strings
today: what are strings and why to use them reading: textbook chapter 8
what are strings
a string in C++ is one of a special kind of data type called a class we will talk more about classes in detail at the end of the term but note that we have already used two classes when we covered les: the ifstream class and the ofstream class a class is a compound data type, unlike the simple, native data types weve already discussed (e.g., int, char, bool, double, etc) a class has members: it has data elds and functions
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strings: declaring and initializing
strings are declared like this: string s where s is a variable whose data type is a string you can set the value of the string using the assignment operator and double quotes ("): s = "hello"; NOTE that you use single quotes for char values and double quotes for string values: char c = A; string s = "hello"; ALSO NOTE that when you use the string class, you also need to include the string header le: #include <string> using namespace std;
strings: output
we have already used strings for output, e.g.: cout << "hello" << endl; but we have not yet output variables that were declared as strings but we can... its just like this: #include <iostream> #include <string> using namespace std; int main() { string s = "hello"; cout << s << endl; } // end of main()
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strings: input
there are two ways to read input values from the keyboard (or from a le) into a string variable: (1) using the >> operator (2) using the getline() function the rst way, using the >> operator, will only read until the rst whitespace character is read (the term whitespace refers to characters like blank spaces, tabs and newlines) when reading a string variable using the >> operator, the input will stop as soon as the rst whitespace character is read example (on the next page):
#include <iostream> #include <string> using namespace std; int main() { string s; cout << "please enter your name:"; cin >> s; cout << "s = " << s << endl; } // end of main() if the user enters david ortiz when the program asks please enter your name: then the value of s will be david
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HOWEVER, when reading a string variable using the getline() function, the input will stop as soon as the rst newline character is read (i.e., the user hits the ENTER key on the keyboard), e.g.: #include <iostream> #include <string> using namespace std; int main() { string s; cout << "please enter your name:"; getline( cin, s ); cout << "s = " << s << endl; } // end of main() here, if the user enters david ortiz when the program asks please enter your name: then the value of s will be david ortiz
note that the getline() function can optionally take a third argument, a delimiter this would be for cases where you wanted to stop the input not at whitespace or at a newline, but at some other character called the delimiter suppose you wanted to enter several commands for a robot to follow using one string, like this: forward|wait|backward|turn left|stop you could do this using the | (vertical bar) character as the delimiter and the getline() function with three arguments, e.g.: string s; cout << "enter some commands: "; getline( cin, s, | ); If the user entered forward|backward when prompted by the above program, the value of s would be forward, since the input would stop at the delimiter (|). You would have to call getline( cin, s, | ); again to read the next command (e.g., wait).
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strings: operators
there are several operators that work with strings the plus sign (+) is the concatenation operator, e.g.: string s1, s2, s3; s1 = "david "; s2 = "ortiz"; s3 = s1 + s2; After the above code fragment, the value of s3 will be david ortiz the comparison operators also work with strings (==, <, <=, >, >=)
the double equals sign (==) compares the value of two strings and returns true if they are the same, e.g.: string s1, s2, s3; bool a1, a2; s1 = "david "; s2 = "ortiz"; s3 = "david "; a1 ( = s1 == s2 ); a2 = ( s1 == s3 ); After the above code fragment: the value of a1 will be false and the value of a2 will be true
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the inequality operators (<, <=, >, >=) perform a lexical comparison between two strings a lexical comparison is like checking if two strings are in alphabetical order: one is less than the other if it comes before the other alphabetically EXCEPT, the lexical comparison is case sensitive and uses the ASCII table, which means that all the upper case letters (A..Z) come before (are less than) all the lower case letters (a..z), e.g.: string s1, s2, s3; bool a1, a2; s1 = "ABC"; s2 = "DEF"; s3 = "abc "; a1 = ( s1 < s2 ); a2 = ( s3 < s2 ); After the above code fragment: the value of a1 will be true because "ABC" < "DEF" and the value of a2 will be false because "abc" > "DEF"
strings: indexes
a string is like an array of char so you can use the index of the individual characters of the string just like you can use the indexes of the individual elements of an array, like the arrays of ints you created for the last homework assignment if you have: string s = then: s[0] is s[1] is s[2] is s[3] is s[4] is "ortiz"; assigned the assigned the assigned the assigned the assigned the
value value value value value
o (the letter oh) r t i z
you can also use the member function at() to nd the value of an individual character of a string e.g., instead of using s[3], you can use s.at(3)
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strings: length
if you have: string s = "ortiz"; then the length of the string is 5 there are two member functions of the string class that will tell you the length of a string: length() and size() (they do the same thing) you call them like this: string s1; int n1, n2; s1 = "ortiz"; n1 = s1.length(); n2 = s1.size(); After this code fragment, the value of n1 will be 5 and so will the value of n2
strings: searching
the find() member function is used to locate a substring within a primary string the function returns the value of the index in the primary string at which the substring starts, if the substring exists in the primary string; or else the function returns the constant string::npos for example: string s1 = "david ortiz"; int n1, n2; n1 = s1.find( "avid", 0 ); n2 = s1.find( "ask", 0 ); After the above code fragment: the value of n1 will be 1 the value of n2 will be string::npos the rst argument to the find() function is the substring to search for the second ar...