why does the value of d do not change?

[Trib]

#include <iostream>

using namespace std;

int main()
{
int a,b, c, p, x, x1, d;
int nr=0;
cin>>a>>b>>c>>x;
x=x1;
while (x1>0)
{
p = x1 / 10;
x1 = p;
nr++;
}
d = 0;
if ((a > 1 && a < 9) && (0 < b && b < 10) && (0 < c && c < 10))
{
if (a==nr)
{
d = d + 1;
}
if (x/10==b)
{
d = d + 1;
}
if (x%10==c)
{
d = d + 1;
} }

if (d==3)
cout<<"DA";
else
cout<<"NU";
cout<<d;
return 0;
}

[Trib]

ok i found out that the mistake was i initialized the x = x1 instead of x1 = x, as x1 had no value what so ever..but it still tells me d == 2, the "if x/10==b" condition doesn t work, any suggestions?

[coder777]

the "if x/10==b" condition doesn t work

What make you think that it doesn't work? What are you trying to do?

I suggest that yu use debug cout to figure ot whether something does what you want or not.

[Repeater]

x1 had no value what so ever

That's not true. In C++, if you create an int, it has a value. You just don't know what that value is.

any suggestions?

Get the value of x. Get the value of b. Think about them.

[Peter87]

x1 had no value what so ever

That's not true. In C++, if you create an int, it has a value. You just don't know what that value is.

Not sure if it should be called a value, but it certainly isn't a normal value. Using an uninitialized variable is UB.

[Repeater]

It is UB, although in practical terms I'm yet to find a compiler that did anything other than simply read whatever happens to be in that memory like any other int. Some of them can be encouraged to warn about it. It would be nice if they did something spectacular instead.

[Peter87]

#include <iostream>

void f(int x)
{
	int a;
	std::cout << "a is "<< a << "\n";
	if (x == 0)
	{
		a = 1;
		std::cout << "a is set to 1\n";
	}
	std::cout << "a is " << a << "\n\n";
}

int main()
{
	for (int i : {0, 1})
	{
		f(i);
	}
}

When I compile and run the above program with GCC 7.3 using the -O2 optimization flag I get the following output.

a is 0
a is set to 1
a is 1

a is 0
a is 1

In the second case (f(1)) it looks like the value of a magically changed without being set.

[Ganado]

Wow, that happens to me, too. Nice example.

[E0p9LyTq]

@Peter87,

Visual Studio 2017 won't compile your code example, errors out line 7. /W3 is default setting.

error C4700: uninitialized local variable 'a' used

TDM-GCC 5.1.0 at default warning level gives a warning about the uninitialized variable, but compiles.

Personally I'd rather have an uninitialized variable stopping compilation instead of allowing it.

[Ganado]

You can make compiler-catchable uninitialized variable warnings be errors in gcc as well, I don't think that was really the point (yes, VS's project defaults are more convenient).
I totally agree it is better to make uninitialized variables be compiler errors when possible, but the point he was showing was that uninitialized variables can cause more than just a variable to be assigned -393745 instead of an actual value (the UB actually changed the apparent runtime logic of the program, halfway through the program).

[E0p9LyTq]

I know one can manually change the warning levels. I just noticed when using the default levels there is a difference in how two different compilers handle uninitialized variables.

An interesting bit of trivia how different implementations vary in small details. Nothing more.

[Peter87]

Yeah, I actually did get a warning. The purpose of the program was just to give a counterexample to what Repeater said, and hopefully convince people to not rely on UB.

[E0p9LyTq]

@Peter87, I guess I am just too a-retentive to ignore/miss a warning and think the code works as I expect.

You convinced me even more to be aware of such pitfalls. :)

[Repeater]

Fair enough. What I was really trying to communicate was that bytes of memory have numbers in them even if you didn't put them there.

[Trib]

after meditating a bit yesterday i found the mistakes by myself, thanks for telling me about debugging, and about the "not known" value of a variable that uninitialized.
Thank you all guys for taking your time to help me out! :)