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Can someone explain why the answer is B? I am using Java and I do not understand

Which of the following statements are the same?

(I) x - = x + 4

(II) x = x + 4 - x

(III) x = x - (x + 4)

and. (I) and (II) are the same

B. (I) and (III) are the same

C. (II) and (III) are the same

D. (I), (II) and (III) are the same

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4 answers 


x -= y is equivalent to x = x - y

therefore

x -= x + 4


equivalent to

x = x - (x+4)

Suppose it (II) x = x - (x + 4)

should have been (III) x = x - (x + 4)

(since you have two options marked as (II)

) (I)

and are the (III)

same.

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This is due to operator priority. Java evaluates that as if it were

x - = (x + 4)



so it first calculates (x+4)

and then subtracts from x

- what the -

part means -=

and then updates x

what the part =

means.

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  • Case (I) expands to x = x - (x + 4) according to the Java operator -=

    ,
    and mathematically simplifies to x = -4.
  • Case (II) is mathematically simplified to x = 4.
  • Case (III) is mathematically simplified to x = -4.

Therefore (I) and (III) are the same, which means that the answer is (B).

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-=

is a so-called compound destination.

They are just short cuts and merging of atomic operations.

x -= y

means x = x-y

x += y

means x = x+y

x++

means x = x+1

x--

means x = x-1

There are also ++ x and -x, which do the same as x ++ / x--, except that they return the value of x before it is incremented / decremented.

Official Java Tutorial:

"You can also combine arithmetic operators with a simple assignment operator to create complex assignments. For example, x + = 1; and x = x + 1; both increase x by 1."

https://docs.oracle.com/javase/tutorial/java/nutsandbolts/op1.html

I think the same works for *=

, /=

and%=

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