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CSCI-1200 Computer Science II — Spring 2006
Lecture 3 — Strings
Review from Lectures 1 and 2
- Standard library and I/O streams
- Expressions and statements
- Variables, objects, types
- Loops (for & while)
- If-else conditionals, logic, switch statements
- Arrays
- Functions and parameter passing (value & reference)
- Scope
- Algorithm analysis and order notation
What’s Next? C++ classes and the C++ standard library
- Strings
- Vectors
- Classes
- Lists and iterators
- Maps
It will take us several weeks to cover all of this material, but by the time we are done you will have much of the preparation you need to solve challenging computational problems.
Readings for Today
- Accelerated C++ (Koenig and Moo): Chapters 1 and 2
- C++ Programming (Malik): pp 399-
3.1 Example from Chapter 1: Strings
// ask for a person’s name, and generate a framed greeting #include #include
int main() { std::cout << "Please enter your first name: "; std::string name; std::cin >> name;
// build the message that we intend to write const std::string greeting = "Hello, " + name + "!";
// build the second and fourth lines of the output const std::string spaces( greeting.size(), ’ ’ ); const std::string second = "* " + spaces + " *";
// build the first and fifth lines of the output const std::string first(second.size(), ’*’);
// write it all std::cout << std::endl; std::cout << first << std::endl; std::cout << second << std::endl; std::cout << "* " << greeting << " *" << std::endl; std::cout << second << std::endl; std::cout << first << std::endl;
return 0; }
3.2 About string Objects
- A string is an object type defined in the standard library to contain a sequence of characters.
- The string type, like all types (including int, double, char, float), defines an interface, which includes construction (initialization), operations, functions (methods), and even other types(!).
- When an object is created, a special function is run called a “constructor”, whose job it is to initialize the object. The greeting example code exhibits three ways of constructing string objects: - By default to create an empty string - With a specified number of instances of a single char - From another string
- The notation
greeting.size()
is a call to a function size that is defined as a member function of the string class. There is an equivalent member function called length.
- Input to string objects through streams includes the following steps:
- The computer inputs and discards white-space characters, one at a time, until a non-white-space character is found.
- A sequence of non-white-space characters is input and stored in the string. This overwrites anything that was already in the string.
- Reading stops either at the end of the input or upon reaching the next white-space character (without reading it in).
- The (overloaded) operator ’+’ is defined on strings. It concatenates two strings to create a third string, without changing either of the original two strings.
- The assignment operation ’=’ on strings overwrites the current contents of the string.
This seems like a lot to remember. Do I need to memorize this? Where can I find all the details on string objects?
3.6 Example Problem: Writing a Name Along a Diagonal
We will spend the rest of lecture on the following problem: read in a name and then write it along a diagonal, framed by asterisks. Here’s how the program might behave:
What is your first name? Peter
We will start by solving a simpler versions and then look at two ways to solve the whole problem.
3.7 Exercises: Writing the Name Diagonally
Write a program to output the name diagonally, so that the program interaction looks like this:
What is your first name? Juanita J u a n i t a
Here is a start to the code:
#include #include
using std::cin; using std::cout; using std::endl; using std::string;
int main() { cout << "What is your first name? "; string first; cin >> first;
// Fill in here...
return 0; }
You may need to use nested for loops here, although it is possible to use just a single loop if you exploit properties of the string class.
3.8 Thinking About the Whole Problem
Now we are better prepared to address the original problem.
- Here are the two main difficulties:
- Making sure that we can put the characters in the right places on the right lines.
- Getting the asterisks in the right positions and getting the right number of blanks on each line.
- In the previous example, we addressed a simplified version of the first. With that practice, we are ready to try the full problem, and we will look at two different ways of solving it.
- It is good practice in general to start by solving simpler problems that address the core issue (or issues) in a larger problem.
3.9 Basic Approach for 1st Solution
- Initial stuff: read the name, and output a blank line, as in an earlier example.
- Let’s think about the main output: think of the output region as a grid of rows and columns:
- How big is this region?
- What gets output where?
- This leads to an implementation with two nested loops, and conditionals used to guide where char- acters should be printed.
3.10 1st Solution
// Program: diagonal_name // Author: Chuck Stewart // Purpose: A program that outputs an input name along a diagonal.
#include #include using namespace std;
int main() {
cout << "What is your first name? "; string first; cin >> first;
const string star_line( first.size()+4, ’’ ); const string blanks( first.size()+2, ’ ’ ); const string empty_line = ’’ + blanks + ’*’;
cout << endl << star_line << endl << empty_line << endl;
3.12 L-Values and R-Values
string a = "Kim"; string b = "Tom"; a[0] = b[0];
String a is now "Tim".
- No big deal, right? Wrong!
- Let’s look closely at the line:
a[0] = b[0];
and think about what happens.
- In particular, what is the difference between the use of a[0] on the left hand side of the assignment statement and b[0] on the right hand side?
- Syntactically, they look the same. But,
- The expression b[0] gets the char value, ’T’, from string location 0 in b. This is an r-value
- The expression a[0] gets a reference to the memory location associated with string location 0 in a. This is an l-value.
- The assignment operator stores the value in the referenced memory location.
The difference between an r-value and an l-value will be especially significant when we get to writing our own operators.
- Anyone seen the error message: “non-lvalue in assignment”?
std::string foo = "hello"; foo[2] = ’X’; cout << foo; ’X’ = foo[3]; cout << foo;
3.13 Ideas for a 2nd Solution
Here are ideas for a second solution:
- Think about what changes from one line to the next.
- Suppose we had a “blank line” string, containing only the beginning and ending asterisks and the intervening blanks.
- We could overwrite the appropriate blank character, output the string, and then restore the blank character.
3.14 Exercise: Finish the 2nd Solution
#include #include
using std::cin; using std::cout; using std::endl; using std::string;
int main() { cout << "What is your first name? "; string first; cin >> first;
const string star_line( first.size()+4, ’’ ); const string blanks( first.size()+2, ’ ’ ); const string empty_line = ’’ + blanks + ’*’; string one_line = empty_line;
cout << endl << star_line << endl << empty_line << endl;
// Fill in here....
cout << empty_line << endl << star_line << endl;
return 0; }
3.15 Thinking About Problem Solving
- We began by working on simplified versions of the problem to get a “feel” for the core issues.
- Then we introduced two solution approaches:
- Thinking of the output as a two-dimensional grid and using logical operations to figure out what to output at each location.
- Thinking of the output as a series of strings, one string per line, and then thinking about the differences between lines.
- There are often many ways to solve a programming problem. Sometimes you can think of several, while sometimes you struggle to come up with one.
- When you have finished a problem or when you are thinking about programming examples, it is useful to think about the core ideas used. If you can abstract and understand these ideas, you can later apply them to other problems.
