fundamentals of c and c++ programming simple data structures pointers

Fundamentals of C and C++ Programming

Simple data structuresPointers

Simple Data Structures

ArraysStructures

Unions

EEL 3801 – Lotzi Bölöni

Simple Data Structures

It would be limiting to have to express all data as variables.

It would be desirable to be able to group data into sets of related data.

This can be done two main ways:–arrays (all data of the same type)–structures (data may be of different types).


Type Definitions

Special data types designed by the programmer can be defined through the typedef keyword.

For example, if we want to define a data type that is to be defined only once and then used thereafter:

typedef unsigned long int Myvar


Type Definitions

So, Myvar can now be used to indicate an unsigned long int wherever used.

Myvar n;

is the same as:

unsigned long int n;

But it can be used for far more. (later)


Arrays

Left-most symbol indicates the name of the array. This is common for all its elements.

Individual data identified by distance from first one in array.

Within square brackets is the cell number (how many cells away from the first one).

Individual cells can be used as regular variables.


Arrays

-45

6

0

72

1543

-89

0

62

-3

c[0]

c[1]

c[2]

c[3]

c[4]

c[5]

c[6]

c[7]

c[8]

For array c:


Declaring Arrays

The declaration allows the compiler to set aside sufficient contiguous memory for the size of array

The type of data to be stored must be identified so that sufficient space is allocated.

Arrays allocated statically - remain the same size throughout program execution.


Declaring Arrays

int c[12];

float a[100];

char b[15];

Can be automatic or external.

Size typically done through a macro.

#define SIZE 10


Initializing Arrays

Not automatically initialized. Can be initialized during declaration or within the program in a loop.

int n[10] = {32,27,64,18,95,14,90,70,60};

If more elements than initialized, others = 0. If less elements than initialized - error.int n[] = {32,27,64,18,95,14,90,70,60,37};


Passing Arrays to Functions

Arrays passed by reference - actual variable address passed. The called function can modify the original array’s values.

Pass name without brackets. Include the size of the array as a passed value.

Function header and prototype must indicate that an array is being passed.


Passing Arrays to Functions

#define SIZE 5

void function1(int [],int);void function2(int);

main(){

int a[] = {0, 1, 2, 3, 4};function1(a,SIZE);function2(a[3]);

}


Multi-dimension Arrays

Arrays can have an arbitrary number of dimensions.

Indicated by multiple bracket pairs.int a[5][10];int b[10][12][20];

Can be called in same way as vector arrays.First bracket is the row scriptSecond is the column script.


Initializing Multi-dim. Arrays

Initialization by row in braces. First brace equates to first row, 2nd to

2nd,.int c[2][2] = {{1,2} {3,4}};

Initializes b[0][0]=1, b[0][1]=2, b[1][0]=3, and b[1][1]=4.

But what if int c[2][2] = {{1} {3,4}}; Initializes b[0][0]=1, b[0][1]=0 , b[1][0]=3, and b[1][1]=4.


Arrays and Strings

Strings are in reality arrays of charactersEach element contains one character.Each cell is one byte in size.More about strings and string operations later.


Structures

A collection of related, but dissimilar variables under one name.

Provides great flexibility that an array does not.

Used to define records to be stored in files.Also used to form dynamic data types such as linked lists, linked stacks and linked queues.


Structure Definitions

Declared as follows: struct planet {char *name;int nummoons;double dist_from_sun;float dist_from_earth;

};

This creates a definition of the structure.planet is the structure tag.


Structures Components

The variables are called members.They can be accessed individually using:–The structure member operator (also called the

dot operator).–The structure pointer operator (also called the

arrow operator).

See Figure 10.2, page 400 of textbook.


Structure Operators

The dot operator accesses the contents of the member using the member name and the structure variable name.

planet.nummoons

directly accesses the contents of the member nummoons

Can be used as a regular integer variable.


Structure Operators

The arrow operator accesses the contents of the member using a pointer to the structure variable and the member name.

planet_ptr->nummoons

directly points to the contents of the member nummoons

equal to (*planet_ptr).nummoons


Structure Variables

The struct keyword defines a “model” of the desired structure.

It is not a real variable per se.A real variable is created by creating an instance of the structure model.

Also referred to as “instantiating”.


Structure Variables

To make instances of the definition:–Instance name(s) can be added after the

definition.–Can be defined as a data type to be instantiated

separately.–The struct keyword can be used along with

the tag to instantiate.

See examples next.


Structure Variables

Instances added after the definition:struct planet {

char *name;int nummoons;double dist_from_sun;float dist_from_earth;

} earth, mars, solar[9], *ptr;

solar[9] is an array of 9 structures of type planet. ptr is a pointer to a planet type.


Structure Variables

The tag is optional. The following code is equivalent to the one in the last slide:

struct {


} earth, mars, solar[9], *ptr;Only way to instantiate is in the definition.


Structure Variables

Defined as a datatype:typedef struct planet Planet;

Planet earth, mars;

Planet *ptr;

Planet solar[9];

This assumes that the structure definition is as before.


Structure Variables

Can also be done directly in the definition:

typedef struct planet {


} Planet;

The planet tag is not necessary in this case.


Structure Variables

The struct keyword can also be used to instantiate.

struct planet {


};struct planet earth;


Initializing Structure Members

Like in arrays.Use values inside braces.Only when variable being instantiated.struct planet earth = {earth,1,1.0e+6,0}

If less values than members, then only the first few are initialized. Others = 0.

Must be constant values or expressions.


Structures and Functions

Structures can be passed to functions as:–Individual structure members.–An entire structure variable.–Pointer to a structure variable.

Passed by value if the individual structure member or the entire structure is passed.

Passed by reference if a pointer to the structure is passed.


Structures

Arrays can be assigned to a structure member.

There can be arrays of structures.Structure members can be other structures.

Structure members can be self-referencing structures - pointers that point to similar structures as itself.


Unions

Same as structures, except members share same storage space.

Saves space when some members are never used at the same time.

Space for a member must be large enough to accommodate the largest of the data types to be stored in that member.


Unions

Unions are declared and defined in a way similar to structures.

The keyword union replaces the keyword struct.

Not highly recommended except when memory management is critical.


Enumeration Constants

Allows a set of integer constants to be represented by identifiers.

Symbolic constants whose value can be set automatically.

Values start with 0 (unless otherwise noted by programmer) and are incremented by 1.

Uses the enum keyword for definition.


Enumeration Example

#include <stdio.h>enum months {JAN=1, FEB, MAR, APR, MAY, JUN, JUL, AUG, SEP, ACT, NOV, DEC}

main(){ enum months month; char *monthName[] = {“”, “January”,..};for(month=JAN;month<=DEC;month++)

printf(……….monthName[month];}

Pointers


Pointer Variables

Conventional variables contain values.Pointer variable contains memory address of variable that contains values (or pointers)

Allows call by reference.Permits creation of dynamic data structures.

Permits dynamic allocation of memory.Difficult to understand and use.


Pointer Variables

Conventional variable names directly reference a value.

Pointer variables indirectly reference a value

Referencing a value through a pointer variable is called indirection.

Pointer variables = pointers


Declaration of Pointer Variables

Pointers must be declared like regular variables.

It must be stated which type of variable they point to.

Declarations use * to indicate “pointerhood”

int *ptr;pointer ptr points to an integer variable.


Pointer Variables

Pointers should be initialized.The * does not distribute.Can be set to NULL or to 0, but NULL is preferred.

NULL is a symbolic constant defined in <stdio.h>

Pointers assigned a value of 0 actually have the value 0 and not an address.


Address-of Pointer Operator

Address-of operator (&) is a unary operator returning the address of its operand.

The basic operator used to assign values to pointers.

int y = 5;int *ptr;ptr = &y;

ptr points to y (contains its address).


Indirection Pointer Operator

Indirection operator (*), or dereferencing operator is also unary and returns the value of the variable pointed at by the pointer.

In the previous example:y = 5

*ptr = 5

Not to be confused with the declaration operator - very confusing!!!.


Pointer Example

main(){

int a;int *aptr;a = 7;aptr = &a;printf(“The address of a =

%d“,&a);printf(“The value of aptr =%d“,

aptr);printf(“The value of a = %d“,

*aptr);}


Call by Reference with Pointers

By passing a variable’s address to a function, we give that function the ability to modify the value of the original value.

This is a simulation of call by reference.


Call by Value - Example

void value_funct1(int);

main(){

int number = 5;printf(“Original value =“, number);value_funct(number);printf(“New value =“, number);

}

void value_funct(int n);{

n = n * n;}


Call by Value - Example

Original value = 5

New value = 5

The call to function value_funct did not change the original variable number in main().


Call by Reference - Example

void value_funct2(int *);

main(){

int number = 5;printf(“Original value =“, number);value_funct(&number);printf(“New value =“, number);

}

void value_funct(int *nptr);{

(*nptr) = (*nptr) * (*nptr);}


Call by Reference - Example

Original value = 5

New value = 25

The call to function value_funct changed the original variable number in main().

A similar effect can be obtained by value_funct returning a value to main()


Functions Returning Pointers

Functions can also return pointers to variables.

int* function1(int, int);

is the prototype for a function that returns a pointer to an integer variable.

Is easily done by simply returning the value of a pointer variable - an address.


The const and Pointer Passing

The const qualifier tells the compiler that the variable following it is not to be changed by any program statements.

Provides a measure of security when passing addresses of variables whose values are not to be modified (for example, arrays).

When passing pointers, 4 possibilities exist:


Pointer Passing

Non-constant pointer to non-constant data –Declaration does not include const in any way.–Data can be modified through the pointer.–Pointer can be modified to point to other data.

Highest level of data access to called function.

This is what we have been doing up to now.


Pointer Passing

Non-constant pointer to constant data:–Pointer can be modified to point to any data.–Data that it points to cannot be modified–May be used to protect the contents of a

passed array.–Read as “a is a pointer to an integer constant”

void funct(const int *a)


Pointer Passing

Constant pointer to non-constant data:–Pointer always points to same memory

location.–Data that it points to can be modified.–Default value for a passed array.–Pointer must be initialized when declared.–Read “aptr is a constant pointer to an integer ”

int x;int * const aptr = &x;


Pointer Passing

Constant pointer to constant data:–Pointer always points to same memory

location.–Data that it points to cannot be modified.–Read “aptr is a constant pointer to an integer

constant” - right to left

int x = 5;const int* const aptr = &x;


Pointer Arithmetic

Pointers are valid operands in mathematical operations, assignment expressions and comparison operations.

But not all operators are valid with pointers.

Operators that are do not always work the same way.


Pointer Arithmetic

A pointer can be incremented (++).A pointer can be decremented (--).An integer may be added to, or subtracted from a pointer (+, +=, -, -=).

One pointer may be subtracted from another.

But this can be misleading.


Pointer Arithmetic

When adding integers to pointers, the value of the integer added is the number of memory elements to be moved.

The actual answer depends on the type of memory element being pointed to by the pointer.

Assuming int = 4 bytes (32 bits):


Pointer Arithmetic

int *yptr = 3000;yptr += 2;

In reality, yptr = 3008, because 2*4=8 bytes. In other words, the pointer moved two integer data “spaces” away from its original address.

Since an integer data space is 4 bytes, it moved 8 bytes.


Pointer Arithmetic

Since character variables are 1 byte in size, the arithmetic will be normal for pointers that point to characters.

The ++ and -- operators work the same way.

They add one data space to the address.

int *ptr = 3000;ptr++;

ptr = 3004, assuming integer takes 4 bytes.


Pointer Arithmetic

Subtraction works the same way.

int x;x = v1ptr - v2ptr;

where v1ptr=3008 and v2ptr=3000;

==> x = 2 if int is 4 bytes.


Pointers and Arrays

The name of an array is in reality a pointer to its first element.

Thus, for array a[] with, for instance, 10 elements, a = &(a[0]).

This is why when an array is passed to a function, its address is passed and it constitutes call by reference.


Pointers and Arrays

a[3] can be also referenced as *(a+3).The 3 is called the offset to the pointer.Parenthesis needed because precedence of * is higher than that of +.

Would be a[0]+3 otherwise.a+3 could be written as &a[3].See Fig. 7.20, page 284 in textbook.


Pointers and Arrays

The array name itself can be used directly in pointer arithmetic as seen before.

Pointer arithmetic is meaningless outside of arrays.

You cannot assume that a variable of the same type will be next to a variable in memory.


Pointers and Strings

Strings are really pointers to the first element of a character array.

Array is one character longer than the number of elements between the quotes.

The last element is “\0” (the character with the ASCII code zero).


Arrays of Pointers

Arrays may contain nearly any type of variable.

This includes pointers.Could be used to store a set of strings.char *suit[4] = {“hearts”, “diamonds”, “spades”, “clubs”};

The char * says that the elements of the array are pointers to char.


Arrays of Pointers

H e a r t s \0

D i a m o n d s \0

C l u b s \0

S p a d e s \0

Suit[0]

Suit[1]

Suit[2]

Suit[3]


Pointers to Functions

Contains address of the function in memory.

This is now addressing the code segment.Can be –passed to functions–returned from functions–stored in arrays–assigned to other function pointers


Pointers to Functions

Pointer contains the address of the first instruction that pertains to that function.

Commonly used in menu-driven systems, where the choice made can result in calling different functions.

Two examples follow:


Example 1

Writing a sorting program that orders an array of integers either in ascending or descending order.

main() asks the user whether ascending or descending order, then calls the sorting function with the array name, its size and the appropriate function (ascending or descending).

See Fig. 7-26, page 292 in textbook.


Example 1 - continued

int ascending(int,int);int descending(int,int);void sort(int *, const int, int (*)(int,int));main(){

. . .sort(array,10,ascending); orsort(array,10,descending);

. . .

}



void sort(int *arr, const int size, int (*compare_func) (int, int));

{if ((*compare_func)(arr[i], arr[i+1]))

do something;}

int ascending(const int a, const int b){

return b < a;}



main() calls sort() and passes to it the array, its size, and the function to be used.

sort() receives the function and calls it under a pointer variable compare_func, with two arguments.

The arguments are elements of the array, arr[i] and arr[i+1].

compare_func returns 1 if true,0 if false.


Example 2

Functions are sometimes represented as an array of pointers to functions.

The functions themselves are defined as they would normally.

An array of pointers is declared that contains the function names in its elements.

Functions can be called by dereferencing a pointer to the appropriate cell.



void function1(int);void function2(int);void function3(int);main(){

void (*f[3])(int) = {function1, function2, function3};

}

“f is an array of 3 pointers to functions that take an int as an argument and return void”



Such functions can be called as follows:(*f[choice]) (choice));

Can be interpreted as calling the contents of the address located in the choice cell of array f, with an argument equal to the value of the integer variable choice.

The parenthesis enforce the desired precedence.


Double Pointers

Double pointers are commonly used when a call by reference is desired, and the variable to be modified is itself a pointer.

A double pointer is a pointer to a pointer to a variable of a particular type.

Declared as int **ptr

Read as a pointer to a pointer to an integer.


Double Pointers

int

ptr


Double Pointers

Deferencing a double pointer results in an address.

Derefencing it again results in the value of the ultimate variable

var = *(*dbl_ptr);

Memory Allocation


Static Memory Allocation

Variables declared at compile time (in the source code) are called static variables.

It is necessary to know how many of these variables will be necessary prior to compilation.

They cannot be undeclared (other than by functions exiting).

Easy to use but are not very flexible.


Static Memory Allocation

Advantages:–System does not run out of memory–Easy to keep track of them–Can be referenced directly by the variable name–Limited to size of data segment in machine

Disadvantages–Must know required amount at compile time–Cannot be generated at run time


Dynamic Memory Allocation

Addresses the disadvantages of static memory.–Only limited by the amount of memory in

machine, not by pre-determined size of array.–Can be created and deleted at runtime.–Can result in memory leaks.–Harder to keep track of the variables



The C function call malloc() creates a block of memory of the size and shape designated by the call.

Its argument is the size of the desired block.Returns a pointer of type void * which points to the block of memory.

Returns a NULL pointer if memory not sufficient.



Uses the sizeof() operator to determine the size of the data type to be represented by the newly allocated block of memory.

The call to malloc() should be cast so as to force the pointer returned to be of the proper type - but not necessary.



Good idea to always use the sizeof() operator, even though technically, you can simply place a number there. This increases portability.

Memory block can be returned to the free memory heap by using the free()function

free(ptr);



Dynamically allocated memory can only be “found” through its pointer.

Pointer must be cast to its correct data type.Cannot be referenced any other way.Typically used with structures and unions.Thus, arrow operator becomes important in dynamically allocated structures.



Arrays are static variables.But can also be dynamically created.Use the calloc() function call for arrays.Requires two arguments:–the number of elements in the array–the size of each element in the array



calloc() also returns a pointer to the first element of the array.

Can be scripted just like a regular array.Must be deleted when no longer needed.Cannot be extended at run time.But its element size can be changed.



realloc() allows the modification of the size of a memory block previously allocated through malloc() or calloc().

Will keep data intact if size is larger.Otherwise, it will become corrupted.Requires two arguments:–Name of pointer to block to be re-allocated.–New size of the memory element.



In C++, it is somewhat easier:–the new operator takes as an argument the data

type name - new <data type name>–returns a pointer of the correct type to a block

of memory of the correct size (does not need sizeof(datatype) ).–delete <pointer name> de-allocates the

memory block and returns it to heap.



new also used to dynamically allocate arrays.delete also used to de-allocate arrays, but empty brackets are needed.

delete can only be used to delete memory allocated with new.

Do not mix and match malloc(), free(), new and delete.


Example

typedef struct name_tag {int blah;float blah_blah;

} Typename;

Typename *ptr;ptr = (Typename *) malloc(sizeof(Typename));

ptr->blah = 2;..

free(ptr);


Example


} Typename;

Typename *ptr;

ptr = (Typename *) calloc(20,sizeof(Typename));

ptr[12]->blah = 10;free(ptr);


Example


} Typename;

Typename *ptr;ptr = new Typename;ptr->blah = 23;..

delete ptr;


Example


} Typename;

Typename *ptr;

ptr = new Typename[20];ptr[15]->blah = 19;.

delete [] ptr;

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