Ch 8 Characters and Strings Timothy Budd Characters

Ch 8. Characters and Strings Timothy Budd

Characters and Literals Strings • Char in C++ is normally an 8 -bit quantity, whereas in Java it is a 16 -bit value. • Char can be declared either as signed or unsigned. • wchar_t represents a wide character, 16 -bit character. 2

Character Literals and the cctype Library isalpha(c) isupper(c) islower(c) isdigit(c) isxdigit(c) isalnum(c) isspace(c) isprint(c)} True if c is alphabetic True if c is upper case True if c is lower case True if c is decimal digit char True if c is hexidecimal digit True if c is alphabetic or numeric True if c is whitespace (space, tab or newline) True if c is a printable character (8 bit only) 3

String Literals • A literal string value has type array of character in C++, whereas it has type string in Java. • Always remember the null character at the end of a string literal. 4

char * text = "A Literal Text"; int vowel. Count (char const * p) { // procedure to count vowels in text int sum = 0; while (1) { // will break out of loop in switch statement switch (*p++) { // case '': return sum; case 'a': case 'e': case 'i': case 'o': case 'u': sum++; break; } } return sum; } 5

The cstring Library • defined by header file string. h, should not be confused with the header file string. strcpy(dest, src) strncpy(dest, src, n) strcat(dest, src) strncat(dest, src, n) strcmp(s 1, s 2) strncmp(s 1, s 2, n) strlen(s) Copy characters from source to destination Copy exactly n characters Append characters from source onto destination Append only n characters Compare strings s 1 and s 2 Compare first n characters in s 1 and s 2 Count number of characters in s 6

The cstring Library • Comparison between pointer values is determined by the relative placement in memory of the locations being pointed to. • To determine a lexicographic comparison of two character pointer values, an explicit comparison function must be used. • If the first string is lexicographically smaller, return less than zero, if equal, returns zero, 7 otherwise larger than zero.

int strcmp (const unsigned char * p, const unsigned char * q) { while (*p && (*p == *q)) { p++; q++; } return *p - *q; } char buffer[20]; char * text = "literal"; strcpy(buffer, literal); // copy literal into buffer for (int i = 0; i < 5; i++) strcat(buffer, literal); // append literal to buffer char buffer[7]; strcpy (buffer, "literal"); // error! - copies eight values, not seven 8

Constant and Mutable Values • String literals in Java are immutable, the same is not true in C++. char * text = "literal "; // note space at end text[0] = 'm'; // change l to m text[2]--; // t becomes s strcpy (text + 6, "ble"); // replace l with ble 9

Constant and Mutable Values • Immutable strings can be formed with the const modifier. • But, this modifier can imply multiple meanings, depending on its placement, and the actual meaning may not be intuitive. 10

Constant and Mutable Values • Placing the const at the front: cannot be modified, but it can be reassigned to a new constant string: const char * a = "literal"; a[2] = 'z'; // error -- cannot modify a a = "new literal"; // ok -- can change to new // constant value 11

Constant and Mutable Values • Placing the const in the middle: the pointer cannot be changed, but the value it references can. char * const b = "literal"; b[2] = 'z'; // ok -- can change what it points to b = "new literal"; // error -- cannot change pointer itself 12

Constant and Mutable Values • A value cannot be changed and be reassigned, can only be formed by using two occurrences of the const modifier: const char * const c = "literal"; c[2] = 'z'; // error -- cannot be modified c = "new literal"; // error -- cannot change pointer itself 13

The string Data Type • The string data type is a recent addition to C++ and is still not widely used. • The string abstraction is similar to combination of the String and String. Buffer data types in Java. 14

string a; string b = "initial text"; string c("more text"); string d(b); // copy of b a = "some text"; // assignment to a • Subscript index values are not checked for validity. // see if string b is prefix to a if (a. substr(0, b. length()) == b). . . 15

Table 8. 1 Comparison of string functionality in C++ and Java Operation C++ Java Number of Characters length () String. length() Readjust length resize (newsize, pad) String. Buffer. set. Length(newsize) Assign s = s 2 Append s +=s 2 String. Buffer. apend(s 2) Catenation s + s 2 String. concat(s 2) Character access s[index] String. char. At(index) Insertion insert(location, s 2) String. Buffer. insert(location, s 2) Removal remove(location, length) Replacement replace(location, length, s 2) Substring substring(location, length) Comparison S < s 2 Comparison strcmp(s. c_str, s 2. c_str) String. compare. To(s 2) Equality comparison s == s 2 String. equals(s 2) Substring search find(text, start) String. index. Of(s 2) Character search find_first_of(text, start) String. substring(start, end) 16

Example - Split a Line into Words void split (const string & text, const string & separators, list<string> & words) // split a string into a list of words text and separators are input, // list of words is output { int text. Len = text. length(); // find first non-separator character int start = text. find_first_not_of(separators, 0); // loop as long as we have a non-separator character while ((start >= 0) && (start < text. Len)) { // find end of current word int stop = text. find_first_of(separators, start); // check if no ending character if ((stop < 0) || (stop > text. Len)) stop = text. Len; // add word to list of words. push_back (text. substr(start, stop - start)); // find start of next word start = text. find_first_not_of (separators, stop+1); } } 17

int main() { string text = "it was the best of times, it was the worst of times. "; list<string> words; string separators = ". , !? : "; split(text, separators, words); string smallest = words. front(); string largest = words. front(); list<string>: : iterator current; list<string>: : iterator stop = words. end(); for (current = words. begin(); current != stop; ++current) { if (*current < smallest) smallest = *current; if (largest < *current) largest = *current; } cout << "smallest word " << smallest << endl; cout << "largest word " << largest << endl; return 0; } 18