Rank: ? (39)
Member #: 14820

ok i m revising for exams and was wondering how to use struct! i have an example in front of me but can anyone explain in english what each line does, (not EACH Line but like important lines marked // i have also included what I THINK they do so please correct me too! )
thanks
also the confusion is how does it know the address of each chunk, etc
Code:

Rank: ? (37)
Member #: 15158

Okay, I'm not sure that you even know what structures are used for. I'll explain to you, since you seem rather confused in this area. Structures are a way of grouping and organizing code to make it more organized, not to mention other reasons, like code reuse. Anyway, when you create a structure, like:
struct AccNode {
//...
};
This is telling the compiler that everything inside the { }; is part of the structure. So, now you must instantiate (create an instance of) AccNode, like in this line:
AccNode *p_head = 0, *p_temp;
This creates two instances of AccNode, both pointers. p_head and p_temp. So, now you can use anything inside AccNode's { }; using either instance. For example,
p_head->p_next != p_temp->p_next

But, I think what you were trying to do with structures was wrong. I think that you were trying to do stuff like AccNode = 2; That is not a proper way to use structures. You must first instantiate a structure, then you can use and modify it's contents on an instance - by - instance basis. I'm sorry that this post was short and choppy and probably still a bit confusing, I didn't have much time to write it. So, if you're still confused, I can clarify for you.

Rank: ? (614)
Member #: 9832

To me this appears to be simple linked list (there are tutorials&thread about them). This means that every AccountNode contains the actual account data (the account struct) and the address of the next account in the list.

AccNode *p_head = 0, *p_temp;

This line creates two pointers of type AccNode, but they point to nowhere right now. No instances of any struct yet.

case 'A'_temp = p_head; //makes p_temp same as p_head
p_head = new AccNode; //p head points to a new location
p_head ->p_next = p_temp;

First line makes p_head to point to same address (same struct) as p_temp. At the first time this appears to make no sense as both pointers point to nothing...

second line finally creates something, a struct of AccNode type that actually contains one account struct. p_head now points to this struct. And next, the p_next member of the struct pointed by p_head is set to point p_temp (still zero)

Now lets say the user creates a second node: p_temp is made to point to first node (not zero anymore), 2nd node is created, and the p_next of the 2nd node is made to point to the first node. This happens every time a node is created so 3rd points to 2nd and so on. In other words, linked list.

cout<<"enter pin ";
cin>>p_head->acc.pin;
cout<<"enter balance ";
cin>>p_head->acc.balance;
cout<<"enter rate ";
cin>>p_head->acc.rate;
break;



Remember that struct AccNode contains Account struct inside it? These lines of code insert some data into member variables of that Account struct. for example:
cin>>p_head->acc.pin means in English:

put user input into pin-variable of acc-struct inside AccNode struct pointed by p_head. "." is used to access members of a class/struct and "->" is used to access members of class/struct through a pointer.

[(*p_head).acc.balance] would do the same thing I think.


case 'P'_temp = p_head; // ptemp is p head
while (p_temp != 0){
cout<<"PIN "<<p_temp->acc.pin<<'\n';
cout<<"Balance "<<p_temp->acc.balance<<'\n';
cout<<"Rate "<<p_temp->acc.rate<<'\n';
p_temp = p_temp->p_next; //ptemp is pnext

this prints everything in the whole list, in other words every account. p_temp=p_head makes p_temp to point to the last created node, next lines print all data and p_temp = p_temp->p_next makes p_temp to point to next node (it kind of goes backwards from last to first...).

while (p_temp != 0){ this checks if p_temp is zero, which would mean p_next of the node just printed was zero. In other words, when the last (well first actually) node is printed and there is nothing more, the loop stops.
hope I didn't confuse you more...

Rank: ? (39)
Member #: 14820

thanks ever so much, but you mean tutorials on HERE or other sites?
why didnt I THINK OF THAT!! sorry! would have saved you a LOT of writing.

Rank: ? (39)
Member #: 14820

case 'A'_temp = p_head; //makes p_temp same as p_head
p_head = new AccNode; //p head points to a new location
p_head ->p_next = p_temp;

the key is these lines then? and p_next is actually linked to each new struct instance?? like in my example i will have a pin, balance, rate AND a p_next which is pointing to the previous instance??

but what i m confused about is that where is it saved?? i mean for the first instance of structure will have a pointer to the next instance, but where will that instruction be saved?

Also can you tell me what this means:

struct AccNode{
Account acc;
AccNode *p_next;

Rank: ? (614)
Member #: 9832

Tutorials here is what I meant but there are tuts on other sites as well. And lots of stuff about linked lists in the c++ forum too. There's couple examples about how to do it with classes too. But you were interested in structs, not linked lists right?

Rank: ? (39)
Member #: 14820

 Crypdoctor


yes i was interested in structs.

Rank: ? (39)
Member #: 14820

case 'A'_temp = p_head; //makes p_temp same as p_head
p_head = new AccNode; //p head points to a new location
p_head ->p_next = p_temp;

the key is these lines then? and p_next is actually linked to each new struct instance?? like in my example i will have a pin, balance, rate AND a p_next which is pointing to the previous instance??

but what i m confused about is that where is it saved?? i mean for the first instance of structure will have a pointer to the next instance, but where will that instruction be saved?

Also can you tell me what this means:

struct AccNode{
Account acc;
AccNode *p_next;

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Member #: 14393

 loverboy260

The key would be these lines, they create what is called a linked list. p_head = new AccNode creates a new object of AccNode type in memory.


No, p_next points to the instance created the last time around the loop.


Yes, unless this is the first time around which then p_next is 0.


In heap memory.


It is not saved in any variable it is just in memory, that you can access through the pointers (p_next) of the AccNodes before it.


This defines a user defined object, called a struct. This struct has two pieces of information a variable that stores an Account object, and a pointer that contains the location in memory where the next AccNode is located.

Hope that helps!