Welcome to Lesson 8!

• How do you handle decision points involving exactly two alternatives using an else clause?
  
• How do you compare data of the char and string types?
• How to make decisions when there are multiple alternatives?
• What is the proper ordering of if statements when order of the statements matter?

Announcements

• Midterm 1 one week from today
  
• Study over your old quizzes, in-class review activities and homework
• On material through if statements 
• Midterm review posted as homework tonight (practice quiz)
• Material from Lesson 2 through if statements (Lesson 8)
  
• Quiz 3 after the break
  
• Don't forget Lab 4 due Friday!
  

Review Activity

With a partner, answer the following questions:

1. Write one line of code to declare a String variable (name of your choice) and assign it the value $.

2. Write one line of code to declare a char variable (name of your choice) and assign it the value of $.

3. What does the following output to the console:

string info = "Why hello, I'm Emily!";

cout << info.length() << ": " << info.substr(1,1) << info.substr(11, 1) << info.substr(14,3);

4. Write an if statement to check if the value stored in the variable age is equal to 5. If so, print out the age.

Using if-else Statements

• Sometimes we want to choose between two actions
• If a condition is true
  • then do this
• Otherwise it is false
  • so do something else
• To make this type of selection we use an if...else statement
• Syntax:

  if (test) {
     statements1
  } else {
     statements2
  }
  

• Where:
  • test: the test condition to evaluate
  • statementsX: the statements to execute depending on the test

(from http://www.tutorialspoint.com/cplusplus/cpp_if_else_statement.htm)

• For example:

  if (7 == guess) {
      cout << "*** Correct! ***\n";
  } else {
      cout << "Sorry, that is not correct.\n";
      cout << "Try again.\n";
  }
  

• ****Note that there is no test condition for the else clause****
• The decision on which set of statements to use depends on only one condition
• Note that you could write an if-else as a pair of complementary if statements instead, like:

  if (7 == guess) {
      cout << "*** Correct! ***\n";
  }
  if (7 != guess) {
      cout << "Sorry, that is not correct.\n";
      cout << "Try again.\n";
  }
  

• However, it is easier and clearer to write an if-else statement instead
• For clarity, write the if and else parts on different lines than the other statements
• Also, indent the other statements
• You can see an example of an if-else statement in the following example

Example Program With an if-else Statement

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#include <iostream>
using namespace std;


int main() {
    int guess = 0;
    cout << "I'm thinking of a number between"
         << " 1 and 10.\nCan you guess it?\n\n"
         << "Enter your guess: ";
    cin >> guess;

    if (7 == guess) {
        cout << "*** Correct! ***\n";
    } else {
        cout << "Sorry, that is not correct.\n";
        cout << "Try again.\n";
    }
    return 0;
}

Comparing Characters and Strings

• Character data can be evaluated using relational operators as well
• Comparing characters works because C++ stores characters as numbers using ASCII codes
• Note that letters nearer to the start of the alphabet have lower numerical values
• Thus a numerical comparison can decide the alphabetical order of characters

Example Program Comparing Characters

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#include<iostream>
using namespace std;

int main() {
    cout << boolalpha; // output true or false
    cout << "'A' < 'B': " << ('A' < 'B') << endl;
    cout << "'A' > 'B': " << ('A' > 'B') << endl;
    cout << "'A' <= 'Z': " << ('A' <= 'Z') << endl;
    cout << "'X' >= 'Y': " << ('X' >= 'Y') << endl;
    cout << "'X' == 'X': " << ('X' == 'X') << endl;
    cout << "'X' != 'X': " << ('X' != 'X') << endl;
}

Comparing Strings

• We can compare strings using relational operators as well
• C++ compares two strings using lexicographical order (a.k.a. alphabetic order)
• For example, "car" is less than "cat":

  c	a	r
  c	a	t

• Also, "car" is less than "card"

  c

  a

  r

  c

  a

  r

  d

Example Program Comparing Strings

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#include<iostream>
using namespace std;

int main() {
    string s1, s2;
    cout << "Enter two strings: ";
    cin >> s1 >> s2;
    cout << boolalpha; // output true or false
    cout << s1 << " <= " << s2 << " : " << (s1 <= s2) << endl;
    cout << s1 << " > " << s2<< " : " << (s1 > s2) << endl;
}

Activity 8.2: Let's Alphabetize! (10 pts)

• Open up Eclipse and create a new C++ project Alphabetize with a file called alphabetize.cpp.
• Our program will take in two string inputs from the user, compare then and the output the two strings in alphabetical order.
• At the top of your program, declare a string variables called word1.

• Now declare a second string variable called word2.
• Next, write a cout statement welcoming your user to the program and letting them know that this program will alphabetize two words.

cout << "Welcome! Give me two words and I will return them to you in alphabetical order!\n";

• Run your program to make sure it is giving you the output you expected.
• Let's prompt the user for the first word and store the result as word1.

cout << "Please enter the first word: ";
cin >> word1;

• Do the same for the second word.
• Now, let's create an if-else statement to determine the ordering of the two words. And, then output the result to our user. The if-else statement will need to use string comparison as discussed above.

• The code that you have written could be a useful part of a larger program. 
  
• Both partners should submit to Canvas when finished.

• Assignment 8 due Tuesday
  
• Lab 4 due Friday at midnight