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Programming Fundamentals using C Arrays and Pointers

Programming Fundamentals using C Arrays and Pointers. By TungTT2. Training objectives. Arrays Array Definition & Declaration Array Representation Multi-dimensional Arrays Passing arrays to function. Training objectives. Pointers Pointer Arithmetic Swap Example w/Pointers Null Pointers

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Programming Fundamentals using C Arrays and Pointers

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  1. Programming Fundamentals using CArrays and Pointers By TungTT2

  2. Training objectives • Arrays • Array Definition & Declaration • Array Representation • Multi-dimensional Arrays • Passing arrays to function

  3. Training objectives • Pointers • Pointer Arithmetic • Swap Example w/Pointers • Null Pointers • Arrays and Pointers • Pointers to Pointers • Function Pointers • Dynamic Memory Allocation

  4. Arrays

  5. Arrays • A data structure is a collection of data types designed to store information in some optimal way.  • Data structures improve readability and simplify coding considerably.  • The simplest example of a data structure is an array. 

  6. The solution without array syntax (1)

  7. The solution without array syntax (2)

  8. The solution without array syntax (3)

  9. The solution without array syntax (4) What are the disadvantages of this solution?

  10. Arrays in C Compare: C: int array[10]; Java: int[] array = new int[10]; All elements of same type – homogenous Unlike Java, array size in declaration int array[10]; int b; array[0] = 3; array[9] = 4; array[10] = 5; array[-1] = 6; First element (index 0) Last element (index size - 1) No bounds checking! Allowed – usually causes no error array[10] may overwrite b

  11. Array Representation a[2] 0x1008 a[1] 0x1004 a[0] 0x1000 • Homogeneous  Each element same size – s bytes • An array of m data values is a sequence of ms bytes • Indexing: 0th value at byte s0, 1st value at byte s1, … • m and s are not part of representation • Unlike in some other languages • s known by compiler – usually irrelevant to programmer • m often known by compiler – if not, must be saved by programmer int a[3];

  12. Array Representation i 0x1014 c2 0x1010 a[2] 0x100C a[1] 0x1008 a[0] 0x1004 c1 0x1000 char c1; int a[3]; char c2; int i; Could be optimized by making these adjacent, and reducing padding (by default, not) Array aligned by size of elements

  13. Array Sizes • What is • sizeof(array[3])? • sizeof(array)? int array[10]; 4 returns the size of an object in bytes 40

  14. Multi-Dimensional Arrays • Multi-dimensional arrays declared with multiple indexes • arr[i][j] /* RIGHT*/ • arr] /* WRONG! */ • Array elements are stored by rows • The rightmost subscript varies the fastest • Array name “points” to 1st element

  15. Multi-Dimensional Arrays matrix[1][2] 0x1014 matrix[1][1] 0x1010 matrix[1][0] 0x100C matrix[0][2] 0x1008 matrix[0][1] 0x1004 matrix[0][0] 0x1000 int matrix[2][3]; matrix[1][0] = 17; Recall: no bounds checking What happens when you write: matrix[0][3] = 42; “Row Major” Organization

  16. Passing Arrays To Functions (1) Arguments • The syntax of a function call that passes an array is functionIdentifier (arrayIdentifier, ... ) • Where functionIdentifier is the name of the function and arrayIdentifier is the array name without brackets. 

  17. Passing Arrays To Functions (2) Parameters • The syntax of a function header that receives an array address is dataType functionIdentifier ( dataType arrayIdentifier [ ], ... ) • The brackets following arrayIdentifier inform the compiler that the parameter holds the address of a one-dimensional array • Multi-dimensional arrays passed to a function must declare the number of elements for every subscript except the first dataType functionIdentifier ( dataType arrayIdentifier [ ][13], ... )

  18. Pointers

  19. Pointers • A pointer is a variable that contains an address. • With pointers • functions can indirectly access variables. • functions can modify the arguments passed by the caller function. • sophisticated data structures can grow and shrink at run-time. • Arrays and pointers are closely related. • Array pointers enable us to conveniently process groups of data such as vectors, lists, and strings.

  20. Pointers • Special case of bounded-size natural numbers • Maximum memory limited by processor word-size • 232 bytes = 4GB, 264 bytes = 16 exabytes • A pointer is just another kind of value • A basic type in C int *ptr; The variable “ptr” is a pointer to an “int”.

  21. Pointer Operations in C • Creation &variable Returns variable’s memory address • Dereference *pointer Returns contents stored at address • Indirect assignment *pointer=val Stores value at address • Assignment pointer=ptr Stores pointer in another variable

  22. Using Pointers … 0x1014 ptr2: 0x1010 … 0x100C ptr1: 0x1008 i2: 0x1004 i1: 0x1000 int i1; int i2; int *ptr1; int *ptr2; i1 = 1; i2 = 2; ptr1 = &i1; ptr2 = ptr1; *ptr1 = 3; i2 = *ptr2; 0x1000 0x1000 2 3 3 1

  23. Using Pointers (cont.) int int1 = 1036; /* some data to point to */ int int2 = 8; int *int_ptr1 = &int1; /* get addresses of data */ int *int_ptr2 = &int2; *int_ptr1 = int_ptr2; *int_ptr1 = int2; What happens? Type check warning: int_ptr2 is not an int int1 becomes 8

  24. Using Pointers (cont.) int int1 = 1036; /* some data to point to */ int int2 = 8; int *int_ptr1 = &int1; /* get addresses of data */ int *int_ptr2 = &int2; int_ptr1 = *int_ptr2; int_ptr1 = int_ptr2; What happens? Type check warning: *int_ptr2 is not an int * Changes int_ptr1 – doesn’t change int1

  25. Pointer Arithmetic char *p; char a; char b; p = &a; p += 1; int *p; int a; int b; p = &a; p += 1; pointer + numberpointer – number E.g., pointer+ 1 adds 1 something to a pointer In each, p now points to b (Assuming compiler doesn’t reorder variables in memory) Adds 1*sizeof(char) to the memory address Adds 1*sizeof(int) to the memory address Pointer arithmetic should be used cautiously

  26. The Simplest Pointer in C • Special constant pointer NULL • Points to no data • Dereferencing illegal – causes segmentation fault • To define, include <stdlib.h> or <stdio.h>

  27. Generic Pointers • void *: a “pointer to anything” • Lose all information about what type of thing is pointed to • Reduces effectiveness of compiler’s type-checking • Can’t use pointer arithmetic type cast: tells the compiler to “change” an object’s type (for type checking purposes – does not modify the object in any way) Dangerous! Sometimes necessary… void *p; int i; char c; p = &i; p = &c; putchar(*(char *)p);

  28. Pass-by-Reference 1 a 2 b x y 1001 1002 void set_x_and_y(int *x, int *y) { *x = 1001; *y = 1002; } void f(void) { int a = 1; int b = 2; set_x_and_y(&a,&b); }

  29. Arrays and Pointers

  30. Dirty “secret”: Array  pointer to the initial (0th) array element a[i]  *(a+i) An array is passed to a function as a pointer The array size is lost! Usually bad style to interchange arrays and pointers Avoid pointer arithmetic! Arrays and Pointers Passing arrays: Must explicitly pass the size Really int *array int foo(int array[], unsigned int size) { … array[size - 1] … } int main(void) { int a[10], b[5]; … foo(a, 10)… foo(b, 5) … }

  31. Arrays and Pointers int foo(int array[], unsigned int size) { … printf(“%d\n”, sizeof(array)); } int main(void) { int a[10], b[5]; … foo(a, 10)… foo(b, 5) … printf(“%d\n”, sizeof(a)); } What does this print? 8 ... because array is really a pointer What does this print? 40

  32. Arrays and Pointers • Given the previous declarations, each of the following lines are equal. char word[10]; char *cptr; cptr = word; // points to word[0] cptr word &word[0] address of word[0] (cptr + n) word + n &word[n] address of word[n] *cptr *word word[0] value of word[0] *(cptr + n) *(word + n) word[n] value of word[n]

  33. Arrays and Pointers int i; int array[10]; for (i = 0; i < 10; i++) { array[i] = …; } int *p; int array[10]; for (p = array; p < &array[10]; p++) { *p = …; } These two blocks of code are functionally equivalent

  34. Strings • In C, strings are just an array of characters • Terminated with ‘\0’ character • Arrays for bounded-length strings • Pointer for constant strings (or unknown length) char str1[15] = “Hello, world!\n”; char *str2 = “Hello, world!\n”; C, … H e l l o , w o r l d ! \n terminator C terminator:’\0’ length H e l l o , w o r l d ! \n Pascal, Java, …

  35. String length • Must calculate length: • Provided by standard C library: #include <string.h> can pass an array or pointer int strlen(char str[]) { int len = 0; while (str[len] != ‘\0’) len++; return (len); } Check for terminator array access to pointer! What is the size of the array???

  36. Pointers and more

  37. Pointers to Pointers ptrArr[0] ptrArr[0] FSO FSO ptrArr[1] ptrArr[1] 123 123 ptrArr[2] ptrArr[2] abc abc … … Pointers to Pointers • Since pointers are variables themselves, they can be stored in arrays just as other variables can. Example: char* ptrArr[8];

  38. Pointers of Pointers  int var = 5; int* ptr; int** pptr; int*** ppptr; int**** pppptr; ptr = &var; pptr = &ptr; ppptr = &pptr; pppptr = &ppptr; var, *ptr, **pptr, ***ppptr, ****pppptr all = 5 &var=? &ptr = ? *ptr = ? &pptr = ? *pptr = ? &ppptr = ? *ppptr = ? &pppptr = ? *pppptr = ?

  39. Pointer to Pointer (char **argv) Passing arguments to main: size of the argv array/vector int main(int argc, char **argv) { ... } an array/vector of char * Recall when passing an array, a pointer to the first element is passed Suppose you run the program this way UNIX% ./program hello 1 2 3 argc == 5 (five strings on the command line)

  40. char **argv “3” argv[4] 0x1020 These are strings!! Not integers! “2” argv[3] 0x1018 argv[2] “1” 0x1010 argv[1] 0x1008 “hello” argv[0] 0x1000 “./program”

  41. Function Pointers Function Pointers • In C, a function is not a variable, but it is possible to define pointers to functions which can be: • assigned, • placed in arrays, • passed to functions, • returned by functions, and so on. int function1(int a, char* s) { … } int function2(int a, char* s) { … } int (*f[2])(int, char*); f[0] = function1; f[1] = function2; (*f[n])(10, "hello");

  42. Complicated Declarations Function Pointers • C is sometimes castigated for the syntax of declarations, particularly ones that involve pointers to functions: int *f(); // f: function returning pointer to int int (*pf)(); // pf: pointer to function returning int • What do these do? char **argvargv: pointer to pointer to char int (*daytab)[13]daytab: pointer to array[13] of int int *daytab[13]daytab: array[13] of pointer to int char (*(*x())[])()x: function returning pointer to array[ ] of pointers to function returning char char (*(*x[3])())[5]x: array[3] of pointer to function returning pointer to array[5] of char

  43. Dynamic memory allocation

  44. Definition — The Heap • A region of memory provided by most operating systems for allocating storage not in Last in, First out discipline • I.e., not a stack • Must be explicitly allocated and released • May be accessed only with pointers • Remember, an array is equivalent to a pointer • Many hazards to the C programmer

  45. Static Data Allocation 0xFFFFFFFF stack (dynamically allocated) SP heap (dynamically allocated) address space static data program code (text) PC 0x00000000

  46. Allocating Memory in The Heap • See <stdlib.h> void *malloc(size_t size); void free(void *ptr); void *calloc(size_t nmemb, size_t size); void *realloc(void *ptr, size_t size); • malloc() — allocates size bytes of memory from the heap and returns a pointer to it. • NULL pointer if allocation fails for any reason • free() — returns the chunk of memory pointed to by ptr • Must have been allocated by malloc or calloc

  47. Notes • calloc() is just a variant of malloc() • malloc() is analogous to new in C++ and Java • new in C++ actually calls malloc() • free() is analogous to delete in C++ • delete in C++ actually calls free() • Java does not have delete — uses garbage collection to recover memory no longer in use

  48. Typical usage of malloc() and free() char *getTextFromSomewhere(…); int main(){ char * txt; …; txt = getTextFromSomewhere(…); …; printf("The text returned is %s.", txt); free(txt); }

  49. Typical usage of malloc() and free() char * getTextFromSomewhere(…){char *t;...t = malloc(stringLength);...return t; } int main(){ char * txt; …; txt = getTextFromSomewhere(…); …; printf("The text returned is %s.", txt); free(txt); }

  50. Definition – Memory Leak • The steady loss of available memory due to forgetting to free() everything that was malloc’ed. • Bug-a-boo of most large C and C++ programs • If you “forget” the value of a pointer to a piece of malloc’ed memory, there is no way to find it again! • Killing the program frees all memory!

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