Consider this code
class Program
{
static void Main(string[] args)
{
string str;
int x;
for (x = 1; x < 10; x++)
{
str = "this";
}
Console.WriteLine(str);
Console.ReadLine();
}
}
When i compile i get: Error Use of unassigned local variable 'str' (I understand this part)
If i change for loop to if then it works fine. why so (confused here) ??
class Program
{
static void Main(string[] args)
{
string str;
int x;
if (true)
{
str = "this";
}
Console.WriteLine(str);
Console.ReadLine();
}
}
What is the reason for this different behavior?. I expected it should give same result in both situations.
What am i doing wrong ?
With static analysis, the compiler knows for sure that your if statement will run and that str will be assigned.
Change your 2nd example to
class Program
{
static void Main(string[] args)
{
string str;
int x;
bool b = true; // With "const bool" it would work, though
if (b)
{
str = "this";
}
Console.WriteLine(str);
Console.ReadLine();
}
}
and you will have the same behavior as the for loop.
The compiler doesn't know for sure that your for loop will be executed even if you know it will be, so that's why it's telling you about the unassigned variable. A more sophisticated compiler might be able to see that your variable is fine in this case, but handling all such cases is a very complicated problem.
If x was a constant (which makes no sense in a for loop since you want to increment it...) the compiler would be able to see that 1 is indeed smaller than 10 and it wouldn't warn you about the unused variable. Of course the loop would run forever now, but I'm saying this just to highlight that the compiler can only be sure about constants.
The reason is that in the first case the compiler considers a case where loop is never executed:
for (x = 1; x < 10; x++)
{
str = "this";
}
So it assumes that str may stay uninitialized.
In the second case, the condition is always true, so compiler considers that str is always initialized:
if (true)
{
str = "this";
}
The compiler cannot be certain that a for loop will actually iterate. It could loop 0 times. An if(true) statement is known to the compiler to unconditionally execute.
While, in theory, a suitably advanced compiler could reason that the first code block does in fact unconditionally execute, solving the problem in the general case is impossible (you run into the Halting Problem). The compiler is forced to use heuristics to take a reasonable guess at whether or not a given statement is reachable. If it states that a path is not reachable you can know with certain that it is not reachable. If it says that it is reachable it may be reachable, or it may be a false positive.
Related
When I have a readonly variable:
public readonly bool myVar = true;
And check it in a code like this:
for(int i = 0; i != 10; i++)
{
if(myVar)
DoStuff();
else
DoOtherStuff();
}
Looking at emitted IL, I can see that the check is performed on each iteration of the loop. I would expect the code above to produce the same IL as this:
if (myVar)
{
for(int i = 0; i != 10; i++)
{
DoStuff();
}
}
else
{
for(int i = 0; i != 10; i++)
{
DoOtherStuff();
}
}
So why isn't the check optimised to the outside of the loop, since the field is readonly and can't be changed between iterations?
Your proposed optimization really is a combination of two individual simpler transformations. First is pulling the member access outside the loop. From
for(int i = 0; i != 10; i++)
{
var localVar = this.memberVar;
if(localVar)
DoStuff();
else
DoOtherStuff();
}
to
var localVar = this.memberVar;
for(int i = 0; i != 10; i++)
{
if(localVar)
DoStuff();
else
DoOtherStuff();
}
The second is interchanging the loop condition with the if condition. From
var localVar = this.memberVar;
for(int i = 0; i != 10; i++)
{
if(localVar)
DoStuff();
else
DoOtherStuff();
}
to
var localVar = this.memberVar;
if (localVar) {
for(int i = 0; i != 10; i++)
DoStuff();
}
else {
for(int i = 0; i != 10; i++)
DoOtherStuff();
}
The first one is influenced by readonly. To do it, the compiler has to prove that memberVar cannot change inside the loop, and readonly guarantees this1 -- even though this loop could be called inside a constructor, and the value of memberVar could be changed in the constructor after the loop ends, it cannot be changed in the loop body -- DoStuff() is not a constructor of the current object, neither is DoOtherStuff(). Reflection does not count, while it may be possible to use Reflection to break invariants, it isn't permitted to do so. Threads do count, see footnote.
The second is a simple transformation but a more difficult decision for the compiler to make, because it's difficult to predict whether it will actually improve performance. Naturally you can look at it separately by doing the first transformation on the code yourself, and seeing what code is generated.
Perhaps a more important consideration is that in .NET, the optimization pass takes place in between MSIL and machine code, not during compilation of C# to IL. So you cannot see what optimizations are being done by looking at the MSIL!
1 Or does it? The .NET memory model is considerably more forgiving than e.g. the C++ model where any data race leads very quickly to undefined behavior unless the object is defined volatile/atomic. What if this loop runs in a worker thread spawned from the object constructor, and after spawning the thread, the constructor goes on (which I'll call the "second half") to change the readonly member? Does the memory model require that change to be seen by the worker thread? What if DoStuff() and the second half of the constructor force memory fences, for example access other members which are volatile, or take a lock? So readonly would only allow the optimization in a single-threaded environment.
Because it can be changed using Reflection:
using System;
using System.Reflection;
public class Program
{
public static void Main()
{
var t = new Test();
var field = typeof(Test).GetField("myVar", BindingFlags.Instance | BindingFlags.Public);
Console.WriteLine(t.myVar); // prints True
field.SetValue(t, false);
Console.WriteLine(t.myVar); // prints False
// Trying to use t.myVar = false or true; <-- does not compile
}
}
public class Test
{
public readonly bool myVar = true;
}
Working Fiddle: https://dotnetfiddle.net/W9UO3m
Note there is no way in which the compile time code optimizer can predict or detect with absolute certainty whether or not such reflection code exists, and if it exists, if it will run or not.
A readonly field can be initialized to different values at different points in the code (multiple constructors). It can't be optimized out because multiple constructors allow for multiple values of the field in question.
Now, if you only had one constructor, with no relevant branching, and therefore only one possible value, ever, for that field, I'd expect more from the optimizer. However, that sort of analysis is one reason that C++ takes much longer to compile than C#, and these sorts of tasks aren't typically delegated to the runtime.
For a clearer explanation:
Note
The readonly keyword is different from the const keyword. A const
field can only be initialized at the declaration of the field. A
readonly field can be assigned multiple times in the field declaration
and in any constructor. Therefore, readonly fields can have different
values depending on the constructor used. Also, while a const field is
a compile-time constant, the readonly field can be used for run-time
constants as in the following example:
See https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/readonly
For your specific case, I'd suggest using const instead of readonly, though admittedly I haven't looked for differences in the generated IL.
Honestly, I'm wondering if it really makes a difference. Any CPU that performs branch prediction and has a halfway-decent cache will likely yield the same performance in either case. (Note that I haven't tested that; it's just a suspicion.)
I am trying to replace the conventional if statement with ternary operator, but it does not compile.
First I would like to check if the input is a number, if true then assign Student.ID to input, if falsethen tell a user to try again.
My code:
(Int32.TryParse(Console.ReadLine().Trim().ToLower())) ? student.ID : Console.WriteLine("id isn't a number, try again");
What am I missing?
As I read it, you want to output something if the parsing fails. This can not be turned into a ternary.
There is a lot of things that are wrong with your code:
Tenaries are only for conditional assignments. You do not have a variable you assign anything to.
you never gave TryParse the out parameter, meaning the Compiler can not figure out wich function to even call there
Both options of a ternary must fit type you are doing assignment too. Console.WriteLine returns void, wich is propably not the same type you will have for Stundent.ID (I asume a integer).
So there are at least 3 reasons this can not compile.
Just keep it at the working if/else block. If you want to try ternaries, do stuff like applying a upper/lower bound for a value. That is what it is designed for. Rarely you use it for initialisation work (if == null, make a new instance. Otherwise give the value).
Edit: I just re-read and noticed that a assignment is not mandatory. It is just the very, very common usecase. And in that case, both results (or their return value) must fit the variable.
Christopher answered the question pretty well, so I won't repeat it here.
A workaround for what you're trying to do may be to implement a method that prompts the user for some input and doesn't return until the user enters a string that can be converted to the expected type (and optionally meets some other conditions).
For example:
private static int GetIntFromUser(string prompt, Func<int, bool> validator = null)
{
int result;
var cursorTop = Console.CursorTop;
do
{
ClearSpecificLineAndWrite(cursorTop, prompt);
} while (!int.TryParse(Console.ReadLine(), out result) ||
!(validator?.Invoke(result) ?? true));
return result;
}
private static void ClearSpecificLineAndWrite(int cursorTop, string message)
{
Console.SetCursorPosition(0, cursorTop);
Console.Write(new string(' ', Console.WindowWidth));
Console.SetCursorPosition(0, cursorTop);
Console.Write(message);
}
Now we can call this method in our main program and not worry about validation - our main code is much cleaner.
For example, say a student number must be 5 digits long, we could do something like:
int studentId = GetIntFromUser("Enter a 5-digit student id: ", x => x > 9999 && x < 100000);
I am making an OS with Cosmos and want to use goto to go to the user input but I am getting the error
No such label 'input' within the scope of the goto statement
'input' is a variable in which the user has inputted.
I can understand why this is happening but how do I fix it?
You cannot user variables as scope identifier for goto statement.. you have to use label identifier within scope (namespace) indicating it by ":" ..
for example
using System;
class Program
{
static void Main()
{
Console.WriteLine(M());
}
static int M()
{
int dummy = 0;
for (int a = 0; a < 10; a++)
{
for (int y = 0; y < 10; y++) // Run until condition.
{
for (int x = 0; x < 10; x++) // Run until condition.
{
if (x == 5 &&
y == 5)
{
goto Outer;
}
}
dummy++;
}
Outer:
continue;
}
return dummy;
}
}
method M contains three nested loops. The first loop iterates through numbers [0, 9], as do the two inner loops. But in the third loop, a condition is checked that causes the loop to exit using the break keyword.
For
Break
The code increments the dummy variable after each completion of the inner loop. If the inner loop is exited early, this variable should be left alone. With the goto statement, it is not incremented.
Result:
The value 50 is printed to the console. The int is incremented 10 x 5 times.
However:
If the goto was a break, the result would be 10 x 10 times, or a total of 100.
Hope this Help.. :)
I am making an OS with Cosmos
For getting any remotely useful answers, I think you will have to give some information about the scope of the OS. Are you only fiddling around with COSMOS a bit, or do you have some special use-case you want to serve with a custom COSMOS OS?
and want to use goto to go to the user input
Especially in the latter case (specialized OS) you should clearly refrain from using GOTO, unless you have a very good reason to do so (and in my humble opinion there is no such thing as a really good reason to use GOTO). There are viable alternatives to GOTOs in modern programming languages and you should re-think your design, algorithm, whatsoever.
To answer your question. Here is an example that produces the very error message you are experiencing
private void FirstMethod()
{
goto MyLabel;
}
private void SecondMethod()
{
MyLabel:
return;
}
I have defined a label in Method. Anyway, from Main you cannot simply jump from main to another method, since the compiler would not know where to return to, after the method has finished, since no data would have been pushed to the call stack on GOTO (please see the Wikipedia page about the call stack for further information).
The following, anyway, would work, since the label and the GOTO live within the same scope
void MyMethod()
{
goto MyLabel;
// do something
MyLabel:
return;
}
I'm trying to make a program that finds the factors of a number. I made a fairly simple one but it always repeated the same two factors twice i.e. 1 and 2, 2 and 1. So, to fix that I tried to check if the number had been used before but it keeps saying the bool proceed is unassigned.
using System;
namespace FactorableOrNah
{
class MainClass
{
public static void Main (string[] args)
{
Console.WriteLine ("Enter a whole number to view its factors: ");
int userInput = int.Parse(Console.ReadLine ());
int[] antiDoubler = new int[userInput];
bool proceed;
Console.Clear();
for (int i = 1; i != userInput; i++) {
antiDoubler[i] = userInput / i;
for(int j = 0; j < userInput; j++) {
if (antiDoubler [j] == i)
proceed = false;
else
proceed = true;
}
if ((userInput % i) == 0 && i != 1 && proceed == true)
Console.WriteLine("{0} and {1}", i, (userInput / i));
}
}
}
}
Using uninitialized variables in C# is not allowed. The compilation error can be solved by using either:
bool proceed = false;
or
bool proceed = default(bool);
since the default value of bool is false;
However, the algorithm is too complicated and very hard to read. Just for fun. A recursive example.
static IEnumerable<int> GetFactors(int number)
{
return GetFactors(number, number);
}
static IEnumerable<int> GetFactors(int number, int check)
{
if (check > 0)
{
if (number % check == 0)
{
yield return check;
}
foreach (var f in GetFactors(number, --check))
{
yield return f;
}
}
}
UPDATE:
Local variables cannot be left uninitialized, however class members (static members and instance variables), furthermore array elements are initialized automatically by the memory manager, so they are never uniitialized.
From the specification:
A variable must be definitely assigned (ยง5.3) before its value can be
obtained. As described in the following sections, variables are either
initially assigned or initially unassigned. An initially assigned
variable has a well-defined initial value and is always considered
definitely assigned. An initially unassigned variable has no initial
value. For an initially unassigned variable to be considered
definitely assigned at a certain location, an assignment to the
variable must occur in every possible execution path leading to that
location.
For your case you have an initially unassigned variable. Thus, the variable must be set in every possible execution path. There is one possible execution path to which your variable is not defined - when userInput >= j.
This would happen if userInput is 0. Following your program manually:
The first for case will check if i != userInput. Since i = 1 this is true, thus it will continue in the for loop.
the second for case will check if j < userInput. Since j = 0 this is false, thus it will skip the for case and never set proceed
Now you have arrived to where you check proceed and it was never set. So the compiler tells you that this is not allowed.
To solve your issue, you have to decide whether to:
define a default value for proceed, for instance false and set it at declaration, i.e. bool proceed = false;.
Rewrite your logic so that you do not need the boolean, for instance like Daniel Leiszen suggests.
Why can't i change value with an if statement?
int amount;
string inputbalk = inputbar.Text;
if (inputbalk== string.Empty)
{
amount = Convert.ToInt32(inputbalk);
amount = 1;
}
if (inputbalk != string.Empty)
{
amount = Convert.ToInt32(inputbalk);
amount = 1;
}
int onepercent = amount/= 100;
It will see "amount" as an unassigned variable, even though I set it to 1, by two different if-statements.
If I debug, I get this:
"Error 1 Use of unassigned local variable 'amount'"
Thanks for help guys, It is fixed/solved.
Just change second if to else
if (inputbalk== string.Empty)
{
amount = Convert.ToInt32(inputbalk);
amount = 1;
}
else
{
amount = Convert.ToInt32(inputbalk);
amount = 1;
}
The compiler can't make sure that one of two if statements will work anyway, so it will throw error that your variable amount can be unassigned.
In if/else notation one of two code blocks will be done anyway, so compiler will not throw error and everything will work as you want.
There is chance of having inputbalk as null :)
if is not a loop
considering inputbalk is string, it can be a null, you don't check for it, so could happen that it's your case.
Change your code, like this:
if (string.IsNullOrEmpty(inputbalk))
{
amount = Convert.ToInt32(inputbalk);
amount = 1;
}
else
{
amount = Convert.ToInt32(inputbalk);
amount = 1;
}
Hope this helps.
The problem is that the compiler cannot see that amount is definitely assigned. You know that inputBalk can't change between the first and second time it is evaluated, and that exactly one of the if conditions will be true, but the compiler doesn't check this. It sees only that there is a path where amount isn't assigned before it is used, which is disallowed.
You should use if/else instead of testing the same condition twice.
if (...)
{
amount = 1;
}
else
{
amount = 2;
}
Now the compiler can see that every possible path through the code causes amount to be definitely assigned.
There are also a number of other advantages: the code is more concise, easier to read and more maintainable. Furthermore, there is a slight performance benefit from not doing the same check twice.
You could however also encounter an FormatException with the Convert.ToInt32() method. Consider using int.tryParse as an alternative.
Because the chances are that you won't go into any if statement and thus leaving the amount as unassigned. You will get error on this line:
int onepercent = amount /= 100;
At compile time, compiler will not be able to determine whether any of the if statement will result in true and setting of amount. To avoid this error you could do (at start) :
int amount=-1;
Now you will not get your compiler error and value of the amount will change in the if statement. P.S. ('If' is a statement not a loop)
Give this a blast:
if (!int.TryParse(inputbar.Text, out amount))
amount = 1;