why I can't change the gloable variable

[Sakula]

#define MAX 20

/* includes */


#include<iostream.h>
#include<stdio.h>
#include<malloc.h>
#include<string.h>


struct Student
{
int number;
int age;
int male;
char name[20];


Student * next;



};


Student * Create();
void ShowList(Student * g_head);
Student * Delete(Student * g_head, int age);

Student * g_head;


Student * Create()
{


Student * pS;
Student * pEnd;


pS=new Student;
cout<<"please input number,age ,male and name"<<endl;
cin>>pS->number>>pS->age>>pS->male;
gets(pS->name);


g_head=NULL;
pEnd=pS;


while(pS->number!=0)
{
if(g_head==NULL)
g_head = pS;
else
pEnd->next=pS;


pEnd=pS;
pS=new Student;
cout<<"please input number,age , male and name"<<endl;

cin>>pS->number>>pS->age>>pS->male;
gets(pS->name);
}
pEnd->next=NULL;
delete pS;
return (g_head);



}


void ShowList(Student * g_head)
{
cout<<"now the items of list are \n";

while(g_head)
{
cout<<g_head->number<<" "<<g_head->age<<"
"<<g_head->male<<"
"<<g_head->name<<endl;
g_head=g_head->next;
}



}


Student * Delete(Student * g_head, int age)
{
Student *p=NULL;
p=new Student;
if(!g_head)
{
cout<<"\nList null!\n";
return NULL;
}
if(g_head->age==age)
{
p=g_head;
g_head=g_head->next;
delete p;
cout<<age<<"the g_head of list has been deleted\n";
return g_head;
}
for(Student * pGuard=g_head; pGuard->next;pGuard=pGuard->next)
{
if(pGuard->next->age==age)
{
p=pGuard->next;
pGuard->next=p->next;
delete p;
cout<<age<<"has been deleted\n";
return g_head;
}
}

cout<<age<<"is not found! \n";



}


void main()
{
int age=0;

g_head=Create();
cout<<"now the list are \n";

ShowList(g_head);
cout<<"please input the age you want to delete : \n";
cin>>age;

Delete(g_head,age);

ShowList(g_head);


}


when i delete the head of the linked list,it will be crupt!
I debug this program and find the gloable variable g_head doesn't be
changed by Delete( ). I don't know why this happen.

 

[Walter Roberson]

/* includes */

#include<iostream.h>

That's a C++ program. Please try a C++ newsgroup. C++ is a different
language than C and has some important differences from C.

 

[Martin Ambuhl]

Sakula wrote:
[...]

#include<iostream.h>

This header does not exist in either C or C++. The (offtopic) C++
language has a simliarly named <iostream> header. Similar functionality

#include<malloc.h>

This header does not exist in either C or C++. In the C language, the

struct Student
{
int number;
int age;
int male;
char name[20];
Student * next;

^^^^^^^
This is not legal in C without a prior typedef.

};


Student * Create();

^^^^^^^
This is not legal in C without a prior typedef.
[...]

void main()

^^^^
This is not legal in C or C++.

etc.

You are clearly lost. Not only is your code not C, which would be
topical in comp.lang.c, you have done the nearly impossible: written
gobbledygook that not even a (non-topical) C++ compiler would accept as
standard C++.

If you want to post C++ questions, do so in the appropriate newsgroup.
But learn something about C++ first: if you can't even write a legal
definition for main, it's time for you to start over at page 1.

 

[Richard Heathfield]

Walter Roberson said:

That's a C++ program.

You may be right (although in fact you probably are not), but the evidence
cited above is insufficient to demonstrate this. C implementations are free
to ship headers by that name.

 

[Walter Roberson]

Walter Roberson said:

You may be right (although in fact you probably are not), but the evidence
cited above is insufficient to demonstrate this. C implementations are free
to ship headers by that name.

It was a C++ program. It had numerous cout uses, including at least
one cout <<" " which is not valid in C (wrong type of argument for <<)

 

[CBFalconer]

Walter Roberson said:

You may be right (although in fact you probably are not), but the
evidence cited above is insufficient to demonstrate this. C
implementations are free to ship headers by that name.

Well, later on he is doing such things as calling an undeclared
function 'new', and doing shifts into the undeclared variable
cout. You may want to reconsider your probabilities.

 

[Richard Heathfield]

Walter Roberson said:

Walter Roberson said:

It was a C++ program.

That's debatable, given the inclusion of <iostream.h>, which is not a
standard C++ header.

 

[Richard Heathfield]

CBFalconer said:

Well, later on he is doing such things as calling an undeclared
function 'new', and doing shifts into the undeclared variable
cout. You may want to reconsider your probabilities.

You may want to re-read my reply, and especially the first sentence thereof.

 

[Michal Nazarewicz]

[cut]

Student * g_head; [cut]
Student * Delete(Student *g_head, int age) { [cut]
}

void main() { [cut]
Delete(g_head, age); [cut]

}

when i delete the head of the linked list,it will be crupt!
I debug this program and find the gloable variable g_head doesn't be
changed by Delete( ). I don't know why this happen.

That's because Delete() has it's own g_head variable passed as an
argument which /shadows/ the global g_head variable. If you want to
be able to modify passed argument you'll have to change definition of
Delete to Student *Delete(Student **g_head, int age) and change the
program approprietly.

BTW. your post doesn't belong here as you are writting in C++ not
C (the "new" operator revelas that).

 

[CBFalconer]

CBFalconer said:

You may want to re-read my reply, and especially the first
sentence thereof.

Which is quoted in its entirety above. I see nothing new.

--
Some informative links:
http://www.geocities.com/nnqweb/
http://www.catb.org/~esr/faqs/smart-questions.html
http://www.caliburn.nl/topposting.html
http://www.netmeister.org/news/learn2quote.html

 

[Richard Heathfield]

CBFalconer said:

Which is quoted in its entirety above. I see nothing new.

Your argument rests on "later on he is doing such things as...", whereas my
statement only referred to "the evidence cited above".

 

[Mark McIntyre]

Your argument rests on "later on he is doing such things as...", whereas my
statement only referred to "the evidence cited above".

Then its a nit you should not have picked, it just makes you look anal
about the wrong things.
--
Mark McIntyre

"Debugging is twice as hard as writing the code in the first place.
Therefore, if you write the code as cleverly as possible, you are,
by definition, not smart enough to debug it."
--Brian Kernighan

 

[Richard Heathfield]

Mark McIntyre said:

Then its a nit you should not have picked

No such thing.

 

[Mark McIntyre]

Mark McIntyre said:


No such thing.

I'll just cry "emerson" and pass on.
--
Mark McIntyre

"Debugging is twice as hard as writing the code in the first place.
Therefore, if you write the code as cleverly as possible, you are,
by definition, not smart enough to debug it."
--Brian Kernighan