I'm trying to find out if a number is prime or not. But I've got an error of "unreachable code detected", which I think is effecting the error of "not all code paths return a value". The error seems to occur in the for loop at i++. Can anyone help me please?
static void Main(string[] args)
{
Console.WriteLine(isPrime(10));
}
public static bool isPrime(int n)
{
for (int i = 2; i < n; i++)
{
if (n % i == 0)
{
return false;
}
return true;
}
}
"Unreachable code detected" means that some code can never be executed. Consider:
int something()
{
if (true)
return 1;
else
return 2; //Obviously we can never get here
}
"Not all code paths return a value" means that you've defined a method with a return value (like "bool" in your example) and there is some way for the method to execute without returning a value.
Consider:
int something(bool someBool)
{
if (someBool)
return 1;
//if someBool == false, then we're not returning anything. Error!
}
Your code has two problems:
You have return true inside the for loop (outside of any conditional). Because return immediately exits the function (returning control to the caller) the i++ statement of the for loop will never get executed (hence your bug). You likely intended for that to be outside the for loop.
Another problem with that being in the loop is that the loop is not guaranteed to execute. If the n passed was 2 or less, you would skip the loop entirely, and there is no return statement in that case. This isn't allowed (since you always need to return a value from a non-void function) so you get a compiler error.
Below is an example of how to get this return working with a for loop and embedded If condition.
private bool WinOneLevelOne()
{
//For loop to check all the items in the winOne array.
for (int i = 0; i < winOne.Length; i++)
{
//If statement to verify that all the gameobjects in the array are yellow.
if (winOne[i].gameObject.GetComponent<MeshRenderer>().material.color != Color.yellow)
{
//Keeps the boolean at false if all the gameobjects are not yellow.
return false;
}
}
return true;
Related
I have the below function that iterates over a list of workers, invoking their DoStuff() method. If the first worker fails, I try the next one until I'm out of workers. If they all fail, I re-throw the last exception.
// workers is an IList<>.
public object TryDoStuff()
{
for (int i = 0; i < workers.Count; i++)
{
try
{
return worker[i].DoStuff();
}
catch
{
if (i == workers.Count - 1)
{
throw; // This preserves the stack trace
}
else
{
continue; // Try the next worker
}
}
}
}
When compiling, I get an error that "not all code paths return a value" for this function. Although I can silence the error by adding an explicit return after the for loop, I'm doubting the compiler is accurate here as I don't see how the for loop will be escaped without either returning or re-throwing an exception. And if an exception is re-thrown, not returning a value is valid.
What am I missing? Is csc unable to reason about the conditional in the catch block?
Yes
If there is an exception thrown on the last index and count isn't what you expect it to be (unlikely yet possible)
Or as RAM pointed out if Count is zero
In this case, the static analysis and subsequent compiler error is very justified
As previously mentioned, if workers is empty (Count is 0), there's no valid return path.
There's also another race condition (depending on the full context, obviously) where workers is not empty, an exception is thrown on an element, there are still elements to iterate in workers, but after evaluating if (i == workers.Count - 1) and before the continue statement executes, another thread removes elements from workers (or changes the entire workers variable to a new instance).
In that scenario, the for condition will return false on the next iteration unexpectedly and you'll fall out of the loop with no return statement for the method.
public object TryDoStuff()
{
for (int i = 0; i < workers.Count; i++)
{
try
{
return worker[i].DoStuff();
}
catch
{
if (i == workers.Count - 1)
{
throw; // This preserves the stack trace
}
else
{
// XXX If workers is changed by another thread here. XXX
continue; // Try the next worker
}
}
}
}
I wrote as a comment for you:
What will be happen if the count of the workers list items be
zero?
It seems this is the compiler question and it dose not do more research about your code! :)
Actually this reason is enough for compiler to show the bellow error to you
not all code paths return a value
When the compiler encounters with a loop in the whole of the body of a method it assume that the loop condition cause that the loop body be ignored then it expected any value out of the loop too.
Yes, even if we set the condition of the loop at the way that the loop be executed!
Proof:
With error:
public static object TryDoStuff()
{
var result =0;
for (int i = 0; i < 3; i++)
{
Console.WriteLine("Add 100 unit");
result += 100;
return result;
}
//Console.WriteLine("last line");
// return result;
}
Without error:
public static object TryDoStuff()
{
var result =0;
for (int i = 0; i < 3; i++)
{
Console.WriteLine("Add 100 unit");
result += 100;
// return result; you can un-comment this line too
}
Console.WriteLine("last line");
return result;
}
So, why is it that there is unreachable code detected in this code:
public bool GetKeyPressed(KeyCode key)
{
for (int i = 0; i < keys.Count; i++)
if (keys[i].key == key && keys[i].pressed)
return true;
else
return false;
return false;
}
The index (i) is apparently unreachable ... why?
Your code has a loop that evaluates once, therefore the first iteration will always return.
If that's what you want, just say that,
return keys[0].key == key && keys[0].pressed;
If however (which is what I suspect here), you want to return true if any in the array meets your test, then use LINQ's Any(),
return keys.Any(k => k.key == key && k.pressed);
Your code having two code path, one is through if condition and another is through the else. Which means the control will leave the function in these two ways. So the return statement after else will never triggers. That is why the compiler points it as unreachable code. This can be avoided by using the following code.
public bool GetKeyPressed(KeyCode key)
{
for (int i = 0; i < keys.Count; i++)
if (keys[i].key == key && keys[i].pressed)
return true;
return false;
}
Expanding on jdphenix answer, I think we need to understand why your loop "evaluates once" and only then can we understand why there is "unreachable code".
public bool GetKeyPressed(Keys key)
{
for (int i = 0; i < keys.Count; i++)
if (keys[i].key == key && keys[i].pressed)
return true;
else
return false;
return false;
}
The for loop has a single if-else statement that makes up the body. The if guard if satisfied returns true otherwise it executes the next else statement which returns false. The net result is that at most a single cycle of the loop will be executed before control is returned to the calling method irrespective of the number of items in keys.
It's more obvious if we look at the code via JetBrains Resharper:
The code may as well be written as:
public bool GetKeyPressed(Keys key)
{
for (int i = 0; i < keys.Count; ) // Look Ma, no i++ !!!
if (keys[i].key == key && keys[i].pressed)
return true;
else
return false;
return false;
}
Don't make the mistake of thinking that the very last return false at the end of the method is not required because it is during the scenario where keys.Count == 0
Of course, formatting the code a bit more nicely goes a long way into revealing the problem that the first return false is redundant and could be simplified as per un-lucky's answer:
I'm trying to experiment with recursion so as to grasp the concept. It is language agnostic, so the same concept applies to both C# and Java.
I've got a TreeView which has a number of nodes. I would like to iterate through every single node and count the ones which satisfy a certain condition. If at any time the condition is not satisfied, I would like the algorithm to finally return -1.
Each TreeViewItem will only be considered if it has a Tag named "Condition" (there are 3 types of TreeViewItems in all - I will only consider the "Condition" ones).
Once a TreeViewItem is found to be of the "Condition" type, I would like to check to see that it satisfies a certain condition. As I mentioned before, even if only one TreeViewItem does not satisfy the condition, I want the algorithm to return -1 in the end.
If the algorithm does not return -1, I want it to return the amount of valid conditions which it has found - i.e. an integer is to be incremented each time a condition is successfully passed, and the final count is returned at the end.
This is what I've tried so far:
private int CountConditions(TreeViewItem item)
{
int conditionCount = 0;
foreach (TreeViewItem child in item.Items)
{
int previousCount = CountConditions(child);
if (previousCount == -1)
{
return -1;
}
else
{
return conditionCount += previousCount;
}
}
if (item.Tag.Equals("Condition"))
{
if (/*Condition is not satisfied*/)
{
return -1;
}
else
{
return conditionCount++;
}
}
else
{
return conditionCount;
}
}
My current algorithm does infact return -1 if a condition is not satisfied, however if conditions are satisfied it just returns 0, rather than the amount of valid conditions.
you use
return conditionCount++;
which is bad practice. for good reason. what happens here is
a)return conditionCount (which you set to zero)
b)increment conditionCount
b never happens as its after the return statement, so you always pass 0 to your next recursion step.
you can use
return ++conditionCount;
or the much better
conditionCount++;
return conditionCount;
It is not a trivial recursion, since you have to handle both the error condition, and the normal condition. If you didn't have the error condition, and only needed to count the number of condition nodes, you could write:
private int CountConditions(TreeViewItem item)
{
int currentCondition = CalculateCondition(item)
int childCounts = 0;
foreach (TreeViewItem child in item.Items)
{
int childCount = CountConditions(child);
childCounts += childCount;
}
return currentCondition + conditionCounts
}
private int CalculateCondition(TreeViewItem item)
{
if (item.Tag.Equals("Condition"))
return 1;
else
return 0;
}
But to handle the error, you have to have two checks for the error condition, one for the current node, and one for the child nodes, and return immediately, if you encounter the condition:
int error = -1
private int CountConditions(TreeViewItem item)
{
int currentCondition = CalculateCondition(item)
if (currentCondition == error) // new
return error; // new
int childCounts = 0;
foreach (TreeViewItem child in item.Items)
{
int childCount = CountConditions(child);
if (childCount == error) // new
return error; // new
childCounts += childCount;
}
return currentCondition + conditionCounts
}
private int CalculateCondition(TreeViewItem item)
{
if (item.Tag.Equals("Condition"))
if (((item.Header as StackPanel).Children[2] as TextBox).Text.Equals("")) // new
return error; // new
else // new
return 1;
else
return 0;
}
You should return conditionCount; only at the end of the function. Only the return -1; have to be in the middle of the function.
return stops the whole function at once and returns a value. That's not what you want. You want to accumulate values and return the sum when you have finished.
I think you could simplify your code by using exceptions. What you do is you throw an exception when the condition fails which you can then catch in another function which starts of the recursion. This will automatically unwind the stack and abort the computation. The function which catches the exception can then return whatever you want it to.
Apart from this all you would have to do in the recursion is accumulate 1 for each passed condition.
I know this is a very newbie C# question but I am implementing a small program which does the following:
namespace ConsoleApplication
{
class Program
{
static void Main(string[] args)
{
bool isRun = false;
int number = 0;
while (isRun = (true) && number < 3)
{
++number;
Console.WriteLine("Number = {0}", number.ToString());
Console.WriteLine();
}
Console.WriteLine(isRun.ToString());
Console.ReadLine();
}
}
}
At the end of the while loop, I would have expected the bool value to be true, but is is printed to be false. Why is that? Is this different from C++ where I would have done something like and the same thing in C# is giving me false
while(number<3)
{
is = true;
}
if(is){
cout<<true;
}
The reason you're seeing this behavior is due to the operator precedence involved. Here the && binds more strongly than = so the code in the loop is actually bound as the following
while (isRun = (true && number < 3)) {
...
}
Once number > 3 the && expression is false and is assigned into the isRun value and simultaneously terminates the loop. Hence once the loop exits you will see isRun as false
To get the behavior you are looking for you will need to manually correct the precedence with parens.
while ((isRun = (true)) && number < 3) {
...
}
Note: In general, as #Servey pointed out, the assignment of locals to expressions inside the loop predicate is considered bad practice. Many C# users would actually be surprised that code compiles at all because they've been conditioned to only use == in loops.
It's more idiomatic to simply set isRun to true on the first line of the loop for this pattern.
while (number < 3) {
isRun = true;
...
}
The problem is that you have set you boolean variable to false and without assigning it back to true, in while loop you are matching it against the value true, thus it fails in every case.
I have a listView in my app. I loop through the items to check which one is currently selected and then return a value. As all paths have to return a value I have to return a value outside the loop which overwrites the for loop return, how do I keep this without overwriting it after the loop?
public string GetItemValue()
{
for (int i = 0; i < listView1.Items.Count; i++)
{
if (listView1.Items[i].Checked == true)
{
return listView1.Items[i].Text; // I want to keep this value
}
}
// Without overwriting it with this but the compiler
// requires me to return a value here
return "Error";
}
Any help is most appreciated. Thanks.
P.S I have tried using break after the if but no luck.
On edit: bringing down my comment from above.
You don't need to worry about this. As soon as it hits the first return inside the loop, it will return immediately with that value. No code outside the loop is ever hit in that case.
Incidentally, this code would be cleaner:
public string GetItemValue()
{
foreach (var item in listView1.Items)
{
if (item.Checked) return item.Text;
}
throw new InvalidOperationException("No checked items found");
}
Exceptions are a more idiomatic way of handling errors, and the foreach loop is preferred to a for loop when you're just iterating over collections.
Also using LINQ, you can get even more concise:
public string GetItemValue()
{
return listView1.Items.Cast<ListViewItem>().Single(i => i.Checked).Text;
}
Well, your return outside of the loop, return "Error"; shouldn't get called based on your logic. Since return causes your method to immediately exit, the "Error" return will never happen, that is unless the code never steps into the if inside the loop.
All things considered, this may be an exceptional case in your code. Thus, throwing an exception may be the appropriate thing to do:
public string GetItemValue()
{
for (int i = 0; i < listView1.Items.Count; i++)
{
if (listView1.Items[i].Checked == true)
{
return listView1.Items[i].Text; // I want to keep this value
}
}
throw new InvalidOperationException("Did not find value expected.");
}
Exceptions are usually thrown to indicate that a bug exists in code. A "Hey, that should really not be happening." The application stops, and hopefully the user gets an opportunity to contact support to help you reproduce it.
Based on your comment:
When I run it, it just returns the error text...
That means your check in your if statement is not succeeding.
if (listView1.Items[i].Checked == true)
Which means that none of your items in your ListView are checked.
That's the kind of situations where you are probably better of throwing an exception in order to signal the exceptional situation:
public string GetItemValue()
{
for (int i = 0; i < listView1.Items.Count; i++)
{
if (listView1.Items[i].Checked == true)
{
// Here you are leaving the GetItemValue method
// and the loop stops
return listView1.Items[i].Text;
}
}
// if we get that far it means that none of the items of
// the select list was actually checked => we are better of
// reporting this to the caller of the method
throw new Exception("Please select a value");
}
The return in your for loop isn't overwritten -- the method will return the value in the loop if your conditions are met. Execution of the method ends immediately upon reaching a return statement.
If your method is returning "Error", then I'd recommend looking at your code in a debugger, because it's reaching the end of the loop and returning the value "Error".
If you're returning a value in the loop, it should not reach the return that's outside of the loop. I would check to make sure that the loop is finding the selected item.
The other option is to create a local variable to hold the return value:
string returnValue = "Error";
for (int i = 0; i < listView1.Items.Count; i++)
{
if (listView1.Items[i].Checked == true)
{
returnValue = listView1.Items[i].Text;
break;
}
}
return returnValue;
Lastly, you might also consider returning an exception when no selections are found, and handling the exception from the calling method.
The return "Error" bit will not overwrite your loop return value. When a return is hit, the function exits, so when a value that is selected is found, the function will spit out your data and stop.
The compiler requires that all paths of the function return a value. The compiler cannot know before hand whether your inner loop if condition will be met. You can cache the value at a variable and return this in the end of the function e.g. :
public string GetItemValue()
{
string temp = null;
for (int i = 0; i < listView1.Items.Count; i++)
{
if (listView1.Items[i].Checked == true)
{
temp = listView1.Items[i].Text; // I want to keep this value
break;
}
}
return temp; // Without overwriting it with this but the compiler requires me to return a value here
}
Actually, there is no need for a loop here:
return (listView1.SelectedItems.Count > 0)
? listView1.SelectedItems[0].Text
: "Error";
But, as it was said, the original question is misleading since return does not override values. You might be thinking about assignment, instead of return. In this case a working code can look like this:
string ret = "Error";
foreach(var item in listView1.Items)
{
if(item.Checked) { ret = item.Text; break; }
}
return ret;