1. Lecture 10 Introduction to Programming in C Prof. Dr. Arne Kutzner Hanyang University / Seoul Korea

2. Pointers / Introduction

3. Introduction to C L10.3 Variables and Memory Address Every variable has some location in the memory that holds the value of the variable. This location is called the address of the variable. 1000 01004 1008 1012 1016 1020 int num = 0; num The address of the variable num

4. Introduction to C L10.4 Pointers A pointer is also a variable, but unlike the other types of variables that we have seen, it contains a memory address as its value.

5. Introduction to C L10.5 Pointer Declaration To declare a pointer variable, a asterisk( *) should placed in front of name of the name of the variable. Example: int *p; p is a pointer to an integer value (i.e. p contains the address of some memory cell that contains an integer value)

6. Introduction to C L10.6 Address Operator You can get the memory address of some variable by using the address operator &. Example: int x = 0; int *p; p = &x; The pointer p gets the address of the variable x as its value 1000 01004 1008 1012 10041016 1020 x p

7. Introduction to C L10.7 How to deference pointers? For dereferencing a pointer place a asterisk in front of the pointer variable name. Example: int x = 0; int *p; p = &x; *p = 5; Write the value 5 to the memory location referenced by p 1000 51004 1008 1012 10041016 1020 x p

8. Introduction to C L10.8 Pointers / More explanation So we say: p is a pointer to the integer variable x. x  0 p  1004 *p*p  num  0 &x  1004 The * operator is referred to as indirection or dereferencing operator p 0 x *p is equal to 0 (the value of x). p indirectly references the value 0.

9. Introduction to C L10.9 Pointer Types Pointers can be declared to point to variables of any type. Examples: int *p1; pointer to an integer double *p2; pointer to a floating point number char *p3; pointer to a character

10. Introduction to C L10.10 Example Program 1 int main() { … int x1 = 5; int x2 = 8; int *x1ptr; int *x2ptr; x1ptr = &x1; x2ptr = &x2; *x2ptr = 12; … } x151000 x281004 x1ptr?1008 x2ptr?1012 x151000 x281004 x1ptr10001008 x2ptr10041012 x151000 x2121004 x1ptr10001008 x2ptr10041012

11. Introduction to C L10.11 Example Program 2 1 #include 2 int main() { 3 int num1, num2, product; 4 int *ptr_num1, *ptr_num2; 5 num1 = 5; 6 num2 = 7; 7 ptr_num1 = &num1; 8 ptr_num2 = &num2; 9 product = num1 * (*ptr_num2); … } num151000 num271004 product351008 ptr_num110001012 ptr_num210041014 The resulting memory picture * operator specifies multiplication * operator specifies dereferencing a pointer

12. Introduction to C L10.12 Description of Example Program 2 Line 3: integer variable declaration Line 4: pointer to integer declaration Line 7: The address of num1 is evaluated by the & operator and stored in ptr_num1 Line 9: The value in num1 is multiplied by the value of the address location pointed by ptr_num2

13. Introduction to C L10.13 Pointer type equivalence A pointer can dereference only a variable which is of the same type. Example: int n = 5; double x = 5.0; int *ptr1; double *ptr2; ptr1 = &n; ptr2 = &n; ptr1 = &x; Compile error! ptr1 is a pointer to integer whereas x is a floating point number. ptr2 is a pointer to double whereas n is an integer number.

14. Introduction to C L10.14 Warning: Uninitialized Pointers Dereferencing a pointer that has not been properly initialized Example: int *p; *p = 1; printf("Value is %d", *p); The pointer contains an arbitrary address, so *p=1 may overwrite some arbitrary location in memory which can cause severe problems. Run-time error! No compile error!

15. Introduction to C L10.15 Effects with uninitialized pointers Program attempts to access some random memory location. Possible effects: –Fatal Runtime error (common program run- time error message: "segmentation fault"). –Accidentally modifying other data / variables. Program continues but delivers incorrect result.

16. Functions and Pointers

17. Introduction to C L10.17 Pointers and Functions Function arguments can be of some “pointer type”. Example: void main () { int i; fun (&i); printf("%d", i) } void fun (int *p) { *p = 1; } Type of function arguments is “pointer to int”

18. Introduction to C L10.18 Call by Reference In the example on the slide before the statement *p = 1 in fun changes the value of i defined main in. Because p refers i in fun we call this form of argument passing call by reference.

19. Introduction to C L10.19 Example for the Application of Call by Reference A function fun shall have two input arguments and two output argument. (Remember: Using return we can deliver only one value!) fun x y x + y x * y InputOutput

20. Introduction to C L10.20 Application of Call by Reference int main() { int x, y, sum, product; x = 3; y = 5; fun (x, y, &sum, &product) } void fun (int a, int b, int *p1, int *p2) { *p1 = a + b; *p2 = a * b; } The addresses of sum and product are passed as arguments.

21. Introduction to C L10.21 Application of Call by Reference In the previous example, the addresses (not values) of the variables sum and product are copied to the function pointer arguments p1 and p2. In this way, sum and product have the same memory locations as *p1 and *p2, respectively. x3 y5 p11476 product151468 p21468 sum81476 …. a3 b5

22. Introduction to C L10.22 Comparison of Call by Reference and Call by Value With the function call by value, the values of the function arguments can't be modified by the function. With the function call by reference, the function arguments are the addresses, not the values of the corresponding variables. These addresses are copied into some pointers which are function arguments. Therefore, modifying the values stored in the addresses pointed by these pointers modifies also the values of the variables.

23. Introduction to C L10.23 Comparison of Call by Reference and Call by Value Call by value … int n = 1; int m = 2; swap(n, m); printf("%d, %d", n, m); … void swap(int x, int y) int temp = x; x = y; y = temp; } Call by reference … int n = 1; int m = 2; swap(&n, &m); printf("%d, %d", n, m); … void swap(int *x, int *y) int temp = *x; *x = *y; *y = temp; } What outputs do we get and why?

24. Introduction to C L10.24 Remark: scanf Now you should be able to understand the scanf function: scanf("%d", &num); Memory address of num

25. Pointers and Arrays / Pointer Arithmetic

26. Introduction to C L10.26 Pointers and Arrays In C there is a strong relationship between the concepts of pointers and arrays An array name is basically a const pointer. (A pointer with fixed address) int *x; int a[10]; x = a; This statement is OK!!! It assigns x the address of the first elements array a.

27. Introduction to C L10.27 Pointers and Arrays (cont.) You can even use the [ ] operator with a pointer: int *x; int a[10]; x = &a[2]; for (int i = 0;i < 3 ;i++) x[i]++; x gets “the address of a[2] ” x[i] is identical to a[i+2]

28. Introduction to C L10.28 Pointer arithmetic Integer math operations can be used with pointers. If you increment a pointer, it will be increased by the size of whatever it points to. int a[5]; a[0] a[1] a[2] a[3] a[4] int *ptr = a; *ptr *(ptr+2) *(ptr+4)

29. Introduction to C L10.29 printing an array void print_array(int a[], int len) { for (int i = 0;i < len; i++) printf ("[%d] = %d", i, a[i]); } void print_array(int *a, int len) { for (int i = 0;i < len; i++) printf ("[%d] = %d", i, *a++); } pointer version array version

30. Introduction to C L10.30 Arrays of Pointers Like for any other type you can create arrays of pointers: int a[] = {0, 1, 2}; int b[] = {4, 5, 6}; int* x[2]; x[0] = a; x[1] = b; printf("%d",*(x[1]+1)); printf("%d",*(*(x+1)+1)); x is array of pointers both statements are identical

31. Introduction to C L10.31 Matrices and Pointers … The technique can be extended to multidimensional arrays: int a[2][3] = {{0, 1, 2}, {4, 5, 6}}; int* x[2]; x[0] = &a[0]; x[1] = &a[1]; printf("%d",*(x[1]+1)); printf("%d",*(*(x+1)+1)); printf("%d",a[1][1]); printf("%d",*(a[1]+1)); printf("%d",*(*(a+1)+1)); all statements deliver the same element

32. Introduction to C L10.32 Pointers to Pointers int *x; int **y; x some int some *int some inty Pointer can refer pointers …

33. Strings

34. Introduction to C L10.34 Strings Recall that C data type char is used to represent digits, letters, and characters char letter = 'A'; A string is a sequence of characters. In C we use arrays of type char for the representation of strings. Example: char color[] = "blue"; The above statement declares a character array, that is a string, and initializes it to "blue".

35. Introduction to C L10.35 Strings Let's see what happens to memory after this declaration and initialization blue\0 The null character Note that the string contains the character '\0'; the null character that marks the end of the string. The same string could be also initialized as follows: char color[]={'b','l','u','e','\0'};

36. Introduction to C L10.36 Strings Another possibility is to fix the size of the string in declaration: char color[10]="blue"; blue\0????? Make sure that the size of the character array is larger than the number of characters in the string plus the null character. char color[4] = "blue"; char color[5] = "blue"; No space left for the null character OK

37. Introduction to C L10.37 String Initialization char str1[10] = "green"; char str2[10] = {'g','r','e','e','n','\0'}; char str3[10]; str3 = "green"; /* !! ERROR !! */ OK You can initialize a string only in the context of its declaration.

38. Introduction to C L10.38 Strings with printf and scanf The scanf function, when it scans a string, skips leading whitespace characters such as blanks, new lines and tabs. When it comes across another whitespace character, it stops and places the null character at the end of the string. If one tries to print a string having no null character at the end with printf, printf continues to display the contents of memory locations until it encounters a null character or until it attempts across a forbidden memory cell.

39. Introduction to C L10.39 String Input/Output with printf & scanf #include #define STRING_LEN 10 int add (int a, int b); int main() { char dept[STRING_LEN], days[STRING_LEN]; int course_num, time; printf("Enter department code, course number, days "); printf("time like this: EB 1003 Thur 1000\n>"); scanf("%s%d%s%d", dept, &course_num, days, &time); printf("%s %d is held %s at %d\n", dept, course_num, days, time); return 0; }

40. Introduction to C L10.40 String Library Function: strcpy The C library provides many functions to manipulate strings; #include string.h strcpy: Prototype: char *strcpy (char *dest, char *source); Purpose: Makes a copy of source, a string, in the character array accessed by dest. Example: char str1[10]; strcpy(str1, "EB1003-01") EB1003-01\0 str1:

41. Introduction to C L10.41 String Library Function: strncpy strncpy: Prototype: char *strncpy (char *dest, char *source, unsigned n); Purpose: Makes a copy of up to n characters from source in dest. Example: char str1[10], str2[20]="SKU EB1003-01"; strncpy(str1, str2, 9); str1[9] = '\0'; SKU EB100\0 str1:

42. Introduction to C L10.42 String Library Function: strncpy strncpy can be used to extract a number of characters from a string. For example, you might want to examine course code in the string "EB1003-01" Example: char str1[10] = "EB1003-01"; char course_code[5]; strncpy(course_code, &str1[2], 4); course_code[4]='\0'; EB1003-01\0 str1: 1003\0 course_code:

43. Introduction to C L10.43 String Library Function: strcat strcat: Prototype: char *strcat (char *dest, char *source); Purpose: Appends source to the end of dest. Example: char str1[10] = "EB"; EB\0??????? str1: strcat(str1, "1003-01"); EB1003-01/0 str1:

44. Introduction to C L10.44 String Comparison Standard C library also provides strcmp function to compare strings. strcmp: Prototype: int strcmp(char *s1, char *s2); Purpose: Compares s1 and s2 alphabetically, returns a negative value if s1 should precede s2, zero if s1 and s2 are equal, and a positive value if s2 should precede s1 in an alphabetized list. Example: char s1[10] = "Pusan", s2[10] = "Seoul"; if (strcmp(s1, s2) < 0)

45. Introduction to C L10.45 String Comparisons strcmp ("thrill", "throw"); returns a negative integer strcmp ("read", "readable); returns a negative integer strcmp ("shrimp", "crab"); returns a positive integer strcmp ("end", "end) returns zero

46. Introduction to C L10.46 String Comparison strncmp: Prototype: int strncmp(char *s1, char *s2, unsigned n); Purpose: Compares the first n characters of s1 and s2 returning positive, zero, and negative values as does strcmp. Example: strncmp ("read", "readable", 1); strncmp ("read", "readable", 2); strncmp ("read", "readable", 3); strncmp ("read", "readable", 4); All returns zero

47. Introduction to C L10.47 Scanning a Full Line With scanf, the strings containing spaces can not be input at once. C provides the gets function to read a line of text into a string. char string1[20]; gets(string1); The gets function waits for the user to input some text and finalized by the enter key. For example, if we type "text string" and press enter, string1 will be set to "text string" at once.

48. Introduction to C L10.48 Arrays of Strings Since a string is an array of characters, an array of strings is a two dimensional array of characters where each row is a string. char week[7][10] = {"Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday", "Sunday"}; printf("Today is %s.", week[0]); Monday

49. Introduction to C L10.49 Character Pointers char a[] = "Hello"; Hello\0 a: char *p = "Hello"; Hello\0 p:

50. Introduction to C L10.50 Arrays of Pointers to chars char s1[]="March"; char s2[]="April"; char s3[]="May"; char *string[3]; string[0] = s1; string[1] = s2; string[2] = s3; spring March\0 April\0 May\0

51. Introduction to C L10.51 Character Analysis In many string-processing applications, we need to know if a character is a letter, a digit or a punctuation mark. The library "ctype.h" defines such facilities: isalpha : If argument is a letter of the alphabet. isdigit : if argument is one of the ten decimal digits. islower ( isupper ): if argument is a lowercase (or uppercase) letter of the alphabet. ispunct : if argument is a punctuation character. ispace : if argument is a whitespace character such as a space, newline, or a tab.

52. The Heap / Dynamic Memory Allocation

53. Introduction to C L10.53 sizeof(...) function Using the sizeof function we can get the number of bytes consumed by instances of some datatype Syntax: sizeof( ); Example: printf("%d", sizeof(int));

54. Introduction to C L10.54 The Heap The Heap is a special area of memory that allows the dynamic allocation of memory during Runtime. Possible Application: Arrays with dynamic size, e.g. arrays where we know the size only during runtime (Remember: So far we have to specify the size (number of elements) of some array in the context of its declaration. E.g.: x[10])

55. Introduction to C L10.55 Heap Management in C Three functions for heap management defined in stdlib.h –calloc(n, el_size) – allocation of continuous space for an array of n elements with el_size bytes for each array element –malloc(size) – allocation of an area of size bytes in the heap –free(p) – freeing of allocated heap-space. (C stores internally the amount of memory associated with p )

56. Introduction to C L10.56 Warning: Dangling Pointer Problem Don’t use pointers that refer to some freed memory area Example: int *p; p = malloc(sizeof(int)); … free(p); … *p = 5; p is here a “dangling pointer”. This statement can create serious trouble.

57. Introduction to C L10.57 “void” pointers in C malloc returns a pointer of type void*. The type- checker of C is quite tolerant with this type of pointers. Example: The following code generates no error during compile time (not even a warning)! void* fun () { return NULL; } void main() { int *p; p = fun(); }