Why does " ‘x’ and 'y' was not declared in this scope" this error seem?

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Hello.Please,help me.I want to ask that Why does " ‘x’ and 'y' was not declared in this scope" this error seem(on place of show function)? I think, they were declared like parameters of the function.Weren't they?

C++

#include<iostream>
using namespace std;
template<class T>
class myMax{
 	T a,b;
public:
      void set(T x,T y);
	T show()
{
	return(x>y)?x:y;
}

};

template<class T>
void myMax<T>::set(T x,T y)
{
	a=x;
	b=y;
}


//_____________________________________________________

int main()
{
	myMax<int>a;
	myMax<int>a1;
	myMax<double>b;
	a1.set(5,19);
	a.set(0,-78);
	b.set(90,89.9);	
    cout<<a1.show()<<endl
	<<a.show()<<endl
	<<b.show()<<endl;	
	return 0;
}

What I have tried:

I also tried other variable names,but only a and b worked.

Posted 27-Aug-16 8:36am

Member 12702056

Updated 27-Aug-16 10:14am

v2

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Patrice T · Accepted Answer · 2016-08-27T08:54:00

Solution 1

Quote:
I think, they were declared like parameters of the function.

Where do you see them declared in this function ?

C++

	T show()
{
	return(x>y)?x:y;
}

May be you should use a and b instead ?
[Update]

Quote:
they are declared for set function.
and why can't i use other variables?

They declared in set function for set function only.
[Update]
In show, you can use a and b because you declared them in class definition.

C++

class myMax{
 	T a,b;

Posted 27-Aug-16 8:54am

Patrice T

Updated 27-Aug-16 9:56am

v3

Comments

Dave Kreskowiak 27-Aug-16 15:27pm

The concept is called "scope". Maybe you should read up on it.

Variables are only usable inside the scope they are defined. Since you defined them in the scope of the set method they are not visible anywhere outside of that method (scope).

My C++ is quite rusty but it would appear that you need to explicitly set the values of the fields a and b using the values passed in the set method as x and y. You can then return the maximum value of a and b from your show method instead of using the x and y values.

Member 12702056 27-Aug-16 15:38pm

but why i can't use (not x,y)/for example k,f instead of a and b?i understood that x,y are parametres of another function,so they are used only in their function scope.
but i mean, why i can't use other variable names instead of a and b?
i think,there must be reason for this,but i can't find that reason.
do you know it?If you know,please let me know

Member 12702056 27-Aug-16 15:59pm

Thanks much :)

Member 12702056 27-Aug-16 15:38pm

ppolymorphe ,they are declared for set function.
and why can't i use other variables?
is this related with that we set x to a and y to b,and then must we use for showing only those variables?

Patrice T 27-Aug-16 15:41pm

I just updated my solution.

Member 12702056 27-Aug-16 15:59pm

Thanks a lot :)

Afzaal Ahmad Zeeshan · Accepted Answer · 2016-08-27T09:44:00

The code that you are showing,

C++

T show()
{
	return(x>y)?x:y;
}

Is invalid, by looking at the class definition (have a look at the members!). You have defined only a and b to be properties in the class, x and y are not given to be of any type, and thus no defined in the given scope.

Quote:
only a and b worked.

That is because a and b are defined as properties (members) in your class of type T. That executes perfectly. Because compiler knows what type to use and where they are defined in the scope (as the members).

Solution to this problem is to just use the a and b names. Don't use the x and y; because that would require to change the class definition too!

Patrice T · Accepted Answer · 2016-08-27T10:14:00

Solution 3

Not a solution, but links to good lectures.

Start with the C book, C language is the ancestor of C++. Most of C apply to C++.
The C Programming Language - Wikipedia, the free encyclopedia[^]
https://hassanolity.files.wordpress.com/2013/11/the_c_programming_language_2.pdf[^]
http://www.ime.usp.br/~pf/Kernighan-Ritchie/C-Programming-Ebook.pdf[^]

C++ Programing Language[^]

[Update]
Learn some analyze method, Dijkstra Top-Down method is a good start.
It will help you to device structured algorithms.
https://en.wikipedia.org/wiki/Top-down_and_bottom-up_design[^]
https://en.wikipedia.org/wiki/Structured_programming[^]
https://en.wikipedia.org/wiki/Edsger_W._Dijkstra[^]
https://www.cs.utexas.edu/users/EWD/ewd03xx/EWD316.PDF[^]

Posted 27-Aug-16 10:14am

Patrice T

Updated 27-Aug-16 10:56am

v2

Comments

Member 12702056 27-Aug-16 16:21pm

ppolymorphe,ok,thanks :)
I'll try .

Patrice T 27-Aug-16 16:24pm

Save the books on your PC, and have a good lecture :)

Member 12702056 27-Aug-16 16:40pm

ok :)
but i am beginner in c++ ,i began this year,so beacuse of this i learn from videos about c++ more than books.but i will try books also,thanks :)

Patrice T 27-Aug-16 16:42pm

The books are references
C book is by the authors of C language.
C++ book is by the author of C++ language.

Member 12702056 27-Aug-16 16:45pm

I got what you mean :) that it should be better than videos :)
I saw that you are professional programmer from your profile . Can i ask you that i want to be c++ debugger,what must i do for it? Also for knowing this language better ,what must i do? :) can you give me advice,please?

Patrice T 27-Aug-16 16:52pm

The best is to ask more questions here.
I am not expert with C++, and other experts are answering here

Member 12702056 27-Aug-16 16:56pm

thanks :)
i am new in this website,i don't know how can i ask expert?who are experts?i don't mean meaning,i know meaning of word expert

Patrice T 27-Aug-16 17:01pm

Just post your other questions the same way as this one. Experts are here.

Maciej Los 28-Aug-16 8:04am

You should upgrage your first answer instead posting another one!

Patrice T 28-Aug-16 8:15am

I did it exceptionally because the links are unrelated to first solution and would have possibly made it confused with too many things.
I have this second solution because after talking to OP, I saw some bug lack of background knowledges.

Maciej Los 28-Aug-16 8:18am

I understand that, but in principle, you have to post only one answer.
Cheers,
Maciej

Patrice T 28-Aug-16 8:20am

That is what I usually do.
Cheers