free or delete array of strcut c++ - c++

I made a simple array of struct I made a function to implement the array from users input
but I am struggling to find the right way to free or delete elements in the array ;
here is my code for a better understanding
#include <iostream>
using namespace std;
typedef struct InfStudent
{
int id;
int age;
int lvel;
}studentInfo;
void addElments(studentInfo *s)
{
int i=0;
for(i=0; i<2; i++) {
s[i].id = i;
s[i].age = i * i + 1;
s[i].lvel = i + i + 2;
}
}
int studCounter = 0 ;
void deltetElement(void *studentInfo1 ) {
for (int i = 0; i < 6; i++) {
cout << "empty " << endl;
free(studentInfo1);
}
}
int main()
{
int n;int i;
studentInfo st[2];
addElments(st);
for(i=0; i<2; i++) {
cout<<"enter the Id number of the student "<< endl;
cin >> st[i].id;
cout<<"enter the age of the student "<< endl;
cin >> st[i].age;
cout<<"enter the level of the student "<< endl;
cin >> st[i].lvel;
}
deltetElement(st);
for(i=0; i<2; i++) {
cout << "Id of the student " << i << "\t=" << st[i].id;
cout << "\t Age of the student " << i << "\t=" << st[i].age;
cout << "\tLevel of the student " << i << "\t=" << st[i].lvel;
cout<< endl;
}
return 0;
}
the output
enter the Id number of the student
1234
enter the age of the student
32
enter the level of the student
2
enter the Id number of the student
321
enter the age of the student
2
enter the level of the student
32
empty
it is printing empty, but the code still working like 3 second and then printing empty massage, but I did not understand it does delete or not, or there is a better way to do that;

Since you are using studentInfo st[2]; it's an array of VALUE types, meaning that studentInfo objects are not allocated in heap, and should not be deleted by free() or delete.

free or delete array of strcut c++
You create an array with automatic storage. Automatic objects are destroyed and their storage is released automatically when the variable goes out of scope. You cannot and you must not "free" them in any way other than by letting the execution proceed to the outside of the scope where the automatic object is defined.
Only thing that may be passed to free is a pointer that was returned by malloc (or certain other related C allocation functions) and hasn't previously been freed. Since that doesn't apply to what you pass to free, the behaviour of your program is undefined. That's bad. Don't do that.
P.S. Don't use malloc nor free in C++ if you can avoid it (and it can usually be avoided).
I am struggling to find the right way to free or delete elements in the array ;
The elements of an array are destroyed and their storage released when the array itself is destroyed and its memory is released. There is no way to separate those two.

Related

C++ Pointer and Structure

I have to do the following tasks:
Read information about individual persons in file person.txt (see below) and store to array p. Set the spouse pointer for each person to NULL value first.
Perform marry operation for Mary and Tom. You can marry two people by setting their spouse pointer to point to each other (store address of one another).
Print out the content in array p where you need to print every person variable pointed by array p. If a person spouse pointer is a NULL value, then print Not Married, else print the spouse name. The output of the program is shown below. Make sure your output is the same.
I can do (1), read the text file person.txt, which has the following content:
Mary 012-35678905 20000
John 010-87630221 16000
Alice 012-90028765 9000
Tom 019-76239028 30000
Pam 017-32237609 32000
But I don't know how to do (2) and (3).
This is what I have done so far, based on the template provided with the question and that I'm not supposed to change:
#include <iostream> //>>>>>>> This part is the template given >>>>>>>
#include <cstdlib> //
#include <fstream> //
//
using namespace std; //
//
struct person //
{ //
char name[30]; //
char phone[15]; //
double money; //
person *spouse; //
}; //
//
int main() //
{ //
person *p[10]; //<<<<<<<< This is the end of the template part <<<
ifstream inFile;
inFile.open("person.txt");
if (inFile.fail())
{
cout << "Error in opening the file!" << endl;
exit(1);
}
char name[30], phone[15];
int money;
int number = 5;
for (int i = 0; i < number; i++)
{
inFile >> name >> phone >> money;
cout << "Name:" << name << endl;
cout << "Phone:" << phone << endl;
cout << "Money:" << money << endl;
cout << "Spouse Name:" << endl;
cout << endl;
}
cin.get();
system("pause");
return 0;
}
The expected output should be like this:
Name: Mary
Phone Number:012-35678905
Money: 20000
Spouse Name:Tom
Name: John
Phone Number:010-87630221
Money: 16000
Spouse Name: Not Married
...
Be aware that this exercise shows outdated use of C++
First to your array p that you somewhat forgot to use. p[10] is an array of 10. But of 10 what ? of person*, so of pointers to persons.
This is very old-fashioned C++. If you follow a course on internet, change immediately, because nowadays, we'd use vector<person>, string and nullptr. If it's a class course, you have no choice, so let's go on...
Some hints, based on what you have already done
First simplify the reading loop and don't forget to set the pointer to NULL as requested in the question:
for (int i = 0; i < number; i++)
{
person *r = new person; // allocate a new person
inFile >> r->name >> r->phone >> r->money; // read data into the new person
r->spouse = NULL; // initialize the poitner
p[i] = r; // store the pointer in the array
}
You already almost have the printing part(3). You just have to move it from your reading loop to a new loop, print from the array, and tackle the special case of married people:
for (int i = 0; i < number; i++)
{
cout << "Name:" << p[i]->name << endl;
cout << "Phone:" << p[i]->phone << endl;
cout << "Money:" << p[i]->money << endl;
cout << "Spouse:" ;
if (p[i]->spouse==NULL) {
cout << "Not married" <<endl;
}
else {
cout << p[i]->spouse->name <<endl;
}
cout << endl;
}
Now something to do on your own
Now about marrying Marry and Tom in (2). This is more delicate. I will not do it for you, because now you have all you need to finish homework. But the general principle is:
Create two pointers spouse1 and spouse2 and initialize them to NULL.
Loop through the array to find which person is Tom and which one is Marry, and update the relevant pointer (e.g. spouse1 = p[i]; )
At the end of the loop, check that we have found both spouses (both pointers are not NULL anymore, and both pointers are different, because you cannot marry someone with hi/her-self)
If it's ok, then just marry them: spouse1->spouse=spouse2; spouse2->spouse=spouse1;
Finally, before you end the programme, you need to deallocate all the pointers in the array (with vectors, you wouldn't have to care about this).
Further improvements needed
You still need to improve your reading loop, to make it more dynamic. Because in reality, you don't know how many lines are in the text file. So start with number=0 and read data as long as possible, incrementing number each time, but stoping if not possible to read anymore, or if the maximum size of the array is reached.

How to print all values in a dynamically resizing struct? C++

I have a dynamically resizing struct[]. I ask the user for how many records he wants then make that many struct.Then I store the name and age info in the struct. The problem is when printing out that data i am only printing the last name and age the user input. I would like to print all the values.
struct Records
{
char name [32] ;
int age;
};
void program2()
{
Records rec;
int size;
cout << "How many record would you like?";
cin >> size;
cout << "\n" << size;
Records* a = new Records[size];
for (int i = 0; i < size; i++)
{
cout << "Whats the name for the Record: ";
cin >> rec.name;
cout << rec.name;
cout << "What is the age for this record: ";
cin >> rec.age;
}
for (int i = 0; i < size; i++)
{
cout << "\n" << rec.name << rec.age;
}
}
In your code, the variable rec has nothing to do with the array. To access the element at position i of your array, you'll need to use a[i].
For example:
cin >> a[i].name;
or
cout << "\n" << a[i].name << " " << a[i].age;
No surprise - you are actually printing repeatedly the last record that you created (rec). Your last loop is not actually traversing the array.
Also, the first loop only creates a struct, but does not actually add it to the array.
What you are trying to do is actually easier and cleaner if you use vectors instead of arrays, adding each record to the vector using push_back(). This is the closest thing to the "dynamically resized array" that you are referring to (such a thing does not actually exist in C++, that's what vectors are for, amongst other things).
Have a look into doing it this way, and if you get stuck feel free to ask again, happy to help.

Handle a dynamic table of structures with pointers

I have an exercise to write down data to a dynamic table of structures using a function. Here's my code:
#include <iostream>
#include <cstdlib>
using namespace std;
struct student{ char name[15], surname[20]; int age; };
student * createTab(int tsize)
{
student *t = new student[tsize];
return t;
}
void fill(student *t, int tsize)
{
for (int i = 0; i<2; i++)
{
cout << "Enter a name: "; cin >> t[i].name;
cout << "Enter a surname: "; cin >> t[i].surname;
cout << "Enter age: "; cin >> t[i].age;
}
}
int main()
{
student *t = createTab(10);
fill(t, 20);
cout << t[0].surname << endl;
cout << t[1].name << endl;
system("pause");
delete[]t;
return 0;
}
It works, okay. But here, in fill() function I use the index syntax with student[].name. I always worked on tables with pointers like that: *(table+i) in a for loop. *(t+i).name doesn't work. Can I iterate on structure fields using pointers?
P.S - Am I freeing the memory correctly?
And I guess P.S 2 - How is it possible, that when I insert a pointer to a first element of my table to a function, and then I can operate on whole table with indexes?
The standard defines the subscripting as follows:
5.2.1/1 (...) The expression E1[E2] is identical (by definition) to *((E1)+(E2))
This is why, using a pointer t and an index i, *(t+i) and t[i] is the same. The problem with your code in the context of struct fields, is a question of priority: you may write (*(t+i)).name or better (t+i)->name, or much clearer, as you did: t[i].name.
P.S.: If you allocate a table with new[...] you have to free it with delete[]. So yes: it's ok !

struct + functions code breaks

There is a problem with the code and i could not find it.
i was asked to write a money struct and use functions to manipulate it.
but the code did not work for any function. i tried couting
the array of structers and it came out nicely, for any missing info
please leave a comment and i'll reply shortly.
Money.txt
2
12 20
13 40
#include <iostream>
#include <fstream>
using namespace std;
struct Money { //declaring structure
int dollars;
int cents;
};
Money addMoney(Money *p[], int n) { //adds money data
Money cash{ 0,0 };
int i;
for (int j = 0; j < n; j++) {
cash.dollars = cash.dollars + p[j]->dollars;
cash.cents = cash.cents + p[j]->cents;
}
if (cash.cents >= 100) //100cents = 1 dollar
{
i = (cash.cents) / 100;
cash.dollars = cash.dollars + i;
i = (cash.cents) % 100;
cash.cents = i;
}
return cash;
}
void printMoney(Money *p[], int n) { //printing money data
for (int i = 0; i < n; i++) {
cout << "Dollars: " << p[i]->dollars << endl;
cout << "Cents: " << p[i]->cents << endl;
}
}
Money maxMoney(Money *p[], int n) {
Money cash;
cash.dollars = p[0]->dollars;
cash.cents = p[0]->cents;
for (int i = 0; i < n; i++)
{
if ((p[i]->dollars)>=(cash.dollars))
if ((p[i]->cents)>(cash.cents))
{
cash.dollars = p[i]->dollars;
cash.cents = p[i]->cents;
}
}
return cash;
}
void main() {
Money cash;
ifstream mycin("money.txt");
if (mycin.fail())
cout << "Enable to open file";
int x;
mycin >> x;
Money *arr = new Money[x];
for (int i = 0; i < x; i++)
{
mycin >> arr[i].dollars;
mycin >> arr[i].cents;
}
cout << "The values in money.txt are: ";
printMoney(&arr, x);
cash = addMoney(&arr, x);
cout << "These values added are :";
cout << cash.dollars << " Dollars and " << cash.cents << " cents" << endl;
cash = maxMoney(&arr, x);
cout << "Maximum value is :";
cout << cash.dollars << " Dollars and " << cash.cents << " cents" << endl;
}
These functions appear to accept an array of pointers to Money, but you're trying to use them with an array of Money.
I suggest you play with arrays of pointers to simpler types (like int) until you're comfortable with the concept, before you attempt it with Money.
This sounds a lot like homework so I'm not posting a full solution, but I will explain what appears to be the misunderstanding and give you some pointers.
First you declare your data structure as an array of Money structures, e.g. a continuous series of blocks of memory containing the Money struct, the first of which is pointed to by "arr" in your main program.
But then, in the rest of the program (functions) you seem to expect the data structure being used to be an array of Money pointers. See the difference? They're not the same and this will not work as is. You have to be consistent.
Either you're dealing with an array of structs, in which case you pass effectively a single, simple Money* to your functions everywhere (and you dereference with . not ->)
Or you're dealing with an array of pointers, in which case you pass effectively a pointer to a (Money pointer) and you dereference with -> as you've done. But then you also have to allocate each Money struct individually when you're reading them in in the main program. That is to say, allocating memory for the array of pointers does not automatically allocate memory for each Money pointer reference in the array of pointers and so you need to do this for each entry you're reading in.
So, as you should hopefully now realise, there's multiple ways to fix your program.
As per your later comment, given that the function signatures need to stay as-is, I would suggest you work with an array of Money pointers.
Money** arr = new Money*[x]
Then you need to add a line to your loop during reading, to actually make each Money * point to a Money struct:
for (int i = 0; i < x; i++)
{
arr[i] = new Money
...
Finally then, because "arr" is now a pointer to a pointer to Money, you can directly pass it to your functions, so calling them are just for example:
printMoney(arr, x);

segmentation fault with dynamic allocation and arrays/pointers

I'm working on an assignment that is introducing the principals of dynamic allocation of memory and pointers. I had made a simple program in the past that accepted 5 names and 5 scores and then used a selection sort to put them in descending order. My assignment now is to come back to that same program and ask the user how many scores they would like to input, then use pointers to dynamically allocate the necessary amount of memory. This is my first time working with pointers and these concepts so im still trying to figure it all out.
I got the code to compile but I get a segmentation fault error as soon as i enter any integer number for how many scores i would like to input (which is the first thing the program asks)
Im sure there are a few errors along the way with how i called and declared functions so if theres anything i just desperately change please let me know, but for now I dont understand why my program is crashing where it is crashing.
Here is my code
#include <iostream>
using namespace std;
void initializeData(string *names[], int *scores[], int num);
void displayData(string *names[], int *scores[], int num);
void sortData(string *names[], int *scores[], int num);
int main()
{
int num;
int **intPoint;
string **strPoint;
cout << "How many scores would you like to enter?: ";
cin >> num;
cout << " core dumped? ";
*intPoint = new int[num];
*strPoint = new string[num];
initializeData(strPoint,intPoint,num);
sortData(strPoint,intPoint,num);
displayData(strPoint,intPoint,num);
return 0;
}
void initializeData(string *names[], int *scores[], int num)
{
for(int i=0;i<num;i++)
{
cout << "Please input the name for score: " << i+1 << ": " << endl;
cin >> *(names[i]);
cout << "Please input the score for player: " << i+1 << ": " << endl;
cin >> *(scores[i]);
}
}
void sortData(string *names[], int *scores[], int num)
{
int minIndex,minValue,x;
string stringTemp;
for(int i = 0;i<(num-1);i++)
{
minIndex = i;
minValue = *(scores[i]);
for(x= i+1;x<num;x++)
{
if(*(scores[x]) > minValue)
{
minValue = *(scores[x]);
minIndex = x;
}
}
*(scores[minIndex])=*(scores[i]);
*(scores[i]) = minValue;
stringTemp = *(names[minIndex]);
*(names[minIndex]) = *(names[i]);
*(names[i]) = stringTemp;
}
}
void displayData(string *names[], int *scores[], int num)
{
cout << "Top scorers: " << endl;
for(int i=0;i<num;i++)
{
cout << names[i] <<": ";
cout << scores[i] << endl;
}
}
and my current output:
How many scores would you like to enter?: 10
Segmentation fault (core dumped)
which happens regardless of what int i put there. I put a cout statement after the
cin << num; to see if the program got that far but it never does.
Any help is greatly appreciated. Sorry if this is the most basic error ever.
int **intPoint;
At this point in your code, intPoint doesn't point to anything since you haven't assigned it a value.
*intPoint = new int[num];
Then you dereference it, but it doesn't point to anything.
Try:
int *intPoint;
intPoint = new int[num];
Now you are setting intPoint's value so that it points to the integers you allocated.
The reason you get a segmentation fault is because you dereference an uninitialized pointer.
int **intPoint; // intPoint is declared a pointer to a 'pointer to an int';
// but currently it points to nothing
*intPoint = new int[num]; // *intPoint "dereferences" intPoint, i.e., assigns w/e it
// pointed to (which is nothing) to a pointer.
Like the others have suggested, you didn't need a double pointer here.
int *intPoint; // intPoint is a pointer to an int
intPoint = new int[num]; // notice how we didn't dereference intPoint.
// all we did was assign to our newly minted memory.
Use std::vector in the place of array of int or string.
say,
std::vector<int> scores;
std::vector<string> names;
This way you can avoid all the hassles. This is simple and elegant.