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C-strings

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  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: Execution: [tlab-10] ./a.out B y e [tlab-10] Code snippet: char *message = "Bye"; int size = strlen(message); for (int i = 0; i < size; i++) cout << *(message+i) << endl;

  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.out Hello Bye [tlab-10] dest in memory (after second strcpy): 'B' 'y' 'e' '\0' 'l' 'o' '\0' ? ... generally undefined

  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.out Hello Yallo Ya Haha [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.out Hello Hello 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.out Hello Hello 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 0 if str1 is lexicographically 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: Execution: [tlab-10] ./a.out -1 1 0 1 [tlab-10] Code snippet: cout << strcmp("", "blue") << endl; cout << strcmp("purple", "zoo") << endl; cout << strcmp("Ha", "Ha") << endl; cout << strcmp("Ho", "ho") << endl;

  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: Execution: [tlab-10] ./a.out 1 0 0 [tlab-10] Code snippet: cout << strncmp("purple", "orange", 1) << endl; cout << strncmp("apple", "apply", 4) << endl; cout << strncmp("hem", "hem", 10) << endl;

  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.out Say the not so secret password [tlab-10] if you put message here instead, the loop will print Say 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.out pple b 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 • Example Code 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.out attempted tempted ted par is not in pearl [tlab-10]