1. Strings, Arrays, and Pointers CS-2301, B-Term 20091 Strings, Arrays, and Pointers CS-2301, System Programming for Non-Majors (Slides include materials from The C Programming Language, 2 nd edition, by Kernighan and Ritchie and from C: How to Program, 5 th and 6 th editions, by Deitel and Deitel)

2. Strings, Arrays, and Pointers CS-2301, B-Term 20092 Reading Assignment Kernighan & Ritchie, Chapter 5 –All the way through! Study §5.5 in detail — pages 104-107 –Character pointer and functions –You will use this all the time! Study §5.10 — pages 114-118 –Command line arguments –You will use this a lot!

3. Strings, Arrays, and Pointers CS-2301, B-Term 20093 Review Array – a set of elements all of the same type stored contiguously in memory – e.g., –int A[25]; // 25 integers –struct Str B[15]; /* 15 objects of type struct Str */ –double C[]; /* indeterminate # of doubles */ Pointer – a variable whose value is the location of some other object –float *p; // pointer to a float

4. Strings, Arrays, and Pointers CS-2301, B-Term 20094 Review (continued) int A[25]; // 25 integers Type of A[i] is int (for all i ). Type of A is int * I.e., pointer to int

5. Strings, Arrays, and Pointers CS-2301, B-Term 20095 Review (continued) ' & ' operator generates a pointer to its operand – e.g., –int n; –Type of &n is int * Unary ' * ' operator dereferences a pointer – i.e., accesses the thing pointed to – e.g., –int *p; –Type of *p is int

6. Strings, Arrays, and Pointers CS-2301, B-Term 20096 Review (continued) Arrays and pointers are closely related Let int A[25]; int *p; int i, j; Let p = A; Then p points to A[0] p + i points to A[i] &A[j] == p+j *(p+j) is the same as A[j]

7. Strings, Arrays, and Pointers CS-2301, B-Term 20097 Review (continued) If void f(int A[], int arraySize); Then f(&B[i], bSize-i) calls f with subarray of B as argument Starting at element i, continuing for bSize-i elements

8. Strings, Arrays, and Pointers CS-2301, B-Term 20098 Review (concluded) void f(int A[], int arraySize); and void f(int *A, int arraySize); are identical! Most C programmers use pointer notation rather than array notation In these kinds of situations

9. Strings, Arrays, and Pointers CS-2301, B-Term 20099 Additional Notes A pointer is not an integer … Not necessarily the same size May not be assigned to each other … except for value of zero! Called NULL in C; defined in Means “pointer to nowhere” void * is a pointer to no type at all May be assigned to any pointer type Any pointer type may be assigned to void *

10. Strings, Arrays, and Pointers CS-2301, B-Term 200910 Additional Notes A pointer is not an integer … Not necessarily the same size May not be assigned to each other … except for value of zero! Called NULL in C; defined in Means “pointer to nowhere” void * is a pointer to no type at all May be assigned to any pointer type Any pointer type may be assigned to void * Defeats type-checking in the compiler A easy way to get into big trouble Absolutely necessary in most large C programs

11. Strings, Arrays, and Pointers CS-2301, B-Term 200911 Questions?

12. Strings, Arrays, and Pointers CS-2301, B-Term 200912 Characters in C char is a one-byte data type capable of holding a character Treated as an arithmetic integer type (Usually) unsigned May be used in arithmetic expressions Add, subtract, multiply, divide, etc. Character constants 'a', 'b', 'c', … 'z', '0', '1', … '9', '+', '-', '=', '!', '~', etc, '\n', '\t', '\0', etc. A-Z, a-z, 0-9 are in order, so that arithmetic can be done

13. Strings, Arrays, and Pointers CS-2301, B-Term 200913 Strings in C Definition:– A string is a character array ending in '\0' — i.e., –char s[256]; –char t[] = "This is an initialized string!"; –char *u = "This is another string!"; String constants are in double quotes May contain any characters Including \" and \' — see p. 38, 193 String constants may not span lines However, they may be concatenated — e.g., "Hello, " "World!\n" is the same as "Hello, World!\n"

14. Strings, Arrays, and Pointers CS-2301, B-Term 200914 Strings in C (continued) Let –char *u = "This is another string!"; Then –u[0] == 'T' u[1] == 'h' u[2] == 'i' … u[21] == 'g' u[22] == '!' u[23] == '\0'

15. Strings, Arrays, and Pointers CS-2301, B-Term 200915 Support for Strings in C Most string manipulation is done through functions in String functions depend upon final '\0' So you don’t have to count the characters! Examples:– –int strlen(char *s) – returns length of string Excluding final '\0' –char* strcpy(char *s, char *ct) – Copies string ct to string s, return s s must be big enough to hold contents of ct ct may be smaller than s –See pp. 105-106 for various implementations

16. Strings, Arrays, and Pointers CS-2301, B-Term 200916 Support for Strings in C (continued) Examples (continued) :– –int strcmp(char *s, char *t) lexically compares s and t, returns 0 if s > t, zero if s and t are identical (p. 106) –char* strcat(char *s, char *ct) Concatenates string ct to onto end of string s, returns s s must be big enough to hold contents of both strings! Other string functions –strchr(), strrchr(), strspn(), strcspn() strpbrk(), strstr(), strtok(), … –pp. 249-250

17. Strings, Arrays, and Pointers CS-2301, B-Term 200917 Character functions in C See, p. 248-249 These return or false (0) or true (non-zero) int isdigit(int c)int isalpha(int c) int isalnum(int c)int isxdigit(int c) int islower(int c)int isupper(int c) int isspace(int c)int iscntrl(int c) int ispunct(int c)int isprint(int c) int isgraph(int c) These change case (if appropriate) int toupper(int c)int tolower(int c)

18. Strings, Arrays, and Pointers CS-2301, B-Term 200918 String Conversion Functions in C See, p. 251-252 double atof(const char *s) int atoi(const char *s) long atol(const char *s) double strtod(const char *s, char **endp) long strtol(const char *s, char **endp, int base) unsigned long strtoul(const char *s, char **endp, int base)

19. Strings, Arrays, and Pointers CS-2301, B-Term 200919 Dilemma Question:– –If strings are arrays of characters, … –and if arrays cannot be returned from functions, … –how can we manipulate variable length strings and pass them around our programs? Answer:– –Use storage allocated in The Heap!

20. Strings, Arrays, and Pointers CS-2301, B-Term 200920 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

21. Strings, Arrays, and Pointers CS-2301, B-Term 200921 Static Data Allocation 0x00000000 0xFFFFFFFF address space program code (text) static data heap (dynamically allocated) stack (dynamically allocated) PC SP This is The Heap.

22. Strings, Arrays, and Pointers CS-2301, B-Term 200922 Allocating Memory in The Heap See, p. 251-252 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

23. Strings, Arrays, and Pointers CS-2301, B-Term 200923 Allocating Memory in The Heap See, p. 251-252 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 Segmentation fault and/or big- time error if bad pointer

24. Strings, Arrays, and Pointers CS-2301, B-Term 200924 Allocating Memory in The Heap See, p. 251-252 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 free() knows size of chunk allocated by malloc() or calloc()

25. Strings, Arrays, and Pointers CS-2301, B-Term 200925 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

26. Strings, Arrays, and Pointers CS-2301, B-Term 200926 Typical usage of malloc() and free() char *getTextFromSomewhere(…); int main free(){ char * txt; …; txt = getTextFromSomewhere(…); …; printf("The text returned is %s.", txt); free(txt); }

27. Strings, Arrays, and Pointers CS-2301, B-Term 200927 Typical usage of malloc() and free() char * getTextFromSomewhere(…){ char *t;... t = malloc(stringLength);... return t; } int main free(){ char * txt; …; txt = getTextFromSomewhere(…); …; printf("The text returned is %s.", txt); free(txt); } getTextFromSomewhere() creates a new string using malloc()

28. Strings, Arrays, and Pointers CS-2301, B-Term 200928 Typical usage of malloc() and free() char * getTextFromSomewhere(…){ char *t;... t = malloc(stringLength);... return t; } int main free(){ char * txt; …; txt = getTextFromSomewhere(…); …; printf("The text returned is %s.", txt); free(txt); } Pointer to text is assigned to txt in calling function

29. Strings, Arrays, and Pointers CS-2301, B-Term 200929 Usage of malloc() and free() char *getText(…){ char *t;... t = malloc(stringLength);... return t; } int main free(){ char * txt; …; txt = getText(…); …; printf("The text returned is %s.", txt); free(txt); } main() must remember to free the storage pointed to by txt

30. Strings, Arrays, and Pointers CS-2301, B-Term 200930 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 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!

31. Strings, Arrays, and Pointers CS-2301, B-Term 200931 Questions?

32. Strings, Arrays, and Pointers CS-2301, B-Term 200932 String Manipulation in C Almost all C programs that manipulate text do so with malloc ’ed and free ’d memory No limit on size of string in C Need to be aware of sizes of character arrays! Need to remember to free storage when it is no longer needed Before forgetting pointer to that storage!

33. Strings, Arrays, and Pointers CS-2301, B-Term 200933 Input-Output Functions printf(const char *format,...) Format string may contain %s – inserts a string argument (i.e., char * ) up to trailing '\0' scanf(const char *format,...) Format string may contain %s – scans a string into argument (i.e., char * ) up to next “white space” Adds '\0' Related functions fprintf(), fscanf() – to/from a file sprintf(), sscanf() – to/from a string

34. Strings, Arrays, and Pointers CS-2301, B-Term 200934 Example Hazard char word[20]; …; scanf("%s", word); scanf will continue to scan characters from input until a space, tab, new-line, or EOF is detected An unbounded amount of input May overflow allocated character array Probable corruption of data! scanf adds trailing '\0 ' Solution:– scanf("%19s", word);

35. Strings, Arrays, and Pointers CS-2301, B-Term 200935 Questions? Programming Assignment #4

36. Strings, Arrays, and Pointers CS-2301, B-Term 200936 Command Line Arguments See §5.10 By convention, main takes two arguments:- int main(int argc, char *argv[]); –argc is number of arguments –argv[0] is string name of program itself –argv[i] is argument i in string form i.e., i < argc –argv[argc] is null pointer! Sometimes you will see (the equivalent) int main(int argc, char **argv); An array of pointers to char (i.e., of strings)

37. Strings, Arrays, and Pointers CS-2301, B-Term 200937 Example — PA #4 Instead of prompting user for # of items to read, simply take it from command line % statistics 1000 would create an array of 1000 items, play 1000 games and store the results of those games in the array. argv[0] – name of program

38. Strings, Arrays, and Pointers CS-2301, B-Term 200938 Example — PA #4 (continued) Instead of prompting user for # of items to read, simply take it from command line % statistics 1000 would create an array of 1000 items, play 1000 games and store the results of those games in the array. argv[1] – first program argument

39. Strings, Arrays, and Pointers CS-2301, B-Term 200939 Example — PA #4 (continued) int main(int argc, char *argv[])){ int nElements; double array[]; if (argc <= 1) { printf(“Enter number of games:- "); scanf(“%d”, &nElements); } else nElements = atoi(argv[1]); array = calloc(nElements, sizeof double); /* rest of program using array[i] */ free(array); return 0; }//main(argc, argv)

40. Strings, Arrays, and Pointers CS-2301, B-Term 200940 Example — PA #4 (continued) int main(int argc, char *argv[])){ int nElements; double array[]; if (argc <= 1) { printf(“Enter number of games:- "); scanf(“%d”, &nElements); } else nElements = atoi(argv[1]); array = calloc(nElements, sizeof double); /* rest of program using array[i] */ free(array); return 0; }//main(argc, argv) array of unknown size declared; no storage allocated

41. Strings, Arrays, and Pointers CS-2301, B-Term 200941 Example — PA #4 (continued) int main(int argc, char *argv[])){ int nElements; double array[]; if (argc <= 1) { printf(“Enter number of games:- "); scanf(“%d”, &nElements); } else nElements = atoi(argv[1]); array = calloc(nElements, sizeof double); /* rest of program using array[i] */ free(array); return 0; }//main(argc, argv) Convert argument #1 to int for # of elements Allocate nElements of double, store pointer in array

42. Strings, Arrays, and Pointers CS-2301, B-Term 200942 Example — PA #4 (continued) int main(int argc, char *argv[])){ int nElements; double array[]; if (argc <= 1) { printf(“Enter number of games:- "); scanf(“%d”, &nElements); } else nElements = atoi(argv[1]); array = calloc(nElements, sizeof double); /* rest of program using array[i] */ free(array); return 0; }//main(argc, argv) Be sure to free() the allocated array before exiting!

43. Strings, Arrays, and Pointers CS-2301, B-Term 200943 Questions?