Welcome to Lesson 12!

Learning Objectives

By the end of today's class, you should know...

• What is a loop?
• What is a while loop?
• What is the syntax of a while loop?
• How does the syntax of a while loop compare to an if statement?
• How do you use a while loop to count?
• What are some common errors that are made when writing while loops?

Review Activity

Review of Conditions Involving Logical Operators:

a. if (numCaller = 19 && (pet = "rabbit" || pet = "cat" || pet = "dog") && (age >= 18 && age <= 65))

cout << "You are the winner!";

b. if (numCaller == 19 && (pet == "rabbit" || "cat" || "dog") && (age >= 18 && <= 65))
      cout << "You are the winner!";

c. if (numCaller == 19 || (pet == "rabbit" && pet == "cat" && pet == "dog") || (age >= 18 && age <= 65))
      cout << "You are the winner!";

d. if (numCaller == 19 && (pet == "rabbit" || pet == "cat" || pet == "dog") && (age >= 18 && age <= 65))
      cout << "You are the winner!";

e. if (numCaller == 19 && (pet == "rabbit" || pet == "cat" || pet == "dog") && (age >= 18 || age <= 65))

cout << "You are the winner!";

• What will the following print to the console?

int age = 20;

bool is_student = true;

if (is_student || age > 21) {

    cout << "Fi!";

}

if (!is_student && age < 21) {

    cout << "Fo!";

}

if (is_student && age < 21) {

    cout << "Fum!";

}

Bad Logical Operators Humor

Introducing Loops

• In your daily life, there are many actions that you repeat until some condition is met.
• For example: 
• You might run on the treadmill at the gym until 30 minutes are up.
• You might wash dishes until the pile of dirty dishes is gone.
• You might take classes every quarter until you receive your degree.
• Can you think of any others?
• When we write code, often times we find ourselves repeating statements.
• Loops allow us to carry out repetitive steps in our code without having to re-write the same statements over and over.
• These loops will run until some test condition is met, just like the examples above.
• You wrote your first loops when you did your Hour of Code lab assignment.
• When did you need loops in this assignment?

Simple Loops

• A loop is a block of code that can execute repeatedly
• Whether or not a program repeats a block of code is determined by a test condition
• The test condition is checked each time the loop executes
• In general, loops have three components:
  • Initialization code
  • Test condition -- evaluated during the loop
  • Loop body with some way to change the test condition
• There are three loop statements in C++:
  • for
  • while
  • do...while
• We will start with the simplest of the three -- the while statement

While Loops

initialization statement;

while (test){ 
 
    statements to repeat
}

string repeat = "y";
while ("y" == repeat) {

cout << "Do you want to play again? (y/n): "

cin >> repeat;

}

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#include <iostream>
using namespace std;

int main() {
    string repeat = "y";
    while ("y" == repeat) {
        cout << "\nPlaying an exciting game!\n";
        cout << "Do you want to play again? (y/n) ";
        cin >> repeat;
    }
    cout << "\nThanks for playing!\n";

    return 0;
}

NASA Mission Control readying for liftoff.

Initializing countdown from 10...

10

9

8

7

6

5

4

3

2

1

We have liftoff!

cout << countdown << endl;

countdown--;

• Note how the repeated code is indented inside the loop
• This lets us see easily which code is repeated and which is not
• Also note the placement of curly braces
• Different groups have different practices for placing curly braces in a loop statement

Applications of Loops

1. Indefinite Loops

Recall our looping application that simulated the play of an exciting game

#include <iostream>
using namespace std;

int main() {
    string repeat = "y";
    while ("y" == repeat) {
        cout << "\nPlaying an exciting game!\n";
        cout << "Do you want to play again? (y/n) ";
        cin >> repeat;
    }
    cout << "\nThanks for playing!\n";

    return 0;
}

• Loops of this type are called indefinite loops because you do not know in advance how many time the loop will execute
• This behavior is different from a counting loop where you know how many times the loop will execute before the loop starts
• With an indefinite loop we can solve a new set of problems
• Most problems solved with indefinite loops make use of while statements

• Another common use for indefinite loops is to process a sequence of inputs
• As an example, let us add up (sum) a series of numbers
• Every number is added to the sum
• We use a loop to repeat the input until the user decides to stop
• Since we do not know how many number the user will enter, we use an indefinite loop as shown below

#include <iostream>
using namespace std;

int main() {
    double input = 1;
    double sum = 0;
    string repeat = "y";

    cout << "I will add up numbers for you\n\n";
    while ("y" == repeat) {
        cout << "So far, sum = " << sum << endl;
        cout << "Enter a number: ";
        cin >> input;
        sum = sum + input;

        cout << "Another number? (y/n) ";
        cin >> repeat;
    }
    cout << "Ending sum: " << sum << endl;

    return 0;
}

3. Terminating the Input with a Sentinel

• Whenever we read a sequence of input values, we need to have some way of terminating the input loop
• We could use a separate variable and input statement as we have done before:

  string repeat = "y";
  while ("y" == repeat) {
      // ... statements to repeat
      cin >> repeat;
  }
  

• However, when entering numbers (or other data) repeatedly, answering an extra question each time through the loop becomes annoying
• One way to avoid asking an extra question is to use a sentinel value
• A sentinel is guard who watches for something to happen

        image source

• Similarly, a sentinel in a program watches for a specific sentinel value that signals termination of a loop
• To use a sentinel value, we must have a special value in the input
• Some commonly used sentinel values for numeric input are 0 or -1
• The following program is an update of the previous program to use a sentinel value to end the loop
• Question: What is the sentinel value for this loop?
  

Example Application Using a Sentinel Value for the Loop Test

#include <iostream>
using namespace std;

int main() {
    double input = 1;
    double sum = 0;

    cout << "I will add up numbers for you\n\n";
    while (input != 0) {
        cout << "So far, sum = " << sum << endl;
        cout << "Enter a number or 0 to exit: ";
        cin >> input;
        sum = sum + input;
    }
    cout << "Ending sum: " << sum << endl;

    return 0;
}

4. Error Checking: Input Validation

• Another common use for indefinite loops is input validation
• Input validation combines a loop with one or more if statements
• The input statement is placed inside the loop
• The if-statement tests for an incorrect input value
• The loop repeats while the user's input contains an error
• Since we do not know how many times the loop must execute ahead of time, the loop is indefinite
• For example, the following program uses a loop to ensure a user enters a positive number
• The if statement is used to decide when to output an error message

Example of Input Validation Using a Loop

#include <iostream>
using namespace std;

int main() {
    double input = 0.0; // initialize value
    while (input <= 0) {
        cout << "Enter a positive number: ";
        cin >> input;
        if (input <= 0.0) {
            cout << "You must enter a positive number\n";
        }
    }
    cout << "You entered: " << input << endl;

    return 0;
}

• We will explore all of these applications in our programs as we continue to write loops.

Activity 12.2: Guessing Game The Final Round (10 pts)

• Copy the following program into Eclipse, save it as loopy.cpp, and then compile and run the starter program to make sure you copied it correctly.

  #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;
  }
  

• Add the following code after the statement int guess = 0; and before the cout statement:

  string repeat = "y";
  

  This is the initialization code that we will use for the test condition that comes next.

• We want to repeat all the rest of the code in our program. For this we need to add a while statement such as:

  while ("y" == repeat) {
      // Place the rest of the code after the initialization
      // and before the return 0 between these curly braces.
  }
  

  Statements inside the curly braces repeat while the test condition in the parenthesis, ("y" == repeat), evaluates to true. 
  

• Inside the while loop we need some way to change the test condition. We change the test condition by letting the user enter a value for the repeat variable by adding the following code at the end of the loop just before the closing curly brace:

  cout << "\nDo you want to play again? (y/n) ";
  cin >> repeat;
  

  Without these two statements our loop would have no way to exit. A loop with no way to exit is known as an infinite loop. 
  

• Formatting a loop is important. Indent all the code within the curly braces of the while loop. 
  
• As a final part of our program, we add the infamous phrase: "Game Over". Add the following statement after the closing curly brace of the while loop:

  cout << "Game over\n";
  

• Compile and run your program again and verify the output looks like:

  I'm thinking of a number between 1 and 10.
  Can you guess it?
  
  Enter your guess: 3
  Sorry, that is not correct.
  Try again.
  
  Do you want to play again? (y/n) y
  I'm thinking of a number between 1 and 10.
  Can you guess it?
  
  Enter your guess: 7
  *** Correct! ***
  
  Do you want to play again? (y/n) n
  Game over
  

• Submit your program source code to Canvas.
• If you finish early, answer today's learning questions under Wrap Up or work on your Lab 6
  

Completed Program

• When finished, your application should look like the following. Note especially the extra indentation within the curly braces of the while loop.

Common Loop Pitfalls

• Loops have many pitfalls for the unwary
• The following are the most common problems you should look out for

Infinite Loops

• Common error: unintentional infinite loop
• For example, what is wrong with the following code?

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#include <iostream> using namespace std; int main() { string repeat = "y"; while ("y" == repeat) { cout << "\nPlaying an exciting game!\n"; cout << "Do you want to play again? (y/n) "; } cout << "\nThanks for playing!\n"; return 0; }

Missing Curly Braces

• Technically, the while loop executes a single statement after the parenthesis of the test condition
• However, we usually use curly braces { } to expand the single statement
• The curly braces let us put multiple statements in them
• For example, what is wrong with the following code?

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#include <iostream> using namespace std; int main() { string repeat = "y"; while ("y" == repeat) cout << "\nPlaying an exciting game!\n"; cout << "Do you want to play again? (y/n) "; cin >> repeat; cout << "\nThanks for playing!\n"; return 0; }

Empty Statements

• Remember that statements are terminated by a semicolon
• Is the following a legal statement?

  ;

• This is known as an empty or null statement
• Empty statements are a common source of infinite loops
• For example, what is wrong with the following code?

#include <iostream> using namespace std; int main() { string repeat = "y"; while ("y" == repeat); { cout << "\nPlaying an exciting game!\n"; cout << "Do you want to play again? (y/n) "; cin >> repeat; } cout << "\nThanks for playing!\n"; return 0; }