strings, arrays, and pointers
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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). Reading Assignment. - PowerPoint PPT PresentationTRANSCRIPT
Strings, Arrays, and Pointers
CS-2301, B-Term 2009 1
Strings, Arrays, and Pointers
CS-2301, System Programmingfor Non-Majors
(Slides include materials from The C Programming Language, 2nd edition, by Kernighan and Ritchie and from C: How to Program, 5th and 6th editions, by Deitel and Deitel)
Strings, Arrays, and Pointers
CS-2301, B-Term 2009 2
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!
Strings, Arrays, and Pointers
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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
Strings, Arrays, and Pointers
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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
Strings, Arrays, and Pointers
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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
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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]
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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
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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
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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 <stdio.h>• 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 *
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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 <stdio.h>• 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
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Questions?
Strings, Arrays, and Pointers
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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
Strings, Arrays, and Pointers
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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"
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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'
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Support for Strings in C
• Most string manipulation is done through functions in <string.h>
• 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
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Support for Strings in C (continued)
• Examples (continued):–– int strcmp(char *s, char *t)
• lexically compares s and t, returns <0 if s < t, >0 ifs > 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
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Character functions in C
• See <ctype.h>, 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)
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String Conversion Functions in C
• See <stdlib.h>, p. 251-252double 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)
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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!
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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
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Static Data Allocation
0x00000000
0xFFFFFFFF
address space
program code(text)
static data
heap(dynamically allocated)
stack(dynamically allocated)
PC
SPThis is The Heap.
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Allocating Memory in The Heap
• See <stdlib.h>, p. 251-252void *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
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Allocating Memory in The Heap
• See <stdlib.h>, p. 251-252void *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
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Allocating Memory in The Heap
• See <stdlib.h>, p. 251-252void *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()
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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
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Typical usage of malloc() and free()
char *getTextFromSomewhere(…);
int main free(){char * txt;…;txt = getTextFromSomewhere(…);…;printf("The text returned is %s.", txt);free(txt);
}
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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()
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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
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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
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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!
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Questions?
Strings, Arrays, and Pointers
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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!
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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
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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);
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Questions?
Programming Assignment #4
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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)
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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
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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
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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)
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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
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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
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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!
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Questions?