equals confusion

[Matty]

Hi,

I'm currently learning c# and am confused why the first example below
doesn't work but the second one does. I'd appreciate any feeback. Thanks.

First example using !=

string input = Console.ReadLine();

if ((input != "0") || (input != "1"))
{
Console.WriteLine("not equal to 0 or 1");
}
else
{
Console.WriteLine("is equal to 0 or 1");
}

Console.ReadLine();

Second example using a single ! followed by ==

string input = Console.ReadLine();

if (!((input == "0") || (input == "1")))
{
Console.WriteLine("not equal to 0 or 1");
}
else
{
Console.WriteLine("is equal to 0 or 1");
}

Console.ReadLine();

 

[Hans Kesting]

Hi,

I'm currently learning c# and am confused why the first example below
doesn't work but the second one does. I'd appreciate any feeback. Thanks.

First example using !=

string input = Console.ReadLine();

if ((input != "0") || (input != "1"))
{
Console.WriteLine("not equal to 0 or 1");
}
else
{
Console.WriteLine("is equal to 0 or 1");
}

Console.ReadLine();

Second example using a single ! followed by ==

string input = Console.ReadLine();

if (!((input == "0") || (input == "1")))
{
Console.WriteLine("not equal to 0 or 1");
}
else
{
Console.WriteLine("is equal to 0 or 1");
}

Console.ReadLine();

your line (first example):
if ((input != "0") || (input != "1"))

what if 'input' really is "1"? Then (input != "0") is true, so the
complete test is true!

|| is an "OR" operator, which means that at least one of the tests (on
both sides) should be "true" (with the special case that if the first
(left hand) test already results in "true", the second one never even
is tested: that would not influence the final result).

Hans Kesting

 

[Mick Wilson]

Hi,

I'm currently learning c# and am confused why the first example below
doesn't work but the second one does. I'd appreciate any feeback. Thanks.

Look up De Morgan's Laws. The second example transposed is equivalent
to:

input != 0 AND input != 1

The first example is

input != 0 OR input != 1

http://en.wikipedia.org/wiki/De_Morgan's_Law

 

[rosshinkley]

Hi,

I'm currently learning c# and am confused why the first example below
doesn't work but the second one does. I'd appreciate any feeback. Thanks.

First example using !=

                string input = Console.ReadLine();

                if ((input != "0") || (input != "1"))
                {
                    Console.WriteLine("not equal to 0 or 1");
                }
                else
                {
                    Console.WriteLine("is equal to 0 or 1");
                }

                Console.ReadLine();

Second example using a single ! followed by ==

                string input = Console.ReadLine();

                if (!((input == "0") || (input == "1")))
                {
                    Console.WriteLine("not equal to 0 or 1");
                }
                else
                {
                    Console.WriteLine("is equal to 0 or 1");
                }

                Console.ReadLine();

Matty-

In two words: DeMorgan's Law. Wiki it sometime, it might interest
you.

Basically, the second example works because you have this:

!((a=b) | (a=c))

which is not the same as

(a=!b) | (a=!c)

Try the following:

if((input !="0") && (input!="1")) ...

If you'd like to see a mathematical reason, I'd be happy to show you.

Hope this helps,
Ross

 

[Adam Benson]

Instead of
if ((input != "0") || (input != "1"))
try
if ((input != "0") && (input != "1"))

Your test basically says "If input isn't "0" or it isn't "1" ...
Well it can't be both at the same time so your test is always true.

HTH,

AB
=======

 

[Ignacio Machin ( .NET/ C# MVP )]

Hi,

I'm currently learning c# and am confused why the first example below
doesn't work but the second one does. I'd appreciate any feeback. Thanks.

How it does not work?
What were you expecting?

if ((input != "0") || (input != "1"))

basically returns true IF input is not 0 or input is not 1. That means
that IF input is 0 it's true as 0 != 1 , the same thing the otherway
around. so in other words it will always be true !!!
so the only way it can fail is if input is 0 and 1, get your
conclusions from there.

 

[Cor Ligthert[MVP]]

Matty,

Simple because in the second sample you tell exactly what you want while the
first is rubish,

You cannot be fysical at the same time not a woman or not a man.

Cor