Strings, Arrays, and Pointers CS-2301 D-term 20091 Strings, Arrays, and Pointers CS-2301 System Programming C-term 2009 (Slides include materials from

Strings, Arrays, and Pointers

CS-2301 D-term 2009 1


CS-2301 System Programming C-term 2009

(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)


CS-2301 D-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!


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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 oftype 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


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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


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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?


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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


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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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Segmentation fault and/or big-

time error if bad pointer


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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 *getText(…);

int main free(){char * txt;…;txt = getText(…);…;printf("The text returned is %s.", txt);free(txt);

}


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char *getText(…){char *t;...t = malloc(stringLength);...return t;

}


}

getText() creates a new string using malloc()


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}


}

Pointer to text is assigned to txt in calling function


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Usage of malloc() and free()


}


}

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?


CS-2301 D-term 2009 32

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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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 — Lab 4

• Instead of prompting user for # of items to read, simply take it from command line% lab4 20

would create an array of 20 items, read in twenty numbers from standard input, and store in the array.


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Example — Lab 4



argv[0] – name of program


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Example — Lab 4



argv[1] – first program

argument


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Example – Lab 4 (continued)

int main(int argc, char *argv[])){int nElements;double array[];if (argc <= 1) {

printf("Error: must specify " "number of elements in command line.\

n");return -1;

}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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n");return -1;





array of unknown size declared; no storage allocated


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n");return -1;





Convert argument #1 to int for # of elements


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n");return -1;





Allocate nElements of double,

store pointer in array


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n");return -1;





Be sure to free() the allocated array before exiting!


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Questions?

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Strings, Arrays, and Pointers CS-2301 D-term 20091 Strings, Arrays, and Pointers CS-2301 System Programming C-term 2009 (Slides include materials from