Welcome to Lesson 12!

Learning Objectives

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

• What are 4 applications of while loops?
  
• What are some common errors that are made when writing while loops?
  
• What is a counting loop?
• What are the three important components of a counting loop?
• What is a for loop and why was it designed?
• How does a for loop differ from a while loop?
• What is an off-by-one error and how can you avoid this type of error?

Announcements

• Quiz 4 after the break

Review Activity

With a partner, answer the following questions:

• Label the different parts of the following while loop as: update statement, initialization or test condition.

int count = 1;

while (count <= 10) {

    cout << count << endl;

    count++;

}

• Correct the loops below. What will happen if you run the code BEFORE making the corrections?

Loop 1:

string repeat = "y";

while (repeat == "y") {

    cout << "Playing an exciting game!\n\n";

    cout << "Want to play again? (y/n): ";

Loop 2:

    int counter = 1;

  while (counter <= 10) {

      cout << counter << endl;

  }

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 12.1: Guessing Game Redux (10 pts)

• Copy the following program into a text editor, 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 Catalyst.
• 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?

1 2 3 4 5 6 7 8 9 10 11 12 13

#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?

1 2 3 4 5 6 7 8 9 10 11 12 13

#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; }

Counting Loops

• Counting is a common task in our lives.
• In programming, counting is also very common.
• In programming we often use loops to count when we know exactly how many times we want a specific piece of code to repeat.
• In fact, counting is such a common programming task, that a special type of loop was designed specifically for counting.
• This loop is called a for loop, and we will be learning more about it in class today.

//counting from 10 to 20 using a while loop

int i = 10; 

while (i <= 20)

{

    cout << i << endl;

    i++;

}

//counting from 10 to 20 using a for loop

for (int i = 10; i <= 20; i++)

{

    cout << i << endl;

}