Thread: increment/decrement questions. I want to clarify this.

Originally Posted by nedlig

but a= b-1 + c. so a is 3 b=2 and c is 7
in this case, since a is not given a value, the answer is 9?

It sounds like you have a fundamental misunderstanding of what a code snippet like this means:

Code:

b = 3;
c = 7;
a = b + c;

In the above snippet, the last statement can be equivalently written like this:

Code:

a = (b + c);

That is, the precedence of the = operator is lower than that of the + operator, hence the grouping of subexpressions is equivalent to the above rather than to:

Code:

(a = b) + c;

Consequently, given this:

Code:

a = b-- + c;

it doesn't make sense to say that "a is 3" because b was equal to 3, because you can only say what a is after evaluating (b-- + c), as it is the result of that subexpression that is assigned as the value of a.

Hence, in post #2 of that other thread sepp2k observed that the answer is 10, since post-decrement was used to decrement b. Furthermore, it is clearly wrong to say that "a is not given a value", because indeed a was given a value, and that value is the result of (b-- + c).

Originally Posted by nedlig

i=i+1;
h=j+2*i;
my attempt
h= j+2*i++ ??

This is wrong. You need to pay attention to the difference between pre-increment and post-increment. Also, you shouldn't be asking if your attempt is correct when you can verify that it is wrong by testing it. You should only ask if it is correct if after testing, you have found that it passes your tests, but you are not sure if you missed anything.

Yes, that is correct.

Originally Posted by nedlig

so then I'm not understanding why a would equal to 10 if a = b-- +c; given b=3 c=7. The output would be 9.

Because of the post-decrement, this:

Code:

a = b-- + c;

has the same result as:

Code:

a = b + c;
b = b - 1;

Hence, mathematically: a = 3 + 7 = 10, and b = 3 - 1 = 2.

Originally Posted by nedlig

ok. let's take out +c and solve a=b-- so that would be a=3 and b=2. so then we will add int c here. that would be a=2+7 which would equal 9. what happens to the output of a (3) ?

Refer to my post #4: you cannot "take out +c". At no point does the value of a become 3. The value of b does indeed become 2, but that happens afterwards (post-decrement). Hence, your claim that "a=3" is absurd, and your attempt to "then we will add int c here" is wrong, hence your statement that "a=2+7" is incorrect.