OR or AND

Well which of the followinf is correct for the below statement and why


i want to make sure that the user does not enter the key a b c and d. In case he enters these he should get an error messgae


if(input!='a' && input!='b' && input!='c' && input!='d')
cour<<"Error";

OR

if(input!='a' || input!='b' || input!='c' || input!='d')
cour<<"Error";





well logicaly the furst one seems to be correct saying that if the user enters A or B or C or D give the error message.. But this statement does not work.. The second statement works...



I know it is got to do with the OR gate and AND gates.. But please clarify

In the first you say that if the user does not press a,b,c or d, he will get an error.
Use the second one.

I say: use none... both are not correct.

In the first you say if the use does not press a, b, c or d he or she will het an error.
In the second you say always show an error.

Third statement: if user enter a OR b OR c OR d then show error

Code:

---

if(input=='a' || input=='b' || input=='c' || input=='d')
  cout<<"Error" << endl;

---

Monster is correct...... just read it to yourself and remember

&& - AND
|| - OR

So the code monster put up reads, "If input is equal to a or b or c or d, output error"

Quote:

---

if(input!='a' || input!='b' || input!='c' || input!='d')
cour<<"Error";

---

This will always be true. Using ||, the whole expression will be true if at least one of the parts are true.

Assume Input has the value 'a'.
Then Input != 'b' will be true, since a != b
Assume that Input has any value except 'a'.
Then Input != 'a' will be true according to the assumption above.

Conclusion: The expression will always be true, since one of Input != 'b' and Input != 'a' will always be true.


(I'm learning Logics in school, this was a nice excercise :))

>i want to make sure that the user does not enter the key a b c
>and d. In case he enters these he should get an error messgae

In logic it often helps to describe a problem in a different way. You want an error message appear when the user has entered a or b or c or d. In other words: if the user enters a or b or c or d, then an error message must be printed.

Even I consider this as easy!

The only thing about these logic operators which I can consider as a problem is where to place the !, whenever you want to invert an expression.

If the expression is

a && b

the the inversion is

!(a && B)

but using the rules of logic, it is the same as

!a || !b

So you can place the inverter wherever you want, but always take good care of the used logic operators. If you're not used to the laws of logic, a thruth table is always very useful.