I have been changing some SWIFT code into OBJECTIVE-C, and I am stuck at certain part of the code, where I am unable to understand if it is a condition or something else.
Following is the code and I am stuck on 9th line stating :
if let channel1Buffer = buffer.floatChannelData?[0]
What I do not understand here is the above if condition is checking if "buffer.floatChannelData" is null, and then proceeding to get the first index, or is it something else.
input.installTap(onBus: 0, bufferSize:4096, format:format, block: { [weak self] buffer, when in
guard let this = self else {
return
}
print("Buffer Float Channel Data: ", buffer.floatChannelData as Any);
**if let channel1Buffer = buffer.floatChannelData?[0]** {
print("channel1Buffer: ", channel1Buffer);
/// encode PCM to mp3
let frameLength = Int32(buffer.frameLength) / 2;
print("frameLength: ", frameLength);
let bytesWritten = lame_encode_buffer_interleaved_ieee_float(this.lame, channel1Buffer, frameLength, this.mp3buf, 4096);
// `bytesWritten` bytes stored in this.mp3buf now mp3-encoded
print("\(bytesWritten) encoded");
this.file.append(this.mp3buf, length: Int(bytesWritten));
// #TODO: send data, better to pass into separate queue for processing
}
})
Let's take it part by part - buffer.floatChannelData?[0]
buffer has property named floatChannelData which is optional so it has ? at the end. then it takes that optional which accepts subscription [0] which also returns optional value. So it continues inside {} only if floatChannelData is not nil AND it's first value is not nil
Your Objc should look like
float *const *channelData = [buffer floatChannelData];
if (channelData) {
float *channel1Buffer = channelData[0]; //this might crash if channelData is empty
...
The line tries to assign the variable channel1Buffer the value of buffer.floatChannelData[0], and the code within {} is only executed if that assignment is successful. It may for instance fail if buffer.floatChannelData is nil or buffer.floatChannelData[0] is nil.
Related
I have a file in which each line is of the form :
key$password (key + separator + password)
so that I can do
let (key , password) = line.split_once("$").unwrap() ;** // to get key and password
the first line is special (id, secret_key) and I need this secret key to decode the passwords
in a function where I have declared :
let mount count = 0;
let user_password = String::new();**
Then I run a loop
for line in reader.lines() {
count += 1;
match line {
match count {
1 => { here I split_once and get a value of user_password )
_ => now I want to parse the following lines using user_password obtained from the 1 arm of the match
... etc
}
my problem is that although user_password is valid in arm 1 (or an if then clause), but is not set in the context of arm 2
There is something that I cannot understand : I declare usr_password in the whole context of the function, in the match arm 1 I set the variable to a value ,but when I am in arm 2 (that is for line 2 .. end this variable is not set
any help would be terribly appreciated (I'm a beginner, sorry)
fn logic(file_contents: &str) {
// this is an iterator, it has its own state and you can call
// .next to get one line
let mut lines = file_contents.lines();
// so called `let-else` statement, if `lines.next()` if `None`
// the else block is executed
let Some(header) = lines.next() else {
// `todo!` will terminate your program if reached
// the advantage is that compiler is happy and you
// can debug happy path
todo!("handle missing header");
};
// same as above
let Some((key, password)) = header.split_once("$") else {
todo!("handle invalid header")
};
// now that we taken first line and processed it, you can consume
// the remaining lines by a loop
for line in lines {
// its always good to separate the concerns of your program
let Some(res) = decode(line, key, password) else {
continue; // or handle the error
};
// use `res`
}
}
// TODO: choose appropriate return type, `()` is placeholder for now
// `Option<T>` signifies function can fail and return nothing
fn decode(line: &str, key: &str, password: &str) -> Option<()> {
todo!("implement decode");
}
I have been trying to convert existing swift2.3 to swift3. I got Ambiguous use on intValue error at the following code.
jobPackageVersion.intJobPackageId = (JobPackageVersionDictionary["intJobPackageId"]! as AnyObject).intValue as NSNumber
Here is the full code
if let url = Bundle.main.url(forResource: "tblJobPackageVersion", withExtension: "csv") {
do {
let strData = try String(contentsOf: url)
let csv = CSwiftV(String: strData)
if csv.keyedRows != nil {
for dictionary in csv.keyedRows! { // [Dictionary<String, String>]
let JobPackageVersionDictionary = dictionary as NSDictionary // Cast to NSDictionary
let JobPackageVersionEntity = NSEntityDescription.entity(forEntityName: "JobPackageVersion", in: context)
let jobPackageVersion = JobPackageVersion(entity: JobPackageVersionEntity!, insertInto: context)
// Set object attributes
jobPackageVersion.intJobPackageId = (JobPackageVersionDictionary["intJobPackageId"]! as AnyObject).intValue as NSNumber
jobPackageVersion.intJobPackageVersionId = (JobPackageVersionDictionary["intJobPackageVersionId"]! as AnyObject).intValue as NSNumber
jobPackageVersion.intStatus = (JobPackageVersionDictionary["intStatus"]! as AnyObject).intValue as NSNumber
jobPackageVersion.intVersion = (JobPackageVersionDictionary["intVersion"]! as AnyObject).intValue as NSNumber
do { // Save object to database and clean up memory
try context.save()
context.refresh(jobPackageVersion, mergeChanges: false)
} catch let error as NSError { Logger.sharedInstance.logMessage("\(#function) JobPackageVersion Saving Error: \(error.userInfo)") }
} // for-loop
Logger.sharedInstance.logMessage("\(#function): Loaded \(csv.keyedRows!.count) tblJobPackageVersion records.")
} else { Logger.sharedInstance.logMessage("\(#function) CSV Parser Warning: no CSV data was parsed in tblJobPackageVersion.csv!") }
} catch { Logger.sharedInstance.logMessage("\(#function) Error reading contents of tblJobPackageVersion.csv.") }
} else { Logger.sharedInstance.logMessage("\(#function) Error locating URL for resource tblJobPackageVersion.csv") }
}
Any help would be appreciated.
Thanks.
You're trying to call intValue on an object of type AnyObject. As the error states, this is too ambiguous because both NSNumber and NSString have intValue properties. Xcode doesn't know which intValue to use, because both NSNumber and NSString fall under the AnyObject umbrella. Since Xcode is confused, you need to be more specific about what type your object is. Try something like this:
jobPackageVersion.intJobPackageId = (JobPackageVersionDictionary["intJobPackageId"]! as NSNumber).intValue
Note 1: You're probably going to get the same error with the other objects you call intValue on, but you can fix them accordingly.
Note 2: Be extremely careful about force unwrapping your objects using !. If the dictionary you're using ever returns nil your program will crash. Instead I would safely unwrap them using either an if let or guard statement depending on your use case. Something like this may work a little better:
guard let intJobPackageId = JobPackageVersionDictionary["intJobPackageId"] as? NSNumber,
let intJobPackageVersionId = JobPackageVersionDictionary["intJobPackageVersionId"] as? NSNumber,
let intStatus = JobPackageVersionDictionary["intStatus"] as? NSNumber,
let intVersion = JobPackageVersionDictionary["intVersion"] as? NSNumber
else {
print("one of the dictionary values is nil")
return
}
jobPackageVersion.intJobPackageId = intJobPackageId.intValue
jobPackageVersion.intJobPackageVersionId = intJobPackageVersionId.intValue
jobPackageVersion.intStatus = intStatus.intValue
jobPackageVersion.intVersion = intVersion.intValue
This may not be exactly what you want, but it should give you an idea on how to safely unwrap your objects so your app doesn't crash. You can play around with it and decide what's best for you.
I am trying to iterate over characters in stdin. The Read.chars() method achieves this goal, but is unstable. The obvious alternative is to use Read.lines() with a flat_map to convert it to a character iterator.
This seems like it should work, but doesn't, resulting in borrowed value does not live long enough errors.
use std::io::BufRead;
fn main() {
let stdin = std::io::stdin();
let mut lines = stdin.lock().lines();
let mut chars = lines.flat_map(|x| x.unwrap().chars());
}
This is mentioned in Read file character-by-character in Rust, but it does't really explain why.
What I am particularly confused about is how this differs from the example in the documentation for flat_map, which uses flat_map to apply .chars() to a vector of strings. I don't really see how that should be any different. The main difference I see is that my code needs to call unwrap() as well, but changing the last line to the following does not work either:
let mut chars = lines.map(|x| x.unwrap());
let mut chars = chars.flat_map(|x| x.chars());
It fails on the second line, so the issue doesn't appear to be the unwrap.
Why does this last line not work, when the very similar line in the documentation doesn't? Is there any way to get this to work?
Start by figuring out what the type of the closure's variable is:
let mut chars = lines.flat_map(|x| {
let () = x;
x.unwrap().chars()
});
This shows it's a Result<String, io::Error>. After unwrapping it, it will be a String.
Next, look at str::chars:
fn chars(&self) -> Chars
And the definition of Chars:
pub struct Chars<'a> {
// some fields omitted
}
From that, we can tell that calling chars on a string returns an iterator that has a reference to the string.
Whenever we have a reference, we know that the reference cannot outlive the thing that it is borrowed from. In this case, x.unwrap() is the owner. The next thing to check is where that ownership ends. In this case, the closure owns the String, so at the end of the closure, the value is dropped and any references are invalidated.
Except the code tried to return a Chars that still referred to the string. Oops. Thanks to Rust, the code didn't segfault!
The difference with the example that works is all in the ownership. In that case, the strings are owned by a vector outside of the loop and they do not get dropped before the iterator is consumed. Thus there are no lifetime issues.
What this code really wants is an into_chars method on String. That iterator could take ownership of the value and return characters.
Not the maximum efficiency, but a good start:
struct IntoChars {
s: String,
offset: usize,
}
impl IntoChars {
fn new(s: String) -> Self {
IntoChars { s: s, offset: 0 }
}
}
impl Iterator for IntoChars {
type Item = char;
fn next(&mut self) -> Option<Self::Item> {
let remaining = &self.s[self.offset..];
match remaining.chars().next() {
Some(c) => {
self.offset += c.len_utf8();
Some(c)
}
None => None,
}
}
}
use std::io::BufRead;
fn main() {
let stdin = std::io::stdin();
let lines = stdin.lock().lines();
let chars = lines.flat_map(|x| IntoChars::new(x.unwrap()));
for c in chars {
println!("{}", c);
}
}
See also:
How can I store a Chars iterator in the same struct as the String it is iterating on?
Is there an owned version of String::chars?
Is it valid to rebind a mutable variable in a while loop? I am having trouble getting the following trivial parser code to work. My intention is to replace the newslice binding with a progressively shorter slice as I copy characters out of the front of the array.
/// Test if a char is an ASCII digit
fn is_digit(c:u8) -> bool {
match c {
30|31|32|33|34|35|36|37|38|39 => true,
_ => false
}
}
/// Parse an integer from the front of an ascii string,
/// and return it along with the remainder of the string
fn parse_int(s:&[u8]) -> (u32, &[u8]) {
use std::str;
assert!(s.len()>0);
let mut newslice = s; // bytecopy of the fat pointer?
let mut n:Vec<u8> = vec![];
// Pull the leading digits into a separate array
while newslice.len()>0 && is_digit(newslice[0])
{
n.push(newslice[0]);
newslice = newslice.slice(1,newslice.len()-1);
//newslice = newslice[1..];
}
match from_str::<u32>(str::from_utf8(newslice).unwrap()) {
Some(i) => (i,newslice),
None => panic!("Could not convert string to int. Corrupted pgm file?"),
}
}
fn main(){
let s:&[u8] = b"12345";
assert!(s.len()==5);
let (i,newslice) = parse_int(s);
assert!(i==12345);
println!("length of returned slice: {}",newslice.len());
assert!(newslice.len()==0);
}
parse_int is failing to return a slice that is smaller than the one I passed in:
length of returned slice: 5
task '<main>' panicked at 'assertion failed: newslice.len() == 0', <anon>:37
playpen: application terminated with error code 101
Run this code in the rust playpen
As Chris Morgan mentioned, your call to slice passes the wrong value for the end parameter. newslice.slice_from(1) yields the correct slice.
is_digit tests for the wrong byte values. You meant to write 0x30, etc. instead of 30.
You call str::from_utf8 on the wrong value. You meant to call it on n.as_slice() rather than newslice.
Rebinding variables like that is perfectly fine. The general rule is simple: if the compiler doesn’t complain, it’s OK.
It’s a very simple error that you’ve made: your slice end point is incorrect.
slice produces the interval [start, end)—a half-open range, not closed. Therefore when you wish to just remove the first character, you should be writing newslice.slice(1, newslice.len()), not newslice.slice(1, newslice.len() - 1). You could also write newslice.slice_from(1).
I have an object that returns a value if successful and false (or nil) if it failed.
i want to assign that value to a variable
if(var1 = [object foo])
{
//if the [object foo] returned a variable, goes here
}
else
{
//[object foo] returned FALSE (or nil), go here
}
can an If statement detected if an assignment was valid?
This is all right but will generate a warning, since this is a common typo (= instead of ==). To silence that warning add another set of parentheses like this:
if ((var = [object foo])) ...
Since this easily can lead to misunderstandings a lot of people will advise against doing this. For a simple if statement this is much clearer to do the assignment first:
var = [object for];
if (var) ...
In while loops this is more useful, but also considered harmful by many people.
Not sure I understand your question, but let me try and explain a few situations you can check
1) Property contains value
if ([object foo])
{
// If foo has a value associated to it that is not nil/false/zero
}
else
{
// If foo equals nil, false or zero
}
2) Assignment to a variable was successful
if ((bar = [object myMethod]))
{
// If myMethod returns any non-nil value
}
else
{
// If myMethod returns nil
}
3) Previous assignment of a variable was successful
bar = [object myMethod];
if (bar)
{
// If bar has a value associated to it that is not nil/false/zero
}
else
{
// If bar equals nil, false or zero
}
use == instead of = in the if statement.
before the if statement, you may have var1 = [object foo]
see comparison operators
If you mean by valid that the variable contains an expected result, you can just perform another if on the variable against the expected result, or null to check it.