cin question

[jub4jive]

Hi, I just learned about looping in c++. I came across two questions that I was unable to answer. I need help understanding the idea.

1) Is the following statement valid? If not, why not? If so, what does it do?

int x = (1,024);

What about the following?

int y;
y = 1,024;

2) How does cin >> ch differ from cin.get(ch) and ch = cin.get() in how it views input?

[Werner2]

Normally, you don't have a , in numbers. Otherwise I don't see anything invalid in your first question.

Parentheses are used to change the order of operations. Wrapping a number in them won't do anything.

As for y, it will first allocate memory for y, and it will be filled with some garbage value, and then you assign a value to it. But a compiler will probably optimise that.

As for 2, I'm not that experienced with input streams yet, sorry.

[mutexe]

int x = (1,024);
This is horrible.
It's first assigning 1 to y, and then it's assigning octal 024 to y i.e. decimal 20.


http://en.wikipedia.org/wiki/Comma_operator

OP, you could of course try and compile this yourself.

[helios]

It's first assigning 1 to y, and then it's assigning octal 024 to y i.e. decimal 20.

Not quite. Only one assignment is made.

[mutexe]

ah yes:

In the C and C++ programming languages, the comma operator (represented by the token ,) is a binary operator that evaluates its first operand and discards the result, and then evaluates the second operand and returns this value (and type).

it evaluates both, but only returns the last.

Apologies.

[jub4jive]

so, y just accepts 024 as input? (024 hexadecimal?)

what about the cin question?

[cire]

1) Is the following statement valid? If not, why not? If so, what does it do?
int x = (1,024);

Functionally equivalent to: int x = 024;

What about the following?

int y;
y = 1,024;

Functionally equivalent to: y = 1;

2) How does cin >> ch differ from cin.get(ch) and ch = cin.get() in how it views input?

With formatted input (cin >> ch) all whitespace is skipped. With the form cin.get(ch) you have unformatted input extraction and ch may actually be a whitespace character. The form ch = cin.get() returns an int rather than a char which allows the return value to also become eof.

[jub4jive]

1) Is there any way to remember this, since with the parenthesis it accepts the 3 digits after the comma; but without the parenthesis it accepts the digit before the comma.

2) cin >> ch -- accepts a single character (alphabet or symbol) and doesn't accept white spaces
cin.get(ch) -- accepts anything (alphabet, symbols and whitespaces), and
ch = cin.get() -- accepts anything and stores it as an int.

Did I get it right?

[giblit]

1) Is there any way to remember this, since with the parenthesis it accepts the 3 digits after the comma; but without the parenthesis it accepts the digit before the comma.

It's just operator precedence. http://www.cplusplus.com/doc/tutorial/operators/

Basically the = operator has higher precedence than the , operator and () has a higher precedence than =.

So with the first int x = (1,024);

The highest precedence is () so it will evaluate that first. Inside that there is the comma operator so it checks 1 then discards then returns 024. Now it does int x = 024;


The second example

int y;
y = 1,024;

The = operator has the highest precedence so it does that first y = 1; then it does the comma operator so it just returns the 024 (in other words does nothing).

So it would be like:

y = 1;
024;

You will probably get a warning message anyways something like: warning: right operand of comma operator has no effect [-Wunused-value]

[jub4jive]

@giblit:
I didn't quite follow the first example.
I understand that the comma has least precedence and ( ) has the highest precedence.

So, in the first example, (1,024) is evaluated first since it is in the brackets. Then the contents within the brackets are exactly the same as the second example, right? So, shouldn't x = 1; just like y in the second example?

[helios]

Then the contents within the brackets are exactly the same as the second example, right?

Yes, the subexpressions are the same, but () doesn't evaluate to the internal subexpression (think about it. If it did, it would have no effect. For example, 2*(1+3) would equal 2*1+3). It evaluates to the value of the internal subexpression. The value of "1,024" is "024". Therefore, these two lines are equivalent:

x = (1,024);
x = 024;

but these two are not:

x = (1,024);
x = 1,024;

[jub4jive]

I am sorry. I don't think I understand the explanation.
All I got was the internal expression is not evaluated.

[cire]

Let's take this statement: 1, 024;

According to the way the comma operator works the first operand (1) is evaluated and discarded. Next, the second operand (024) is evaluated and is used for the value of the expression. Now, that doesn't really matter here since the result of the expression is not used. But, if you stick the expression in a set of parentheses, that is the result you will get, and that is why x = (1, 024); results in x equaling 024.

Whereas in y=1, 024; the compiler sees it as the parenthetical expression (y=1), 024 where (y=1) is the first operand of the comma operator (see the precedence rules mentioned by giblit above.) So, y=1 is evaluated and the resulting value is discarded. The expression then takes on the value of second operand. However, that value is never used for anything.

http://ideone.com/GzKvI8