lesson11

Welcome to Lesson 11!

Learning Objectives

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

What are some of the mistakes you could make when you write your conditional statements involving logical operators?
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?

Announcements

Quiz 4 on Thursday
Return midterms before the break.
Excellent midterm results!

As: 25 <-- 4 100% scores
Bs: 8
Cs: 4
Ds: 1
Fs: 3

Lab 6 due Friday at midnight

Pre-Assessment Activity

Review of Numbers, Operators and Precision:

With a partner: Write three equivalent statements to the one below, using the shortcuts described last class:

x = x + 1;

With a partner: Write one statement to print the following value to the console to 5 decimal places:

const double PI = 3.1415927;

Review of Conditions Involving Logical Operators:

Imagine that a local radio station is holding a contest to give away a year's supply of free pet food to one lucky winner.
To win the contest, you must meet the following criteria:

You must be caller 19
The pet your own must be one of the following: dog, cat, rabbit
You must be between the ages of 18 and 65.

Which of the following test conditions will correctly determine the winner of the contest

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!";

}

Wrapping Up Conditionals

Conditional Pitfalls

Unfortunately, you can write many things in C++ that should be incorrect but end up working for some obscure reason
This means that you can code something that should create an error message but does not
Thus, a program may compile and run with no error messages but still be wrong
Since you may not realize that it is wrong, it can be hard to find and correct these types of errors

Strings of Inequalities

One common mistake is to use = when you meant to use ==

For example, look at the test condition in the following code:

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

Notice that the condition is really an assignment statement and not a test
You would think that it would fail to compile -- but it does not
However, it will not work as you might expect
A way to prevent this type of problem is to reverse the order of your test condition:
```
if (7 = guess) {
```

Now the compiler will give you an error message and your code will not compile:

guess.cpp: In function `int main()":
guess.cpp:10: error: non-lvalue in assignment

However, if you correctly use == then your code will compile
```
if (7 == guess) {
```

Strings of Inequalities

Do NOT use a string of inequalities like the following:

int a = 5, b = 1, c = 10;
if (a < b < c) {
    cout << "b is between a and c\n";
} else {
    cout << "b is NOT between a and c\n";
}

Your code may compile and run but give incorrect results
The test condition is evaluated by the computer from left to right
The first condition is a < b which evaluates to 0 (false)
The second condition is then 0 < c which evaluates to 1 (true)
Since the whole test condition evaluates to true you get an incorrect result

Instead, the correct way is to use && as follows:

int a = 5, b = 1, c = 10;
if (a < b && b < c) {
    cout << "b is between a and c\n";
} else {
    cout << "b is NOT between a and c\n";
}

Strings of Logical Operators

Logical expressions often read like "normal" English.
However, C++ requires more exactness than English

For example, the following code will compile and run but give wrong results:

int guess;
cout << "Enter a guess: ";
cin >> guess;
if (guess == 7 || 8) {
    cout << "*** Correct! ***\n";
} else {
    cout << "Sorry, that is not correct.\n";
}

The test condition is evaluated by the computer from left to right
The left hand side is (guess == 7) which can evaluate to either true or false
The right hand side is 8, which is interpreted as true by C++
Since (something or true) is always true, then the test condition always evaluates to true
Instead, the correct way is to use || as follows:

int guess;

cout << "Enter a guess: ";
cin >> guess;
if (guess == 7 || guess == 8) {
    cout << "*** Correct! ***\n";
} else {
    cout << "Sorry, that is not correct.\n";
}

We will work more with test conditions and logical operators as we write programs containing loops.

Hand Back and Go Over Midterms

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

The simplest looping structure is the while statement
A while statement has a test condition and a body, like an if statement
Before each execution of the loop body, the while statement checks the test condition
If the test condition evaluates to true, then our program executes the body of the loop
When the program reaches the end of the loop body, it automatically returns to the while statement
Here is the syntax of a while loop:

initialization statement;

while (test){

statements to repeat
}

For example:

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

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

cin >> repeat;

}

What does the while loop syntax remind you of?
The following flow chart shows how the while loop operates

Diagram of while Loop Operation

Image source.

Let's look at an example.

Understanding the `while` Loop

The following looping application simulates 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;
}

Notice the structure of the code for the looping application

First was the statement to initialize the looping condition:

string repeat = "y"; //gives the variable its starting value

Then came the while statement with the test condition
```
while ("y" == repeat) 
```
The loop executes the body of the loop if and only if the condition evaluates to true
Another important part of the loop is the update statement:

cin >> repeat; //changes the value of the variable

Most loops have these parts:
- Initialization statement
- Test condition
- Update statement (allows the loop the option to fail)
Question: What would happen if I replaced the while loop with an if statement?
We will explore the while loop more in the next exercise.

Group Activity: Halloween Candy

As a class, we are going to act out the following while loops

While Loop 1:

int num_pieces_candy = 10;

instructor.fill_with_candy(Bag, num_pieces_candy);

instructor.hand_bag(first_student);

while (num_pieces_candy > 0)
{
   student.takeOnePieceCandy();
   student.passBag(next_student);
   num_pieces_candy--;
}

With your partner, discuss for the while loop above:

What is/are the initialization statement(s)?
What is the loop test condition?
What are the statements that repeat?
Which statement will ultimately cause the test condition to fail?

Now, let's act out While Loop 1!

While Loop 2

instructor.fill_with_candy(Bag);

instructor.hand_bag(first_student);

while (Bag != empty)

{

student.takeOnePieceCandy();

student.passBag(next_student);

}

With your partner, discuss for the while loop above:

How is the above while loop different from while loop 1
How many times will the statements inside while loop 2 execute?
Were you able to state an exact number for while loop 2?

Now, let's act out While Loop 2!

Activity 11.1: Counting Down...

Find a partner for pair programming.
Open up CodeBlocks and create a new C++ file named countdown.cpp.
Imagine we are at NASA Mission Control.
A new space ship is about to be launched.
Let's write a program that counts down until take off. See the sample output below:

NASA Mission Control readying for liftoff.

Initializing countdown from 10...

10

9

8

7

6

5

4

3

2

1

We have liftoff!

We are going to write this program in two ways.

Part 1: Repetition, Repetition, Repetition (10 pts)

The first program should use 13 cout statements to achieve the above output.
Let's get to work!
Add your name and section in appropriate block comments, include the iostream library and use the standard namespace.
Don't forget a main() function.

Declare an integer variable called countdown at the top of main and set it equal to 10.
Now, write a cout statement to output the following:

NASA Mission Control readying for liftoff.

Add another cout statement to print the following to the console:

Initializing countdown from 10...

Next, in a pair of statements, lets print the contents of countdown and then decrease its value by 1.
Add the following two statements to your code:

cout << countdown << endl;

countdown--;

You will need to repeat these two lines of code 10 times in your program.
Copy the two lines and then paste them into your program 9 more times.
You should have a very repetitive program by now!
Finally, add a cout statement to print out "We have liftoff!" below the numbers.
Run your code to make sure that your output is identical to my output above.
When you are finished and the program is working properly, upload it to Catalyst.

Part 2: Using a While Loop (10 pts)

Let's alter the above program to be written more efficiently with a loop.
First, take a look at your code.
Which part of the code is repetitive?
Hint: The repetitive lines will go inside the curly braces of the while loop.
We need a while loop to count down from the number entered by the user. Create a while loop that looks like this:

while (test will go here) {

//statements
}

What should go inside the test?

countdown > 0

Why do you use > 0 here?
Inside the curly braces of the while loop, we need to print out the contents of the countdown variable and then decrease its value by 1.
Take one of your pairs of cout statement and variable decrement and place it in the curly braces of the while loop.
Now, you can erase the other 9 pairs of statements.
Finally, below your while loop, there should now be only one cout statement
Run your program to make sure it is working the same as before.
When you are finished, upload this new version to Catalyst.
If you finish early, work on your Lab 6 or answer the question from today's Wrap Up

Proper While Loop Formatting

About those Curly Braces

Like the if-statement, the curly braces of a while loop are optional
Technically, the while statement affects only the single statement that follows
We use curly braces to make that one statement into a block of statements
This allows us to put any number of statements within the body
Curly braces are not always required, but the best practice is to always include them

Program Style: Indent Statements Inside a Loop

It is important to format the loop code correctly:

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

Also Accepted Style:

string repeat = "y";

while ("y" == repeat)
{

// ... statements to repeat

cin >> repeat;
}

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

Wrap Up

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 loop below (hint it is missing something!). What will happen if you run the code BEFORE making the correction?

string repeat = "y";

while (repeat == "y") {

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

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

}

Answer the questions from today's learning objectives.

Upcoming Assignments

Assignment 11 due Thursday at 3:20pm
Quiz 4 on Thursday
Lab 6 due Friday at midnight

~See You Thursday!~

De Anza College CIS

Navigation

Pre-Assessment Activity

What will the following print to the console?

Wrapping Up Conditionals

Strings of Inequalities

Strings of Inequalities

Strings of Logical Operators

Introducing Loops

Simple Loops

Diagram of `while` Loop Operation

Let's look at an example.

Understanding the `while` Loop

Group Activity: Halloween Candy

About those Curly Braces

Program Style: Indent Statements Inside a Loop

lesson11

Pre-Assessment Activity

What will the following print to the console?

Wrapping Up Conditionals

Strings of Inequalities

Strings of Inequalities

Strings of Logical Operators

Introducing Loops

Simple Loops

Diagram of while Loop Operation

Let's look at an example.

Understanding the while Loop

Group Activity: Halloween Candy

About those Curly Braces

Program Style: Indent Statements Inside a Loop

Diagram of `while` Loop Operation

Understanding the `while` Loop