SYSC 2006 C Winter 2012 String Processing in C D.L. Bailey, - - PowerPoint PPT Presentation

String Processing in C

D.L. Bailey, Systems and Computer Engineering, Carleton University

SYSC 2006 C Winter 2012

• Hanly & Koffman, Chapter 9
• Some examples adapted from code in The C

Programming Language, Second Edition, Kernighan & Ritchie, Prentice Hall, 1988

References

• Understand how C implements character strings
• Look at a few string functions from the C

standard library (caller's view)

• Illustrate string processing algorithms by

reimplementing some of the standard library functions

Objectives

• Unlike C++, Java and Python, C does not have a

named string type ○ C++: type string ○ Java: type String ○ Python: type str

• C strings are implemented using arrays of

characters

String Types

• A sequence of characters in double quotes is a

string constant or string literal

• Example: "SYSC"
• Stored as an array of characters, terminated by

'\0' (the null character) ○ C compiler creates the array and initializes it

String Constants

• Note 1: '\0' is not the same as '0' (the

character zero)

• Note 2: number of elements in the array is 1

more than the number of chars between the quotes

• Adjacent string constants are concatenated at

compile time ○ "Hello, " "world!" and "Hello, world!" are equivalent

String Constants

• This declaration:

char dept[] = "SYSC"; allocates an array called dept, initialized with 5 chars: 'S', 'Y', 'S', 'C', '\0'

String Variables

• char dept[] = "SYSC";

is equivalent to: char dept[5]; dept[0] = 'S'; dept[1] = 'Y'; dept[2] = 'S'; dept[3] = 'C'; dept[4] = '\0';

String Variables

• We don't need to initialize all the elements in a

character array char dept[5]; dept[0] = 'I'; dept[1] = 'M'; dept[2] = 'D'; dept[3] = '\0';

• dept[4] is uninitialized (that's o.k., because the

string is properly terminated with '\0')

String Variables

• Can't assign a string literal to a character array
• This isn't permitted:

char dept[5]; ... dept = "SYSC"; // Error!

String Variables

• const qualifier tells the compiler that the array

elements should never be altered (compiler should flag any attempt to do so) const char dept[] = "SYSC";

String Variables

• C's operators are not overloaded to support string
• perations
• Example: in C++, Java and Python, + is the string

concatenation operator ○ In C, + cannot be used to concatenate two character strings

String Operations

• C standard library provides several functions that

provide common string operations

• Prototypes are found in <string.h>

<string.h>

• int strlen(const char s[]);
• Returns the length of its character string

argument, excluding '\0' #include <string.h> ... char greeting[] = "Hello"; int len; len = strlen(greeting); // returns 5 (not 6)

strlen

• int strcmp(const char s[],

const char t[]);

• Returns

○ negative value if s < t ○ 0 if s == t ○ positive value if s > t

strcmp

Example: char name1[30]; char name2[30]; // Initialization of name1 and name2 // not shown if (strcmp(name1, name2) != 0) { // strings are different }

strcmp

• char *strstr(const char s[],

const char t[]);

• Returns the location of substring t in string s as a

character pointer (we'll study pointers later)

• If substring t isn't found in s, returns the value

NULL ○ NULL is defined in several header files

• If all you need to know is whether or not t is in

s, but you don't care where, just check if the function returns NULL

strstr

Example: char phrase[] = "quick brown fox jumped";

if (strstr(phrase, "fox") == NULL) { printf("fox is not in the string"); } else { printf("fox is in the string"); }

• Output is: fox is in the string

strstr

• char *strcpy(char s[],

const char t[]); ○ ignore char * return type for now

• Copies all chars in t to s, including '\0'

○ Programmer is responsible for ensuring that s is big enough to hold all chars copied from t

strcpy

• char *strcat(char s[],

const char t[]); ○ ignore char * return type for now

• Concatenates t to end of s, including '\0'

○ Programmer is responsible for ensuring that s is big enough to hold all chars copied from t

strcat

• Loop over the string, counting characters until we

reach null int CU_strlen(const char s[]) { int i = 0; while (s[i] != '\0') { i = i + 1; } return i; }

A strlen Implementation

• Loop, comparing the two strings on a character-

by-character basis, until we find two characters that differ

• Calculate the difference of those two characters

to determine if 1 string is > or < than the other

• If while looping we reach the end of both strings

before finding chars that differ, the two strings are equal

A strcmp Implementation

int CU_strcmp(const char s[], const char t[]) { int i; for (i = 0; s[i] == t[i]; i = i + 1) { if (s[i] == '\0') return 0; } // i is first pos'n where s and t differ return s[i] - t[i]; }

A strcmp Implementation

• Loop over the source string, copying chracters

into the destination string, until we reach the end of the source string

• Null terminate the destination string

A strcpy Implementation

void CU_strcpy(char s[], const char t[]) { int i = 0; while (t[i] != '\0') { s[i] = t[i]; i = i + 1; } // Terminate s s[i] = '\0'; }

A strcpy Implementation

• Loop over the destination string, until we find

null

• Loop over the source string, copying chracters

into the destination string, until we reach the end of the source string ○ 1st character copied from source overwrites null in destination

• Null terminate the destination string

A strcat Implementation

void CU_strcat(char s[], const char t[]) { int i, j; for (i = 0; s[i] != '\0'; i = i + 1) ; // find end of s // Copy t to s, except for null for (j = 0; t[j] != '\0'; j = j + 1) { s[i] = t[j]; i = i + 1; } s[i] = '\0'; // Terminate s }

A strcat Implementation