When debugging i often need to put a conditional breakpoint, ex :
if messageType == 1 {
nop; // <-- breakpoint
}
There is no 'nop' command and am fairly used to just put a dummy line in which is doing, ex :
if messageType == 1 {
x = 1; // <-- breakpoint
}
And then put the breakpoint at the line with x = 1.
Issue is in Go the compiler often starts battling me because im not using x etc - and am just wondering if theres a quick and smarter way of doing it with a oneline statement ? what do you guys do ?
Just don't store it in a real variable:
if messageType == 1 {
_ = 1
}
Related
I was reading some Rust code and I came across this line
if let Some(path) = env::args().nth(1) {
Inside of this function
fn main() {
if let Some(path) = env::args().nth(1) {
// Try reading the file provided by the path.
let mut file = File::open(path).expect("Failed reading file.");
let mut content = String::new();
file.read_to_string(&mut content);
perform_conversion(content.as_str()).expect("Conversion failed.");
} else {
println!(
"provide a path to a .cue file to be converted into a MusicBrainz compatible tracklist."
)
}
}
The line seems to be assigning the env argument to the variable path but I can't work out what the Some() around it is doing.
I took a look at the documentation for Option and I understand how it works when used on the right hand side of = but on the left hand side I am a little confused.
Am I right in thinking this line is equivalent to
if let path = Some(env::args().nth(1)) {
From the reference :
An if let expression is semantically similar to an if expression but
in place of a condition expression it expects the keyword let followed
by a refutable pattern, an = and an expression. If the value of the
expression on the right hand side of the = matches the pattern, the
corresponding block will execute, otherwise flow proceeds to the
following else block if it exists. Like if expressions, if let
expressions have a value determined by the block that is evaluated.
In here the important part is refutability. What it means refutable pattern in here it can be in different forms. For example :
enum Test {
First(String, i32, usize),
Second(i32, usize),
Third(i32),
}
You can check the x's value for a value for 3 different pattern like :
fn main() {
let x = Test::Second(14, 55);
if let Test::First(a, b, c) = x {}
if let Test::Second(a, b) = x {} //This block will be executed
if let Test::Third(a) = x {}
}
This is called refutability. But consider your code like this:
enum Test {
Second(i32, usize),
}
fn main() {
let x = Test::Second(14, 55);
if let Test::Second(a, b) = x {}
}
This code will not compile because x's pattern is obvious, it has single pattern.
You can get more information from the reference of refutability.
Also you are not right thinking for this:
if let path = Some(env::args().nth(1)) {
Compiler will throw error like irrefutable if-let pattern because as the reference says: "keyword let followed by a refutable pattern". In here there is no refutable pattern after "let". Actually this code tries to create a variable named path which is an Option and this make no sense because there is no "If" needed,
Instead Rust expects from you to write like this:
let path = Some(env::args().nth(1)); // This will be seem like Some(Some(value))
The other answers go into a lot of detail, which might be more than you need to know.
Essentially, this:
if let Some(path) = env::args().nth(1) {
// Do something with path
} else {
// otherwise do something else
}
is identical to this:
match env::args().nth(1) {
Some(path) => { /* Do something with path */ }
_ => { /* otherwise do something else */ }
}
This can be done in python / javascript:
# Python:
a = ""
b = "test"
c = a or b # test
// javascript
a = "";
b = "test";
c = a || b; // test
Can the same be accomplished in go without doing some conditional block? The only way I'm able to do this so far is with conditional blocks...
a := ""
b := "test"
var c
if a {
c = a
} else {
c = b
}
I think this is answer is going to be "no, this cannot be done", but I figured I would ask just in case I'm wrong. The example here is simplified. The variables a and b could have been defined long ago...
From golang FAQ, There is no ternary testing operation in Go. You may use the following to achieve the same result:
if expr {
n = trueVal
} else {
n = falseVal
}
I am debugging a kernel crash dump. There seems to be a problem with one process was trying to memory map a new region. The problem is that it was not able to hold the memory map semaphore.
When I looked into process's mm_struct and printed its content. I saw that the struct rw_semaphore mmap_sem were as seen below. Now, does he value of count seem suspicious? It has a negative value, as if there was a race condition where it was decremented twice by two different threads after checking for zero.
mmap_sem = {
count = -4294967295,
wait_lock = {
{
rlock = {
raw_lock = {
slock = 262148
}
}
}
},
wait_list = {
next = 0xffff8801f0113e48,
prev = 0xffff8801f0113e48
}
},
Sorry for the confusion. I thought crash pulls the correct data types and uses that properly when printing out the all the values ...
Looks like crash utility is not read the count member as an int ....
When I print it as int, I get the correct value.
crash> p (int) (((struct mm_struct *) 0xffff8801f15fa540)->mmap_sem).count
$13 = 1
I've actually fixed this problem already (while documenting it for this post), but I still want to know is why it is happening, so that I can understand what I did and hopefully avoid wasting time with it in the future.
In a Swift project, I have a function that parses out a string that I know will be presented in a specific format and uses it to fill in some instance variables.
There is a helper function that is passed the string, a starting index, and a divider character and spits out a tuple made up of the next string and the index from which to continue. Just in case a poorly formatted string gets passed in, I define a return type of (String, Int)? and return nil if the divider character isn't found.
The helper function looks, in relevant part, like this:
func nextChunk(stringArray: Array<Character>, startIndex: Int, divider: Character) -> (String, Int)?
{
[...]
var returnValue: (String, Int)? = (returnString, i)
return returnValue
}
So far, so good. I set a breakpoint, and just before the function returns, I see that all is as it should be:
(lldb) po returnValue
(0 = "21三體綜合症", 1 = 7)
{
0 = "21三體綜合症"
1 = 7
}
That's what I expected to see: the correct string value, and the correct index.
However, when I go back to the init() function that called the helper in the first place, and put a breakpoint immediately after the call:
var returnedValue = self.nextChunk(stringArray, startIndex: stringArrayIndex, divider: " ")
I get a completely different value for returnedValue than I had for returnValue:
(lldb) po returnedValue
(0 = "I", 1 = 48)
{
0 = "I"
1 = 48
}
Now here's the really weird part. After I get the return value, I want to test to see if it's nil, and if it's not, I want to use the values I fetched to set a couple of instance variables:
if(returnedValue == nil)
{
return
}
else
{
self.traditionalCharacter = returnedValue!.0
stringArrayIndex = returnedValue!.1
}
If I comment out both of the lines in the "else" brackets:
else
{
// self.traditionalCharacter = returnedValue!.0
// stringArrayIndex = returnedValue!.1
}
then my original breakpoint gives the expected value for the returned tuple:
(lldb) po returnedValue
(0 = "21三體綜合症", 1 = 7)
{
0 = "21三體綜合症"
1 = 7
}
Again: the breakpoint is set before this if/else statement, so I'm taking the value before any of this code has had the chance to execute.
After banging my head against this for a few hours, I realize that...there isn't actually a problem. If I press the "step over" button in the debugger, the execution pointer jumps back from the if() line to the call to nextChunk. Pressing it again sends it forward to "if" again, and sets the values properly.
This extra double-jump only happens if the assignment code is active, consistently and reproducibly. As I know, since I reproduced it for hours trying to figure out what was wrong before even trying stepping forward and noticing that it "fixed itself."
So my question is: why? Is this a bug in the debugger, or am I using breakpoints wrong? It happens just the same whether I put the breakpoint between the function call and the if() or on the if() line. Can someone explain why the debugger is jumping back and forth and when the value I need is actually getting set?
The most egregiously redundant code construct I often see involves using the code sequence
if (condition)
return true;
else
return false;
instead of simply writing
return (condition);
I've seen this beginner error in all sorts of languages: from Pascal and C to PHP and Java. What other such constructs would you flag in a code review?
if (foo == true)
{
do stuff
}
I keep telling the developer that does that that it should be
if ((foo == true) == true)
{
do stuff
}
but he hasn't gotten the hint yet.
if (condition == true)
{
...
}
instead of
if (condition)
{
...
}
Edit:
or even worse and turning around the conditional test:
if (condition == false)
{
...
}
which is easily read as
if (condition) then ...
Using comments instead of source control:
-Commenting out or renaming functions instead of deleting them and trusting that source control can get them back for you if needed.
-Adding comments like "RWF Change" instead of just making the change and letting source control assign the blame.
Somewhere I’ve spotted this thing, which I find to be the pinnacle of boolean redundancy:
return (test == 1)? ((test == 0) ? 0 : 1) : ((test == 0) ? 0 : 1);
:-)
Redundant code is not in itself an error. But if you're really trying to save every character
return (condition);
is redundant too. You can write:
return condition;
Declaring separately from assignment in languages other than C:
int foo;
foo = GetFoo();
Returning uselessly at the end:
// stuff
return;
}
I once had a guy who repeatedly did this:
bool a;
bool b;
...
if (a == true)
b = true;
else
b = false;
void myfunction() {
if(condition) {
// Do some stuff
if(othercond) {
// Do more stuff
}
}
}
instead of
void myfunction() {
if(!condition)
return;
// Do some stuff
if(!othercond)
return;
// Do more stuff
}
Using .tostring on a string
Putting an exit statement as first statement in a function to disable the execution of that function, instead of one of the following options:
Completely removing the function
Commenting the function body
Keeping the function but deleting all the code
Using the exit as first statement makes it very hard to spot, you can easily read over it.
Fear of null (this also can lead to serious problems):
if (name != null)
person.Name = name;
Redundant if's (not using else):
if (!IsPostback)
{
// do something
}
if (IsPostback)
{
// do something else
}
Redundant checks (Split never returns null):
string[] words = sentence.Split(' ');
if (words != null)
More on checks (the second check is redundant if you are going to loop)
if (myArray != null && myArray.Length > 0)
foreach (string s in myArray)
And my favorite for ASP.NET: Scattered DataBinds all over the code in order to make the page render.
Copy paste redundancy:
if (x > 0)
{
// a lot of code to calculate z
y = x + z;
}
else
{
// a lot of code to calculate z
y = x - z;
}
instead of
if (x > 0)
y = x + CalcZ(x);
else
y = x - CalcZ(x);
or even better (or more obfuscated)
y = x + (x > 0 ? 1 : -1) * CalcZ(x)
Allocating elements on the heap instead of the stack.
{
char buff = malloc(1024);
/* ... */
free(buff);
}
instead of
{
char buff[1024];
/* ... */
}
or
{
struct foo *x = (struct foo *)malloc(sizeof(struct foo));
x->a = ...;
bar(x);
free(x);
}
instead of
{
struct foo x;
x.a = ...;
bar(&x);
}
The most common redundant code construct I see is code that is never called from anywhere in the program.
The other is design patterns used where there is no point in using them. For example, writing "new BobFactory().createBob()" everywhere, instead of just writing "new Bob()".
Deleting unused and unnecessary code can massively improve the quality of the system and the team's ability to maintain it. The benefits are often startling to teams who have never considered deleting unnecessary code from their system. I once performed a code review by sitting with a team and deleting over half the code in their project without changing the functionality of their system. I thought they'd be offended but they frequently asked me back for design advice and feedback after that.
I often run into the following:
function foo() {
if ( something ) {
return;
} else {
do_something();
}
}
But it doesn't help telling them that the else is useless here. It has to be either
function foo() {
if ( something ) {
return;
}
do_something();
}
or - depending on the length of checks that are done before do_something():
function foo() {
if ( !something ) {
do_something();
}
}
From nightmarish code reviews.....
char s[100];
followed by
memset(s,0,100);
followed by
s[strlen(s)] = 0;
with lots of nasty
if (strcmp(s, "1") == 0)
littered about the code.
Using an array when you want set behavior. You need to check everything to make sure its not in the array before you insert it, which makes your code longer and slower.
Redundant .ToString() invocations:
const int foo = 5;
Console.WriteLine("Number of Items: " + foo.ToString());
Unnecessary string formatting:
const int foo = 5;
Console.WriteLine("Number of Items: {0}", foo);