate-takes-a-callback-1f71ad5d2296
https://medium.learnreact.com/setstate-takes-a-function-56eb940f84b6
I would also suggest taking some time to read up about async/await and promises
https://medium.com/#bluepnume/learn-about-promises-before-you-start-using-async-await-eb148164a9c8
And finally this article and SO question on Promise.all are quite good
https://www.taniarascia.com/promise-all-with-async-await/
Using async/await with a forEach loop
Try this instead. Async functions and Promises can be tricky to get right and can be difficult to debug but you're on the right track.
async _getFetchData() {
let channels = "";
let results = await AsyncStorage.getAllKeys();
results.forEach((result) => {
if (result.includes("not") === false) {
let item = await AsyncStorage.getItem(result);
if (item === 'true') {
console.log(`channel: ${result}`)
channels = `channel_id ${result}`;
}
}
});
return channels;
}
_fetchData() {
this._getFetchData().then((channels) => {
console.log(`channel required: ${channel}`);
});
}
what if you wrap the _getFetchData() in a Promise? This would enable you to use
var channel = this._getFetchData().then(console.log("channel required: " + channel));
Otherwise the console.log won't wait for the execution of the _getFetchData().
This is what the console.log is telling you. it just logs the string. the variable is added after the async operation is done.
UPDATE
I would try this:
//_getFetchData()
AsyncStorage.getAllKeys().then((result) => { //get all stored Keys
valuelength = result.length;
if (valuelength !== 0) {
for (let i = 0; i < valuelength; i++) {
if (result[i].includes("not") == false) { //get Keys without not
AsyncStorage.getItem(result[i]).then((resultvalue) => {
if (resultvalue === 'true') {
if (this.state.firstValue) {
this.state.channels = this.state.channels + "channel_id" + result[i];
console.log("channel: " + this.state.channels);
}
else {
this.state.channels = this.state.channels + "channel" + result[i];
}
}
});
}
return new Promise((resolve, reject) => {
this.state.channels !=== undefined ? resolve(this.state.channels) : reject(Error('error '));
}
_fetchData() {
var channel = this._getFetchData().then(console.log("channel required: " + channel));
}
maybe you must change the this.state.channels !=== undefined to an expression that's matches the default value of this.state.channels.
I have this piece of code where I try to convert command line arguments to integers. The problem is, the variables width and height should really be immutable, as I don't plan to change them. Is there any way I can use match like let width = {match....} and initialize them in one step as immutable variables rather than giving them a default value and mutating them? I believe this would be safer and more efficient.
let args: Vec<String> = env::args().collect();
let width_arg = &args[1];
let height_arg = &args[2];
let mut width = 0;
let mut height = 0;
match width_arg.parse::<i32>() {
Ok(w) => width = w,
Err(_) => ask_for_number(),
}
match height_arg.parse::<i32>() {
Ok(h) => height = h,
Err(_) => ask_for_number(),
}
Sure, match is an expression, so you should be able to rewrite it as follows:
let args: Vec<String> = env::args().collect();
let width_arg = &args[1];
let height_arg = &args[2];
let width = match width_arg.parse::<i32>() {
Ok(w) => w,
Err(_) => ask_for_number(),
};
let height = match height_arg.parse::<i32>() {
Ok(h) => h,
Err(_) => ask_for_number(),
};
Yes, just initialise the variable like so: let a = match b { /* your code here*/ };
I'm trying to implement a simple REPL calculator in Rust and I'm hitting brick walls all over the place.
I'm consuming chars while iterating over a hardcoded string. When I hit a numeric character I want to pass control over to a function that will consume the rest of the number (assuming the number has more than one digit) and return the number, converted to an Integer.
I'm having trouble with passing Chars iterator to a function. The error I'm getting is use of moved value: 'iter'.
I understand that I can't mutate something that I gave to someone else - something that had its ownership moved - but I don't know any other way of doing this, especially since the Chars iterator is non-copyable.
#[derive(Clone, Debug)]
enum Token {
Addition,
Substraction,
Multiplication,
Division,
Integer(i32),
Error,
}
fn consume_number(mut iter: std::str::Chars) -> Option<i32> {
while let Some(item) = iter.next() {
println!("{:?}", item);
}
return Some(1337);
}
fn tokenize(line: &str) -> Vec<Token> {
let mut iter = line.chars();
let mut tokens = Vec::new();
let mut token;
while let Some(c) = iter.next() {
if c.is_whitespace() { continue };
if c.is_digit(10) {
token = match consume_number(iter) {
Some(i32) => Token::Integer(i32),
None => Token::Error,
};
} else {
token = match c {
'+' => Token::Addition,
'-' => Token::Substraction,
'*' => Token::Multiplication,
'/' => Token::Division,
_ => Token::Error,
};
};
tokens.push(token);
}
return tokens;
}
fn main() {
let line = "631 * 32 + 212 - 15 / 89";
println!("{:?}", tokenize(&line));
}
The answer is yes, it's done in the FromIterator trait.
What you experience here is much more basic:
fn consume_number(mut iter: std::str::Chars) -> Option<i32> { ... }
while let Some(c) = iter.next() {
...
match_consume_number(iter)
...
}
When calling match_consume_number you are transferring ownership of the iterator to it. It means that at the next iteration of the loop body, this iter variable is no longer available.
If the iterator is meant to still be usable afterward, you should pass a reference to it:
fn consume_number(iter: &mut std::str::Chars) -> Option<i32> { ... }
while let Some(c) = iter.next() {
...
match_consume_number(&mut iter)
...
}
You were close!
I am using this code to append a new line to the end of a file:
let text = "New line".to_string();
let mut option = OpenOptions::new();
option.read(true);
option.write(true);
option.create(true);
match option.open("foo.txt") {
Err(e) => {
println!("Error");
}
Ok(mut f) => {
println!("File opened");
let size = f.seek(SeekFrom::End(0)).unwrap();
let n_text = match size {
0 => text.clone(),
_ => format!("\n{}", text),
};
match f.write_all(n_text.as_bytes()) {
Err(e) => {
println!("Write error");
}
Ok(_) => {
println!("Write success");
}
}
f.sync_all();
}
}
It works, but I think it's too difficult. I found option.append(true);, but if I use it instead of option.write(true); I get "Write error".
Using OpenOptions::append is the clearest way to append to a file:
use std::fs::OpenOptions;
use std::io::prelude::*;
fn main() {
let mut file = OpenOptions::new()
.write(true)
.append(true)
.open("my-file")
.unwrap();
if let Err(e) = writeln!(file, "A new line!") {
eprintln!("Couldn't write to file: {}", e);
}
}
As of Rust 1.8.0 (commit) and RFC 1252, append(true) implies write(true). This should not be a problem anymore.
Before Rust 1.8.0, you must use both write and append — the first one allows you to write bytes into a file, the second specifies where the bytes will be written.