1

C-strings

String = null-terminated array of characters The null character ('\0') specifies where the string

terminates in memory. Example: The string "Hello" is represented as:

A string is accessed via a pointer to its first character.

'H' 'e' 'l' 'l' 'o' '\0'

2

C-strings

Declaring an initializing strings Using arrays:

char message[] = {'H', 'i', '!', '\0'}; char message[] = {"Hi!"}; char message[10]; cin >> message; // CAREFUL: make sure the array

is // large enough for the message

Using pointers const char *message = "Hi!"; char *message = new char[10]; cin >> message; // CAREFUL: make sure the array

is // large enough for the message

3

The cstring library

int strlen (const char *str); Returns the length of string str, without counting the

null character. Example:

Code snippet:

char *message = "Bye";int size = strlen(message);for (int i = 0; i < size; i++)

cout << *(message+i) << endl;

Execution:

[tlab-10] ./a.outBye[tlab-10]

4

The cstring library

char *strcpy (char *dest, const char *src); Copies string src into string dest and returns the

modified dest. CAREFUL: dest must be large enough to fit src. Example:

Code snippet:

char *dest = new char[20];strcpy(dest, "Hello");cout << dest << endl;char *src = "Bye";strcpy(dest, src);cout << dest << endl;

Execution:

[tlab-10] ./a.outHelloBye[tlab-10]

dest in memory(after second strcpy):

'B' 'y' 'e' '\0' 'l' 'o' '\0' ...?

generallyundefined

5

The cstring library

char *strncpy (char *dest, const char *src, int n); Similar to strcpy but only copies first n characters of

src into dest. CAREFUL: make sure dest is null-terminated after

strncpy. If n > strlen(src), the remainder of dest is padded

with nulls.

Code snippet:char *dest = new char[20];strcpy(dest, "Hello");cout << dest << endl;char *src = "Yap";strncpy(dest, src, 2);cout << dest << endl;dest[2] = '\0';cout << dest << endl;strncpy(dest, "Haha", 15);cout << dest << endl;

Execution:

[tlab-10] ./a.outHelloYalloYaHaha[tlab-10]

6

The cstring library

char *strcat (char *str1, const char *str2); Appends str2 to str1 (overwriting str1's null character),

appends a null character at the end and returns the modified str1

CAREFUL: make sure str1 is large enough. Example:

Code snippet:

char *dest = new char[20];strcpy(dest, "Hello");cout << dest << endl;char *src = " there";strcat(dest, src);cout << dest << endl;

Execution:

[tlab-10] ./a.outHelloHello there[tlab-10]

7

The cstring library

char *strncat (char *str1, const char *str2, int n); Similar to strcat but only appends first n characters

of str1 to str2. It again adds a null character at the end.

Example:Code snippet:

char *dest = new char[20];strcpy(dest, "Hello");cout << dest << endl;char *src = " there";strncat(dest, src, 4);cout << dest << endl;

Execution:

[tlab-10] ./a.outHelloHello the[tlab-10]

8

The cstring library

int strcmp (const char *str1, const char *str2); Compares str1 with str2 and returns an integer less

than, equal to or larger than 1 id str1 is less than, equal to or greater than str2.

The function compares the strings character by character until it finds two different characters. It returns a value based on which character has a higher ASCII value.

Example:Code snippet:

cout << strcmp("", "blue") << endl;cout << strcmp("purple", "orange") << endl;cout << strcmp("Ha", "Ha") << endl;cout << strcmp("Ho", "ho") << endl;

Execution:

[tlab-10] ./a.out-1101[tlab-10]

9

The cstring library

int strncmp (const char *str1, const char *str2, int n); Similar to strcmp but compares only the first n

characters. Example:Code snippet:

cout << strncmp("purple", "orange", 1) << endl;cout << strncmp("apple", "apply", 4) << endl;cout << strncmp("hem", "hem", 10) << endl;

Execution:

[tlab-10] ./a.out100[tlab-10]

10

The cstring library

char * strtok (char *str1, const char *delims ); This function can be used to break str1 into tokens (non-

empty subsets of str1 that do not contain any of the characters in delims).

The first call to strtok should have str1 as its first argument. Subsequent calls should replace this with NULL.

Each call returns a pointer to the next token, or NULL if no other tokens are found.

delims may be different in each call. How it works:

The last character of each token is a delimiter, but this is overwritten by a null character.

A pointer to the next character is saved for the next call to strtok.

11

The cstring library

char * strtok (char *str1, const char *delims ); Example

Code snippet:

char message[] = "Say the not-so-secret password";char *tokenPtr;

tokenPtr = strtok(message, " -"); while (tokenPtr != NULL) {

cout << tokenPtr << endl;tokenPtr = strtok(NULL, " -");

}

Execution:

[tlab-10] ./a.outSaythenotsosecretpassword[tlab-10]

if you put message here instead, the loop will printSay over and over (i.e. it will not terminate).

12

The cstring library

char * strchr (const char *str, char c ); Returns a pointer to the first occurrence of character c

in str, NULL if the character does not appear in the string.

Example

Code snippet:

char *ptr;if ( ptr = strchr ("apple", 'p') )

cout << ptr << endl;if ( !(ptr = strchr ("apple", 'b')) )

cout << "b is not in apple\n";

Execution:

[tlab-10] ./a.outppleb is not in apple[tlab-10]

13

The cstring library

char *strstr (const char *haystack, const char *needle); Returns a pointer to the first occurrence of string needle

in string haystack, NULL if the substring is not found. If needle is empty, the function returns haystack ExampleCode snippet:

char *ptr;if ( ptr = strstr ("attempted", "") )

cout << ptr << endl;if ( ptr = strstr ("attempted", "te") )

cout << ptr << endl;if ( ptr = strstr ("attempted", "ted") )

cout << ptr << endl;if (!(ptr = strstr ("pearl", "par")) )

cout << "par is not in pearl\n";

Execution:

[tlab-10] ./a.outattemptedtemptedtedpar is not in pearl[tlab-10]