Im writing a project and its for a car lot and im creating classes. I need to fullfill certain requirements. For accessory descriptions I need to use a pointer to a string that can be used to dynamically allocate an array of strings with the exact number of accessories. Each element will hold the name of the accessory.
If the number of accessories is 0, there is no need to allocate space, set the pointer to null.
And also pointer to a double that can eb used to dynamically allocate an array of doubles with the same number of elements as accessories. Each element will hold the cost of the associated accessory, that is, the cost in element 0 is the cost of the accessory in element 0.
If the number of accessories is zero, set the pointer to null since there is no need to allocate space.
Heres what my class is so far without those last two requirements. Im stumped.
#ifndef VEHICLE_H
#define VEHICLE_H
class Vehicle
{
public:
Vehicle();
protected:
int vin_number;
string manufacturer;
string model;
string color;
double basecost;
int accessories;
string accessory_list;
private:
};
#endif // VEHICLE_H
Please help it's an online course and ive been googling and reading for hours.
You should not dynamically allocate an array of string.
If you decide to use C++, you should be using STL and collections. Like this:
std::list<std::string> accessory_list;
If you decide to use C, a dynamically allocated string list could look like this:
//init
int accessory_count = 0;
int accessory_cap = 20;
char** accessory_list = calloc (sizeof(char*), accessorry_cap);
//add:
if (accessory_count==accessory_cap) {
accessory_cap += 20;
accessory_list = realloc (accessory_list, sizeof(char*)* accessorry_cap);
}
accessory_list[accessory_count++] = new_accessory.
If you really need a dynamic array of strings, you can do:
int accessory_arr_cap = 20;
string* accessory_arr = new string[accessory_arr_cap];
But since there is no realloc possible in this case, you will have to copy the entire array into new one if you need to enlarge it.
If the cost and name of the option are related, put them in a struct:
struct Option
{
char* Name;
double price;
}
Than what you are looking for is a collection, perhaps a std::vector<Option>. I'll leave it up to you to google the std::vector, it's a good learning exercise.
On a side note, do you have to use C++? You might find another language like C#, or Java a little easier to learn to program with.
If you can't use vectors, make your own collection. I won't post the code because I sense this is an assignment but here's how they normally work:
instantiate with a default size array of say 10.
keep a variable with this max in it
keep a variable with the current number of items in it (starts at 0, maybe call it count or something)
when you add a pointer, put it in element 'count' and increment the counter
if count = capacity then allocate a new array 2*capacity, copy all elements into it, delete the old one and assign the new array to the variable that was the old array.
Related
I have class A, in it I have a dynamic array arr and a dynamic variable s which determines the size of the array. At the start of my main I use the constructor for class A which sets s to 2 and fills the arr with dummy objects. Now when I add objects to the array and sort it, objects seem to disappear.
The size of my array (so variable s) gets doubled when there is not enough space in the array, so no free slot available.
The way I display the array is by using a for loop with the limit i < s. So it should display all objects in the array.
Now when I add more objects and the array gets bigger, the previously "lost" objects start to appear again. So it seems like my array is bigger than the variable s, which dynamically determines the size.
How is that possible?
class A
private:
int *s = new int;
obj *arr = new obj[*s];
//triggers when trying to add an object to the array, but every place from
//arr[0] to arr[s-1] is taken by non dummy object
void A::resize {
*s *= 2;
//for loop that fills code with dummy objects
}
void A::display {
for (int i = 0; i < *s; i++) {
displayobject(i);
}
}
I had a very similar code last year in university and everything worked fine, can't find my mistake.
I would just use a vector, where i can easily use a for loop to go through the real size, but our prof wants us to use a dynamically alocated array.
For a project I am working on I need to have a global array of entry structs. I am having trouble though because I can't allocate memory until while running my program I determine the size of a file. The overall goal of this project is to create a word reference. So far how I am doing it is:
struct info{
//stores the specific character
std:: string c;
//stores the amount of times a word has come up in the file
float num;
}
info info_store[];
This project is to learn about arrays so I need to use an array
You can:
- use new/delete[]
info* p_array=new info[100]; // create an array of size 100
p_array[10].num; // member access example
delete[] p_array; // release memory
- use std::unique_ptr
std::unique_ptr<info[]> array(new info[size]);
-> The advantage is that your memory is automatically released when array is destroyed (no more delete[])
First of all, use std::vector or any other STL container.
Second, you can use dynamic arrays.
auto length = count_structs(file);
auto data = new info[length];
Something like this. Then just fill this array.
Ohh, and make sure you have delete [] data to prevent memory leaks.
I have an array of a structure (with the parameters of name and number), and the initial array takes in elements from a document that I've made. The initial list size starts at 1000. When the list fills up, I call another method that I'm struggling with. I would like for it to copy the data into a new array that doubled the size, and then delete the old array.
If I name it: array1 and array2, I have my program use array1 throughout. I need help with the pointers that would get array2 to work as array1.
Is there a way to copy the array to a temp array of the same or new size, and then remake the initial array reassigning back to that? For this exercise, I can't use vectors. While I know how to use them, and that they solve this issue while being better, I'm trying to do it with only arrays.
using namespace std;
struct Information {
char functionality;
int SSN;
string name;
};
int numPeople = 1000;
//Gets called if the initial array (whatever size) is filled
void doubleArray(Information *array){
numPeople = numPeople * 2;
//Will now be the doubled array size
Information temp[numPeople]
for(int i = 0; i < numArray; i++){
temp[i].SSN = array[i].SSN;
temp[i].name = array[i].name;
}
//Normally makes it crash
delete[] array;
}
edit: This is what I currently have
void doubleArray(Information *person){
numPeople = numPeople * 2;
Information* temp = new Information[numPeople];
memcpy(temp, person, numPeople);
delete[] person;
person = temp;
}
It gets to numPeople = 1000 (the initial list size) but then crashes shortly after. Is the doubling array correct?
Arrays are fixed size. You cannot change the capacity of the original array.
{Use std::vector}
You can have a pointer to an array. And use the same pointer. When the array is full, you can allocate another array, copy old array items to new array, delete the old array and assign your array pointer to the new array.
{Did I mention std::vector?}
By the way, there is a data structure that performs resizing as necessary. If I recall correctly, it is std::vector. Try it out. :-)
Assuming you are using std::array (which you should be), then copying the array is very easy.
std::array<myStruct, 1000> array1{};
std::array<myStruct, 2000> array2{};
// codes...
std::copy(array1.begin(), array1.end(), array2.begin())
However, this is a specific scenario in which you only use these two arrays. It will not dynamically double the size of the array as you simply cannot do this dynamically with stack-based arrays, just like c arrays[].
What you can, and should, be using is std::vector<myStruct>. This will dynamically grow as you need it. Until you provide us with code and a more specific issue, this is the best advice that I can offer with the information provided.
If you aren't allowed to use std::vector, as one of your comments stated, then you'll want to look at dynamic allocation.
size_t sz = [whatever];
// Dynamically allocate an array of size sz.
T* T_array = new T[sz];
// Do whatever...
delete[] T_array; // new[] needs to be paired with delete[].
T_array = nullptr; // Not strictly necessary, but a good idea if you have more code after.
As the size doesn't need to be constant for a dynamic array, this will allow you to allocate memory as necessary. You can then use std::copy() to copy data from one array to the other, as Goodies mentioned.
[For more information on dynamic allocation, see here.]
I'm trying to use an array in C++ that changes in size. For some reason the size does not change, it only ever holds 1 string. The difficult part is that the user cannot input the number of courses they are going to add, instead the addCourse function is called until the user stops. A vector cannot be used (this is for a school assignment, and a resizing array is required). I'm stuck as to why the array only seems to hold one string, I would think it to hold the equivalent of numCourses strings. How would I go about resizing to hold multiple strings after each call to the function?
void Student::addCourse(string* courseName)
{
int x;
numCourses += 1;//increments number of courses
string newCourse = *courseName;
string* newCourses = new string[numCourses];//temporary array
for(x=0; x<numCourses - 1; x++)//fills temp array with the values of the old
{
newCourses[x] = courses[x];
}
newCourses[numCourses - 1] = newCourse;//adds extra value
delete[] courses;//removes original array
courses = newCourses;//sets the new course list
}
Edit: For those asking why a vector cannot be used because the point of the assignment is to actively avoid memory leak using the heap. Using an array like this forces intentional delete of stored values.
The comment should have answered your question: there is no way for the debugger to know that a pointer to a string is pointed to an array, nor does it know its bounds, because no such information is kept at runtime (a std::vector will show its whole contents in the debugger, in contrast).
Your method prototype should read:
void Student::addCourse(const string& courseName);
If you don't want to have a memory leak, declare a pointer to courses in your class:
private:
string* courses;
Allocate space for an array of strings in your constructor:
Student::Student()
{
courses = new String[5];
}
Then deallocate in the destructor:
Student::~Student()
{
delete[] courses;
}
This gives you room for up to 5 courses. If you need more you need to adjust the size of the array of strings at run time:
void Student::ExtendArray()
{
delete[] courses;
courses = new String[10];
}
Note this code is not exception safe, but will give you the basic idea.
suppose I declare a dynamic array like
int *dynArray = new int [1];
which is initialized with an unknown amount of int values at some point.
How would I iterate till the end of my array of unknown size?
Also, if it read a blank space would its corresponding position in the array end up junked?
Copying Input From users post below:
Thing is:
a) I'm not allowed to use STL (means: no )
b) I want to decompose a string into its characters and store them. So far I wanted to use a function like this:
string breakLine (string line){
int lineSize = line.size();
const char *aux;
aux=line.data();
int index=0;
while (index<=lineSize){
mySynonyms[index]=aux[index];
index++;
}
I thought that the array aux would end up junked if there was a large blank space between the two numbers to be stored (apparently not). And I was wondering if there was a way to iterate till an undefined end in this type of array. Thanks for you answers.
You don't: wrap the array into a structure that remembers its length: std::vector.
std::vector v(1);
std::for_each( v.begin(), v.end(), ... );
No portable way of doing this. Either pass the size together with the array, or, better, use a standard container such as std::vector
Short answer is that you can't. If you have a pointer to the first element of an array, you can't know what the size of the array is. Why do you want to use a array in the first place. You would be much better off using a std::vector if your array can change size dynamically, or a boost::Array if it will be a fixed size.
I don't understand your second question.
Your code needs to keep to track of the array, so the size would never be unknown. (Or you would have to use some library with code that does this.)
I don't understand the last part of your quesiton. Could you elaborate?
You explained in your post below that you want to look at the guts of a std::string.
If you are expecting your stirng to be like a c-string (aka doesn't contain NULLs), then use line.c_str() instead of line.data(). This will guarantee that aux points to a null terminates c-style string.
After that you can iterate until aux[index] == '\0';
Otherwise, you can use line.data() and string.length/size to get it's size like in your example.
However, "decomposing a string into its characters" is pretty pointless, a string is an array of characters. Just make of copy of the string and store that. You are allowed to do:
char ch = line[index];
Better yet, use iterators on the original string!
for(std::string::const_iterator it = line.begin(); it != line.end(); ++it) {
const char ch = *it;
// do whatever with ch
}
a) I'm not allowed to use STL (means:
no )
What?? Who's moronic idea was that?
std::vector isn't part of the "STL" (which is a copyrighted product of HP), but is (and has been for nearly a decade) part of the C++ Language Standard.
If you're not allowed to use the STL (for whatever reason), the first thing you want to do is actually to implement your own version of it – at least the parts you need, with the level of customizability you need. For example, it's probably overkill to make your own vector class parametrizable with a custom allocator. But nevertheless do implement your own lightweight vector. Everything else will result in a bad, hardly maintainable solution.
This smells like homework, and the teacher's objective is to give you a feeling of what it takes to implement dynamic arrays. So far you're getting an F.
You need to realize that when you allocate memory like this
int *dynArray = new int [1];
you allocate precisely one integer, not an indefinite number of integers to be expanded by some unidentified magic. Most importantly, you can only say
dynArray[0] = 78;
but you cannot say
dynArray[1] = 8973;
The element at index 1 does not exist, you're stepping into memory that was not reserved for you. This particular violation will result in a crash later on, when you deallocate the array, because the memory where you stored 8973 belongs to the heap management data structures, and you corrupted your heap.
As many other responders mention, you must know how many elements you have in the array at all times. So, you have to do something along the lines of
int arraySize = 1;
int *dynArray = new int [arraySize];
arraySize goes together with the array, and is best combined with dynArray in one C++ object.
Now, before you assign to dynarray[1], you have to re-allocate the array:
if (index > arraySize) {
int newSize = index+1;
int *newArray = new int[newSize]
// don't forget to copy the data from old array to new
memcpy(newarray dynArray, sizeof *newArray * arraySize);
arraySize = newSize;
dynArray = newArray;
}
// now you're ready!
dynArray[index] = value;
Now, if you want to make it a bit more efficient, you allocate more than you need, so you don't have to allocate each time you add an element. I'll leave this as an exercise to the reader.
And after doing all this, you get to submit your homework and you get to appreciate the humble std::vector that does all of this for you, plus a lot more.
Use a vector, which has a vector.size() function that returns an integer and a vector.end() function that returns an iterator.
You could create a simple Vector class that has only the methods you need. I actually had to recreate the Vector class for a class that I took this year, it's not very difficult.
If there's a value that cannot be valid, you can use that as a sentinel, and make sure all of your arrays are terminated with that. Of course, it's error-prone and will cause hard-to-find bugs when you happen to miss doing it once, but that's what we used to do while reading files in FORTRAN (back in the all-caps days, and before END= became standard).
Yes, I'm dating myself.