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Where an array is declared in the main function it will usually have details of dimensions included. It is possible to use another type called a pointer in place of an array. This means that dimensions are not fixed immediately, but space can be allocated as required. This is an advanced technique which is only required in certain specialised programs.
Another feature of arrays is their passing as function arguments. A function can be passed an array as an argument without knowing the size of the array's final dimension. So for example if we have a function which inverts a matrix (represented by an array) then the function will be able to invert matrices of different sizes. The drawback is that the function is unable to determine what size the matrix is, so this information will have to be passed as an additional argument.
NO DIRECT EQUIVALENT IN C. WORK AROUND THIS BY DECLARING:
int c[10];
AND LETTING SUBSCRIPTS RANGE FROM 0..9
C does not distinguish between packed and unpacked arrays.
Here you see that the loops work well because they increment the variable for you, and you only
need to increment by one. Its the easiest loop to read, and you access the entire array.
An example shown here is a simple function to add up all of the integers in a single dimensioned array.
Pointers are variables which refer to the memory locations of other variables.
So how about an analogy for this. Let's take a few houses. Each house is a memory address, the value in the house is the person that lives there. Note that people will move, but the houses always stay there. It might look something like it shown in example above. Think of "647" as the person that lives at address "0x5FA70". x could be the car that gets you to see that nice person, "647". If you put an ampersand(&) in front of x you are looking for the house address and you could care less about who's inside. A memory address is a location within your computers memory. Its where something is. And each one of your variables has a memory address of its own. Which means that at that address in memory is the value of that variable. Say for instance you have a single integer named x and it has a value of 654. Let's pretend that x's memory address is "0x5FA70" (hexidecimal). At the location "0x5FA70" in computers memory is the number 654. At the memory address "0x5FA70" is the number 654. "..." in the other boxes means that we don't know what's at those memory locations. The memory addresses are defined in hexidecimal because its easier to represent them that way. Also, the addresses in this depiction increase in increments of 4. That is because the size (in bytes) of an integer is usually four (unless you're working with a crummy ol' 16-bit compiler).
Pointer syntax can be confusing, because pointers can both give the memory location and give the actual value stored in that same location. When a pointer is declared, the syntax is this: variable_type *name; Notice the *. This is the key to declaring a pointer, if you use it before the variable name, it will declare the variable to be a pointer.
There are two ways to use the pointer to access information about the memory address it points to. It is possible to have it give the actual address to another variable, or to pass it into a function. To do so, simply use the name of the pointer without the *. However, to access the actual memory location, use the *. The technical name for this doing this is dereferencing.
In order to have a pointer actually point to another variable it is necessary to have the memory address of that variable also. To get the memory address of the variable, put the & sign in front of the variable name. This makes it give its address. This is called the reference operator, because it returns the memory address.
The cout outputs the value in x. The integer is called x. A pointer to an integer is then defined as "pointer". Then it stores the memory location of x in pointer by using the ampersand (&) symbol. If you wish, you can think of it as if the jar that had the integer had a ampersand in it then it would output its name (in pointers, the memory address) Then the user inputs the value for x. Then the cout uses the * to put the value stored in the memory location of pointer. If the jar with the name of the other jar in it had a * in front of it would give the value stored in the jar with the same name as the one in the jar with the name. It is not too hard, the * gives the value in the location. The unastricked gives the memory location.
The pointer can be initialised using free memory. This allows dynamic allocation of array memory. It is most useful for setting up structures called linked lists.
The pointer can be initialised using free memory. This allows dynamic allocation of array memory. It is most useful for setting up structures called linked lists.
The final implication of NULL is that if there is no more free memory, it is possible for the ptr after being "new"-ed to point to NULL. Therefore, it is good programming practice to check to ensure that the pointer points to something before using it. Obviously, the program is unlikely to work without this check.
Your program should always be prepared to catch the bad_alloc exception before trying to access the new object (unless you use a new-handler). A request for allocation of 0 bytes returns a non-null pointer. Repeated requests for zero-size allocations return distinct, non-null pointers.
Here you see that the loops work well because they increment the variable for you, and you only
need to increment by one. Its the easiest loop to read, and you access the entire array.
In addition, efficient logarithmic algorithms exist that permit a new element to be added to a heap and the largest element removed from a heap. For these reasons, a heap is a natural representation for the priority queue datatype.
In addition, efficient logarithmic algorithms exist that permit a new element to be added to a heap and the largest element removed from a heap. For these reasons, a heap is a natural representation for the priority queue datatype.
COMMENT TO EXAMPLE.
database employee; //There is now an employee variable that has modifiable variables inside it. The struct database declares that database has three variables in it, age, id_number, and salary.
You can use database like a variable type like int. You can create an employee with the database type as I did above. Then, to modify it you call everything with the 'employee.' in front of it. You can also return structures from functions by defining their return type as a structure type. Example:
struct database fn();
In each step of the loop, the current value of most is compared with the salary field of the current record. The operation
aux++;
moves aux to the next record because the increment is scaled by the size of the structure person. Comparing the efficiency of two programs one can notice that the efficiency of the second program is higher. This technique can not be used in Pascal because a pointer cannot be mode to point to a particular element of an array.
Think of it like a train. The programmer always stores the first node of the list. This would be the engine of the train. The pointer is the connector between coaches of the train. Every time the train ads a car, it uses the connectors to add a new car. This is like a programmer using the keyword new to create a pointer to a new struct or class.
An example shown here is a simple function to add up all of the integers in a single dimensioned array.
A structure type is usually defined near to the start of a file using the typedef statement. typedef defines and names a new type, allowing its use throughout the program. typedefs usually occur just after the #define and #include statements in a file.
Here is an example structure definition.
typedef struct {
char name[64];
char course[128];
int age;
int year_of_study;
} student;
This defines a new type student which can be used to declare variables as follows.
student st_rec;
This creates a variable called st_rec of type student. The variable has members called name, course, age and year_of_study.
Think back to the train. Lets imagine a conductor who can only enter the train through the engine, and can walk through the train down the line as long as the connector connects to another car. This is how the program will traverse the linked list. The conductor will be a pointer to node, and it will first point to root, and then, if the root's pointer to the next node is pointing to something, the "conductor" (not a technical term) will be set to point to the next node. In this fashion, the list can be traversed. Now, as long as there is a pointer to something, the traversal will continue. Once it reaches a NULL pointer, meaning there are no more nodes (train cars) then it will be at the end of the list, and a new node can subsequently be added if so desired.
COMMENT TO THE CODE: That is the basic code for traversing a list. The if statement ensures that there is something to begin with (a first node). In the example it will always be so, but if it was changed, it might not be true. If the if statement is true, then it is okay to try and access the node pointed to by conductor. The while loop will continue as long as there is another pointer in the next. The conductor simply moves along. It changes what it points to by getting the address of conductor->next.
An example shown here is a simple function to add up all of the integers in a single dimensioned array.
The final output is necessary because the while loop will not run once it reaches the last node, but it will still be necessary to output the contents of the next node. Consequently, the last output deals with this.
The standard header <string> must be #included to allow this.
Note: there is no provision for an explicit tag field in C unions, analogous to the tag field of a Pascal variant record.
C++ only:
The field name Borr for the union can be omitted; the variants can then be referenced as in Pascal.
In C++, initialisation of a struct this way is not allowed if any field is of class type, requiring a constructor. Thus, in particular, this example would not work correctly if name were declared as string, since string is a library class.