Could you please help me understand why variable a is not incremented in the first case but it is in the second case?
Case 1:
int a = 10;
a = a++;
Console.WriteLine(a); //prints 10
Case 2:
int a = 10;
int c = a++;
Console.WriteLine(a); //prints 11
I've gone through other similar questions but couldn't find any specifics.
UPDATE 1: How I think the program flows
Case 1:
1. 'a' is assigned 10
2. 'a' is assigned 10 before increment happens
3. 'a' is incremented by 1 (Why doesn't this step affect the final value of 'a'?)
4. 'a' is printed --> 10
Case 2:
1. 'a' is assigned 10
2. 'c' is assigned 10 before 'a' is incremented
3. 'a' is incremented by 1 (Why does the increment of 'a' work here?)
4. 'a' is printed --> 11
UPDATE 2: Thanks to all the answers, i think i've understood it, please correct me if i'm wrong.
Case 1:
1. `a` is assigned 10
2. Compiler evaluates `a++`, stores old value 10 and new value 11 as well. Since it's a post increment operation, assigns the old value to `a`. What i thought was, compiler would assign the old value 10 first and evaluate the `++` operation later. This is where i was wrong, compiler evaluates the RHS beforehand and assigns the value based on the operator.
4. 'a' is printed --> 10
Case 2:
1. `a` is assigned 10
2. Compiler evaluates `a++`, stores old value 10 and new value 11 as well. Since it's a post increment operation, assigns the old value to `c` but value of `a` is preserved with `11`.
4. 'a' is printed --> 11
For me the best way to understand some behaviour is to check IL generated. In your first case it's
IL_0001: ldc.i4.s 0A // stack: 10
IL_0003: stloc.0 // a = 10, stack: empty
IL_0004: ldloc.0 // stack: 10
IL_0005: dup // stack: 10, 10
IL_0006: ldc.i4.1 // stack: 10, 10, 1
IL_0007: add // stack: 10, 11
IL_0008: stloc.0 // a = 11, stack: 10
IL_0009: stloc.0 // a = 10, stack: empty
IL_000A: ldloc.0 // stack: 10
IL_000B: call System.Console.WriteLine
You can see that there's still that original value hanging on the stack and so the created 11 gets overwritten in the end.
Let me try to explain it in plain words.
When you are assigning a value to a variable (a = a++) the whole right side of the assignment gets evaluated first in order to guarantee the correct value, that's how it is. So nothing like you get 10, application goes on and increments value when you execute the next line.
Now, imagine post-increment as someone, who first increments a value, but gave you his world that you will get back the original value from the expression. And now you should see why the 11 is overwritten. Increment goes first and in the end, you get the promise original value (as IL proves).
The first case a = a++ is post-increment. Which says add 1 to a but return the previous value of a and then store the previous result back into a. This is basically a no-op.
If it was pre-increment, a = ++a, then a would be 11.
There is nothing undefined behavior here as someone describes in comment.
This is clearly defined behavior. To understand what is happening, you must first understand how the pre increment and post increment operator works.
Case1:
a++(post increment) will increment the value of a and stores it in a then returns the value before it was incremented.
So after executing a++;, value of a will be 11 but the operator will return 10.
Then a = a++; the assignment part becomes a = 10;.
Case2:
Sameway a++; will increment the value of a to 11 and returns the previous value (10). which will be assigned to c. c will be 10 but a will be 11 because you're not overwriting the value of a in this case.
Your Case1 is equal to:
int a = 10;
int temp = a;//10
a = a + 1; //11
a = temp; //10
Console.WriteLine(a);//10
and Case2 is equal to:
int a = 10;
int temp = a;//10
a = a + 1; //11
int c = temp;//10
Console.WriteLine(a);//11
I hope now that should be clear why you see what you see.
This is not undefined behavior nor is it a bug. From the MSDN Documentation:
The increment operator (++) increments its operand by 1.
The increment operator can appear before or after its operand.
The first form is a prefix increment operation. The result of the operation is the value of the operand after it has been incremented.
The second form is a postfix increment operation. The result of the operation is the value of the operand before it has been incremented.
So literally, MSDN tells you that if you use this syntax (postfix):
a = a++;
Then the result of the operation will assign a to a then increment. However, since the assignment operation already took place, you are losing the result of the increment.
Using it like this (prefix):
a = ++a;
This will increment a first, then assign the incremented value to a.
EDIT
I will try to break this down so hopefully you understand better.
First of all, know that ++ always returns a value. If you use the prefix version (e.g. ++a), it returns the value of a+1. If you use the postfix version (e.g. a++), it returns the value of a, before the increment occurs.
When you execute this code:
int a = 10;
a = a++;
You are telling the compiler to assign a the value of a before the increment. Therefore a equals 10 after this executes. The incremented value of 11 gets lost in the "nether". Note that you are not assigning 11 to a. You are assigning the old value, before the increment, which is why you are getting an output of 10.
When you execute this code:
int a = 10;
int b = a++;
After the last line executes, it is incrementing a to equal 11 and assigning 10 to 'b'. Since you are assigning to a different variable, then a is not getting overwritten by its original value of 10 as in the first example.
To make this even more visual, look here:
a = a++;
^ a is increased to 11, but the postfix increment returns the old value (10)
This line effectively becomes:
a = 10;
int b = a++;
^ a is increased to 11, but b gets assigned a's old value
This line effectively becomes:
int b = 10;
a = a + 1;
Does this clear it up?
I don't think that this is an undefined behavior , since in the first case :
int a = 10;
a = a++;
Console.WriteLine(a); //prints 10
The value of a is the value before the increment action , meaning that if we simplify a = a++ , then :
a = a ; // `a` has `10`
a++; // `a` has `11`
and only then a has the value 11 .
The 2nd case :
int a = 10;
int c = a++;
Console.WriteLine(a); //prints 11
Now c has the value 10 since the assignment operation makes the value on the left side to get the value of the variable (on the right side) before the increment action , and only then the value on the right side goes up by 1 .
To make it clear, there is a different between ++a, and a++.
Both two of them produces the same result, that is to increment a by 1, but the process is a little bit different and it is only seen by assigning it to another variable.
This is difference between pre and post increment that you might looking for
Case 1
int a,b;
a = 10;
b = a++; //Here b is assigned by a post-incremented a, therefore b value is still 10
Case 2
int a,b;
a = 10;
b = ++a; //Here b is assigned by a pre-incremented a, b value is now 11
Taken from http://en.wikipedia.org/wiki/Increment_and_decrement_operators
UPDATE
In reply to this problem
int a = 10;
a = a++;
the process is as followed:
1. variable 'a' with data type integer is created
2. variable 'a' value is assigned with 10
3. check the availability and the data type correctness of the right side. This means all operation on the right side (increment 'a') is being executed. 'a' is now 11
4. assign the post-incremented value to 'a', which is 10, 'a' is now 10
You should print out c in case 2.
The post incement operator result is always the value before increment. So first give the value 10 back then increment it to 11 and after that assign the operator result (10) to the left operand.
Related
I have two questions about this code...
Why is that at line 10 it starts keeping the current value. For example,
int a = 7
(a += 4) Which is 11 is brought over to the next line of code (a -= 4) now making it 7. Instead of just using it initial declaration for the variable a which is 7. How come I don't get 3 ? Is the = in the += operator changing what I initially declared it in the beginning of the code ? Does a still hold the value 7 in memory, or does those statements change that?
At the last MessageBox.Show() statement. I increment a by 1 using a++. However, I get the same value I had for the previous MessageBox.Show(). How come it didn't increment ??
This is the code:
private void button1_Click(object sender, EventArgs e)
{
int a = 7;
int b = 3;
MessageBox.Show((a + b).ToString(), "a+b");
MessageBox.Show((a - b).ToString(), "a-b");
MessageBox.Show((a * b).ToString(), "a*b");
MessageBox.Show((a / b).ToString(), "a/b");
MessageBox.Show((a += 4).ToString(), "a+=4"); //adds 4 to a
MessageBox.Show((a -= 4).ToString(), "a-=4"); //substracts 4 from a
MessageBox.Show((a *= 4).ToString(), "a*=4"); //multiplies 4 from a
MessageBox.Show(a++.ToString(), "a++"); //adds 1 to a
}
How come I don't get 3 ? Is the "=" in the "+=" operator changing what
I initially declared it in the beginning of the code ?
The += operator is equivalent to:
a = a + 4
Effectively assigning a new value to a.
Does "a" still hold the value 7 in memory, or does those statements
change that?
It doesn't. After your first assignment, it changes.
At the last MessageBox.Show() statement. I increment "a" by 1 using
"a++". However, I get the same value I had for the previous
MessageBox.Show(). How come it didn't increment ??
That's what happens when you use ++ as a postfix. The docs say:
The second form is a postfix increment operation. The result of the
operation is the value of the operand before it has been incremented.
But, if you use it as a prefix:
MessageBox.Show((++a).ToString(), "++a");
You'll see the updated value, again as the docs say:
The first form is a prefix increment operation. The result of the
operation is the value of the operand after it has been incremented.
MessageBox.Show((a += 4).ToString(), "a+=4"); //adds 4 to a
This sentance result is 7+4=11 so in a it takes 11 value
after that
MessageBox.Show((a -= 4).ToString(), "a-=4"); //substracts 4 from a
for this sentance it take a value i.e current value it 11
i.e 11-4=7 so a value now is 7;
MessageBox.Show(a++.ToString(), "a++"); //adds 1 to a
in this it post-increment condition so in this it use that value and for next loop it increment by 1
For Above condition you can use
MessageBox.Show((++a).ToString(), "a++"); //adds 1 to a
MessageBox.Show((a *= 4).ToString(), "a*=4"); //multiplies 4 from a
and for this also it take 7
and += mean
a=a+b;
for post incerement and pre incerement condition go through this What is the difference between pre increment and post increment operator
(a += 4) increments the value of a by 4 and returns the incremented value.
a++ increments the value of a by 1 but still returns the original value.
++a increments the value of a by 1 and returns the incremented value.
try to add this line and you will notice it:
MessageBox.Show((++a).ToString(), "a++");
Hay I didn't know even If this question has asked before but my problem is as following.
In my c# console application I had declared a variable i with assigning a value as
int i = 0 and now I want increment i by 2, obviously I can use following cede.
int i = o;
i += 2;
Console.WriteLine(i);
Console.ReadLine();
//OUTPUT WILL BE 2
but this one is my alternate solution. As my lazy behavior I refuse to use this code and I had used following code.
int i = 0;
i += i++;
Console.WriteLine(i);
Console.ReadLine();
In above code I had accepted FIRST i++ will increment by one and than after it will again increment by i+=i but this thing is not happen.!!!
I doesn't know why this thing is happening may be I had done something wrong or some compilation problem.?????
Can any one suggest me why this happening????
I just want to know why code no 2 is not working? what is happening in there?
The i++ returns the value of i (0) and then adds 1. i++ is called post-increment.
What you are after is ++i, which will first increase by one and then return the increased number.
(see http://msdn.microsoft.com/en-us/library/aa691363(v=vs.71).aspx for details about increment operators)
i needs to start off with 1 to make this work.
int i = 1;
EDIT::
int i = 0;
i += i++;
Your code above expresses the following:
i + 0 then add one
if you use i += ++i; then you'll get i + 1 as it processed the increment beforehand.
What your code is doing:
"i++" is evaluated. The value of "i++" is the value of i before the increment happens.
As part of the evaluation of "i++", i is incremented by one. Now i has the value of 1;
The assignment is executed. i is assigned the value of "i++", which is the value of i before the increment - that is, 0.
That is, "i = i++" roughly translates to
int oldValue = i;
i = i + 1
//is the same thing as
i = oldValue;
The post-increment operator increments the value of your integer "i" after the execution of your i++
For your information:
i++ will increment the value of i, but return the pre-incremented value.
++i will increment the value of i, and then return the incremented value.
so the best way of doing the 2 step increment is like that:
i +=2
I have this sample code
public class MyClass
{
private static int tempKey = 0;
public MyClass()
{
this.Key = --tempKey;
}
public int Key {get; set;}
}
What does --tempkey do exactly?
It decrements tempKey and returns new value. Compare with tempKey--, which also decrements tempKey, but returns the original value.
See Microsoft documentation here.
The increment operator (++) increments its operand by 1. The increment
operator can appear before or after its operand:
++ var
var ++
Where:
var An expression that denotes a storage location or a property or an
indexer.
The first form is a prefix increment operation. The result of
the operation is the value of the operand after it has been
incremented.
The second form is a postfix increment operation. The result of the
operation is the value of the operand before it has been incremented.
EDIT: This is valid for C#. Visual Basic doesn't have this increment/decrement operator.
In Visual Basic --tempKey is evaluated as -1 * (-1 * tempKey) which is equal to tempKey.
"--x" is pre-decrement arithmetic operator. Meaning the value is decremented by 1 before being used in the statement.
int x = 10;
Console.WriteLine(--x); // This will print 9
"x--" is post-decrement arithmetic operator. Meaning the value is used and then decremented by 1.
int x = 10;
Console.WriteLine(x--); // This will print 10
Console.WriteLine(x): // This will print 9
It decrements the variable by 1 and then evaluates to the decremented value. Therefore, in the example you gave, the constructor would decrement tempKey from 0 to -1 and then set Key to the same value of -1 as well. For instance:
int x = 5;
int y = --x; // x gets set to 4 and they y is also set to 4
int x2 = 5;
int y2 = x2--; // x2 gets set to 4 and then y gets set to 5 (x2's value prior to decrement)
There is no equivalent operator in VB. In VB, --tempKey would have no affect. A single minus sign before a variable name will negate the value. Two minus signs in a row before a variable name will negate the value twice, thereby returning it to the original value. For instance:
Dim x As Integer = 5
Dim y As Integer = --x ' x still equals 5 and y also gets assigned 5
In other words, it's the same as saying y = -1 * -1 * x.
It subtracts one from the value of tempkey.
More info can be found here: http://msdn.microsoft.com/en-us/library/6a71f45d.aspx
Specifically, from http://msdn.microsoft.com/en-us/library/wc3z3k8c.aspx
The decrement operator (--) decrements its operand by 1. The decrement
operator can appear before or after its operand: --variable and
variable--. The first form is a prefix decrement operation. The result
of the operation is the value of the operand "after" it has been
decremented. The second form is a postfix decrement operation. The
result of the operation is the value of the operand "before" it has
been decremented.
The -- operator is the subtraction operator. In this case, it subtracts one from the tempKey variable. Because it is before the variable, it subtracts the value first, before returning it into the this.Key value. If it was after the variable tempkey-- then it would subtract the value after returning the value into this.Key.
--i decrements the number and returns the decremented result, whereas i-- also decrements the number, but returns the pre-decremented result.
It is a pre-decrement operator. Effectively, it subtracts one from tempKey. Thus when the constructor MyClass is first called, tempKey changes its value to -1 and then this.Key gets the value -1. Since tempKey is static, each subsequent call for constructor will decrement tempKey.
I am a little confused about how the C# compiler handles pre- and post increments and decrements.
When I code the following:
int x = 4;
x = x++ + ++x;
x will have the value 10 afterwards. I think this is because the pre-increment sets x to 5, which makes it 5+5 which evaluates to 10. Then the post-increment will update x to 6, but this value will not be used because then 10 will be assigned to x.
But when I code:
int x = 4;
x = x-- - --x;
then x will be 2 afterwards. Can anyone explain why this is the case?
x-- will be 4, but will be 3 at the moment of --x, so it will end being 2, then you'll have
x = 4 - 2
btw, your first case will be x = 4 + 6
Here is a small example that will print out the values for each part, maybe this way you'll understand it better:
static void Main(string[] args)
{
int x = 4;
Console.WriteLine("x++: {0}", x++); //after this statement x = 5
Console.WriteLine("++x: {0}", ++x);
int y = 4;
Console.WriteLine("y--: {0}", y--); //after this statement y = 3
Console.WriteLine("--y: {0}", --y);
Console.ReadKey();
}
this prints out
x++: 4
++x: 6
y--: 4
--y: 2
Lets have a look at the IL that gets generated from that statement
IL_0002: ldloc.0
Loads the value of x onto the stack. Stack => (4)
IL_0003: dup
Duplicates the topmost item on the stack. Stack => (4, 4)
IL_0004: ldc.i4.1
Push 1 onto the stack. Stack => (1, 4, 4)
IL_0005: sub
Subtract the two top values and push result onto the stack. Stack => (3, 4)
IL_0006: stloc.0
Store the topmost value of the stack back to x. Stack => (4)
IL_0007: ldloc.0
Load the value of x back into the stack. Stack => (3, 4)
IL_0008: ldc.i4.1
Load the value 1 onto the stack. Stack => (1, 3, 4)
IL_0009: sub
Subtract the two. Stack => (2, 4)
IL_000A: dup
Duplicate the top value => (2, 2, 4)
IL_000B: stloc.0
Store the top value back to x. Stack => (2, 4)
IL_000C: sub
Subtract the two top values. Stack => (2)
IL_000D: stloc.0
Store this value back into x. x == 2
From your comment:
I thought that post- and pre-increments are executed after / before evaluation of the complete codeline - but they are executed after / before the evaluation of each item in the expression.
Your misunderstanding is an extremely common one. Note that in some languages, like C, it is not specified when the side effect becomes visible and it is therefore legal, but not required, for your statement to be true in C.
This is not the case in C#; in C# side effects of code on the left side of an expression are always observed to happen before code on the right side executes (from a single thread; in multithreaded scenarios all bets are off.)
For a more detailed explanation of what the increment operators do in C#, see:
What is the difference between i++ and ++i?
There are a great many additional links there to articles I've written on this often-misunderstood topic.
The most interesting thing that you'll get a completely different answer with C++.Net compiler.
int x = 4;
x = x++ + ++x; // x = 11
x = 4;
x = x-- - --x; // x = -1
Of course the difference in results is determined by different semantics - it seems normal. But despite the understanding the fact that two .net compilers don't behave in a similar manner for such basic things confuses me too.
In this example,
int x = 4;
x = x++ + ++x;
you can break it like:
x = 4++; which is = 5
x = 4 + ++5; which is 4 + 6
x = 10
Similarly,
int x = 4;
x = x-- - --x;
Here,
x = 4--; which is = 3
x = 4 - --3; which is 4 - 2
x = 2
Simply putting you can say, replace the current value of x, but for every ++ or -- add/subtract a value from x.
I think the explanation for the ++ + ++ case is wrong:
command...........value of x
..................undefined
int x=4 ..........4
x++...............5 (first summand is 4)
++x...............6 (second summand is 6)
x=summand1+summand2 ..4+6=10
Analogous the explanation for the -- - -- case is
command...........value of x
..................undefined
int x=4 ..........4
x--...............3 (subtactor is 4)
--x...............2 (subtrahend is 2)
x=subtractor-subtrahend ..4-2=10
Today I came a cross an article by Eric Lippert where he was trying to clear the myth between the operators precedence and the order of evaluation. At the end there were two code snippets that got me confused, here is the first snippet:
int[] arr = {0};
int value = arr[arr[0]++];
Now when I think about the value of the variable value, I simply calculate it to be one. Here's how I thought it's working.
First declare arr as an array of int
with one item inside of it; this
item's value is 0.
Second get the value of arr[0] --0 in
this case.
Third get the value of arr[the value
of step 2] (which is still 0) --gets
arr[0] again --still 0.
Fourth assign the value of step 3
(0) to the variable value. --value =
0 now
Add to the value of step 2 1 --Now
arr[0] = 1.
Apparently this is wrong. I tried to search the c# specs for some explicit statement about when the increment is actually happening, but didn't find any.
The second snippet is from a comment of Eric's blog post on the topic:
int[] data = { 11, 22, 33 };
int i = 1;
data[i++] = data[i] + 5;
Now here's how I think this program will execute --after declaring the array and assigning 1 to i. [plz bear with me]
Get data[i] --1
Add to the value of step 1 the value
5 --6
Assign to data[i] (which is still 1)
the value of step 2 --data[i] = 6
Increment i -- i = 2
According to my understanding, this array now should contain the values {11, 27, 33}. However, when I looped to print the array values I got: {11, 38, 33}. This means that the post increment happened before dereferencing the array!
How come? Isn't this post increment supposed to be post? i.e. happen after everything else. What am I missing guys?
The postincrement operation occurs as part of evaluating the overall expression. It's a side effect which occurs after the value is evaluated but before any other expressions are evaluated.
In other words, for any expression E, E++ (if legal) represents something like (pseudo-code):
T tmp = E;
E += 1;
return tmp;
That's all part of evaluating E++, before anything else is evaluated.
See section 7.5.9 of the C# 3.0 spec for more details.
Additionally, for assignment operations where the LHS is classified as a variable (as in this case), the LHS is evaluated before the RHS is evaluated.
So in your example:
int[] data = { 11, 22, 33 };
int i = 1;
data[i++] = data[i] + 5;
is equivalent to:
int[] data = { 11, 22, 33 };
int i = 1;
// Work out what the LHS is going to mean...
int index = i;
i++;
// We're going to assign to data[index], i.e. data[1]. Now i=2.
// Now evaluate the RHS
int rhs = data[i] + 5; // rhs = data[2] + 5 == 38
// Now assign:
data[index] = rhs;
The relevant bit of the specification for this is section 7.16.1 (C# 3.0 spec).
For the first snippet, the sequence is:
Declare arr as you described:
Retrieve the value of arr[0], which is 0
Increment the value of arr[0] to 1.
Retrieve the value of arr[(result of #2)] which is arr[0], which (per #3) is 1.
Store that result in value.
value = 1
For the second snippet, the evaluation is still left-to-right.
Where are we storing the result? In data[i++], which is data[1], but now i = 2
What are we adding? data[i] + 5, which is now data[2] + 5, which is 38.
The missing piece is that "post" doesn't mean "after EVERYTHING else." It just means "immediately after I retrieve the current value of that variable." A post increment happening "in the middle of" a line of code is completely normal.
data[i++] // => data[1], then i is incremented to 2
data[1] = data[2] + 5 // => 33 + 5
I would expect the post-increment operator to increment the variable after its value is used.
In this case, the variable is incremented before the second reference to the variable.
If it would not be so, you could write
data[i++] = data[i++] + data[i++] + data[i++] + 5
If it would be like you say, then you could remove the increment operator because it doesn't do actually anything, in the instruction I reported.
You have to think of assignments in three steps:
Evaluate left hand side (=get address where the value should be stored)
Evaluate right hand side
Assign the value from step 2 to the memory location from step 1.
If you have something like
A().B = C()
Then A() will run first, then C() will run, and then the property setter B will run.
Essentially, you have to think of your statement as
StoreInArray(data, i++, data[i] + 5);
The cause might be that some compilers optimize i++ to be ++i. Most of the time, the end result is the same, but it seems to me to be one of those rare occasions when the compiler is wrong.
I have no access to Visual Studio right now to confirm this, but try disabling code optimization and see if the results will stay the same.