Computer Science 1620

Computer Science 1620 Strings

Programs are often called upon to store and manipulate text • word processors • email • chat • databases • webpages • etc

In C++, we refer to sequence of characters as strings • we've been using strings since the beginning of the semester • cout << "Hello world!" << endl;

String storage in C++ • two ways • 1) A character array (C-strings) • 2) The string type • are these the same? • similar, except the string type is much more flexible than the character array

C-Style Strings • C Programmers did not have the string type • to use a string, they had to use a character array • it is useful to be able to use character arrays • you may not always have the string type • you will learn to appreciate the string type

How is a C-string stored? • as an array of chars • index 0 holds first character • index 1 holds second character • … char x[12]; x[0] = 'H'; x[1] = 'e'; x[2] = 'l'; x[3] = 'l'; x[4] = 'o'; x[5] = ' '; x[6] = 'W'; x[7] = 'o'; x[8] = 'r'; x[9] = 'l'; x[10] = 'd'; x[11] = '!'; 'H' 'e' 'l' 'l' 'o' ' ' 'w' 'o' 'r' 'l' 'd' '!' x[0] x[1] x[2] x[3] x[4] x[5] x[6] x[7] x[8] x[9] x[10] x[11] How does C/C++ know how big the string is?

Null-termination • end-of-string symbolic constant • shows program the end of the string • corresponds to value 0 • not character '0', this is value 48 • numeric 0 has no corresponding character counterpart • can be specified in a number of ways • '\0' • 0

char x[13]; x[0] = 'H'; x[1] = 'e'; x[2] = 'l'; x[3] = 'l'; x[4] = 'o'; x[5] = ' '; x[6] = 'W'; x[7] = 'o'; x[8] = 'r'; x[9] = 'l'; x[10] = 'd'; x[11] = '!'; x[12] = 0; Array must be big enough to store all characters plus the terminating null character. 'H' 'e' 'l' 'l' 'o' ' ' 'w' 'o' 'r' 'l' 'd' '!' 0 x[0] x[1] x[2] x[3] x[4] x[5] x[6] x[7] x[8] x[9] x[10] x[11] x[12]

Initializing character arrays • character arrays can be initialized in the same ways as other arrays • C++ allows you to initialize character arrays using a stringliteral • Advantages • automatically puts null at the end • easier to type • Must be big enough to store literal char x[13] = {'H','e','l','l','o',' ','W','o','r','l','d','!', '\0'}; char y[] = {'K', 'e', 'v', 'i', 'n', '\0'}; char x[13] = "Hello world!"; char y[] = "Kevin"; char x[10] = "Hello world!"; // error

String Output • cout is equipped to handle null-terminated character array • outputs each character until it finds the terminating null char x[13] = "Hello world!"; char y[] = "Kevin"; cout << x << endl; cout << y << endl;

String Input • cin is equipped to handle null-terminated character array • programmer must "guess" at a good size of array, to accommodate input • powers of 2 are common char x[256]; cin >> x; cout << x << endl;

Line Input • what is the output of the following program? • assume I type "Kevin John Grant" at the prompt int main() { char name[256]; cout << "Please enter your full name: "; cin >> name; cout << "Name: " << name << endl; return 0; }

cin data tokenized by white space. How do we read in the entire line?

getline • use the cin.getline function • takes (up to) three parameters • 1) the character array to read into • 2) the maximum # of characters you wish to read • typically, the size of the array • 3) an optional delimiting (stopping) character • if omitted, default is '\n' (newline)

Line Input • what is the output of the following program? • assume I type "Kevin John Grant" at the prompt int main() { char name[256]; cout << "Please enter your full name: "; cin.getline(name, 256, '\n'); cout << "Name: " << name << endl; return 0; }

Line Input • previous program could also have been written: since no third parameter included, assumes '\n' int main() { char name[256]; cout << "Please enter your full name: "; cin.getline(name, 256); cout << "Name: " << name << endl; return 0; }

Advantages of Character Arrays • easy to use • setting/retrieving characters can be done using the array indexing syntax • char str[256]; • str[5] = 'K'; • cout << str[6]; • C++ has some conventions for making strings easier to handle • placing string literals in char arrays • handling char array input and output

Disadvantages of Character Arrays • 1) Static Size • the programmer must decide beforehand how much room to give a character array • this leads to two problems: • a) too much space • b) too little space • example: write a program that takes in a name of a person, and prints that name to the screen

Example Program • how big do we make our character array? int main() { char name[???]; cout << "Please enter your name: "; cin.getline(name, ???); cout << "Your name is: " << name << endl; }

Example Program • how big do we make our character array? • how about 10? int main() { char name[10]; cout << "Please enter your name: "; cin.getline(name, 10); cout << "Your name is: " << name << endl; }

Example Program • how big do we make our character array? • to ensure correctness, better make it 50 int main() { char name[50]; cout << "Please enter your name: "; cin.getline(name, 50); cout << "Your name is: " << name << endl; }

Here, we are wasting 44 characters.

Disadvantages of Character Arrays • 2) Difficult to modify • suppose I have a string that contains "Saskatoon" • I want to change that string to "Lethbridge" • how do I do it?

Example Program • how do I make the new string hold "Lethbridge"? int main() { char city[50] = "Saskatoon"; }

Example Program • how do I make the new string hold "Lethbridge"? int main() { char city[50] = "Saskatoon"; city = "Lethbridge"; // will this work? }

Example Program • we can only assign a string literal to a character array upon declaration int main() { char city[50] = "Saskatoon"; // this works city = "Lethbridge"; // this doesn't }

The String type • a programmer-defined type • not part of the actual C++ syntax • rather, defined in a library (like cmath) • must include <string> to use this type

Advantages of String type over char array • do not need to declare a size • the string will "resize" itself as necessary • you will learn how this is accomplished in future courses • can reassign a string after it’s been declared • just like an atomic variable

Example: #include<iostream> #include<string> using namespace std; int main() { string city = "Saskatoon"; cout << city << endl; city = "Lethbridge"; cout << city << endl; return 0; } String size not specified. Strings can be assigned new strings.

String Storage • the string type stores its characters in an array • however, it does not store the null-terminating character • rather, it stores the size (# of characters) of the string • however, these details are transparent to the user

String Operations • 1) Output • strings can be output using cout • the value sent to output will be whatever value is stored in the string #include<iostream> #include<string> using namespace std; int main() { string city = "Saskatoon"; cout << city << endl; // outputs Saskatoon city = "Lethbridge"; cout << city << endl; // outputs Lethbridge return 0; }

String Operations • 2) Input • strings can be input using cin • the value received by the string will be whatever the user types at the prompt #include<iostream> #include<string> using namespace std; int main() { string city; cout << "Where are you from: "; cin >> city; cout << "You are from " << city << endl; return 0; }

String Operations • 2) Input • the string type also allows line input • use the getline function • note: not cin.getline • takes three parameters • cin • the string variable you are sending the value to • an optional delimiting character ('\n' by default) #include<iostream> #include<string> using namespace std; int main() { string city; cout << "Where are you from: "; getline(cin, city, '\n'); cout << "You are from " << city << endl; return 0; }

String Operations • 2) Input • remember that if you want to read until a carriage return, the '\n' is optional #include<iostream> #include<string> using namespace std; int main() { string city; cout << "Where are you from: "; getline(cin, city); cout << "You are from " << city << endl; return 0; }

String Operations • 3) Assignment • a value can be assigned to a string using the assignment (=) operator • very flexible in what you can assign to a string variable • a string literal • a null-terminated character array • a single character • another string

#include<iostream> #include<string> using namespace std; int main() { string str; str = "Hi"; cout << str << endl; char ca[] = "How are ya?"; str = ca; cout << str << endl; str = 'K'; cout << str << endl; string str2 = "Goodbye"; str = str2; cout << str << endl; return 0; }

String Operations • 4) Comparison • we can use a string with one of the relational operators (==, !=, <, <=, >, >=) • compared letter by letter (as discussed previously) • very flexible in what you can compare to a string variable • a string literal • a null-terminated character array • another string variable

#include<iostream> #include<string> using namespace std; int main() { cout << boolalpha; string str = "Hi"; cout << (str == "Hi") << endl; char ca[] = "How are ya?"; cout << (str > ca) << endl; string str2 = "Hello"; cout << (str != str2) << endl; return 0; }

String Operations • 5) Character Access • with character arrays (C-style strings), we could get any character we wished using the subscript operator char x[] = "Kevin"; cout << x[2] << endl; // outputs v • the string type gives us exactly the same feature string x = "Kevin"; cout << x[2] << endl; // outputs v x[0] = 'S'; x[3] = 'e'; cout << x << endl; // outputs Seven

String Operations • 6) Size • to obtain the size of a string, affix a .length() or .size() to the end of the variable name • these types of function calls (class function calls) will be explained later on in this course string x = "Kevin"; cout << x.size() << endl; // outputs 5 x = "Lethbridge"; cout << x.length() << endl; // outputs 10

String Operations • 7) Concatenation • we can concatenate (fuse into one string) two strings using the + operator • the + operator allows several types to be concatenated with a string variable • a string literal • a character array • a single character • another string

Computer Science 1620