lesson9

Welcome to Lesson 9!

Learning Objectives

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

How to make decisions when there are multiple alternatives?
What do you do when order matters when making decisions?

Announcements

Midterm 1 after the a short lesson and our break
Next quiz one week from Thursday
Yes, there is homework for tonight!

Making Decisions Continued

"Nesting" if Statements

The inner if statement is evaluated only if the test condition of the outer if test first evaluates to true
The following code shows an example of a nested if statement

Example Showing a Nested `if` Statement

#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 (guess != 7) {
        if (guess < 7) {
            cout << "Your guess is too low.\n";
        } else {
            cout << "Your guess is too high.\n";
        }
    } else {
        cout << "*** Correct! ***\n";
    }

    return 0;
}

Nesting in the `else` Clause

You can also nest if statements in the else clause
When used this way, the computer can make only one selection
As soon as a condition is found to be true, the rest of the selections are ignored
The following code shows an example of a nested else if statement

For example:

if (guess < 7)
{
    cout << "Your guess is too low.\n";
}
else if (guess > 7)
{
    cout << "Your guess is too high.\n";
}
else
{
    cout << "*** Correct! ***\n";
}

The trick in understanding this type of logic is to remember:
- You start at the top
- The computer makes only one selection
- Once the selection is made and processes, the computer skips the rest of the options

Example Showing a Nested `else if` Statement

#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 (guess < 7) {
        cout << "Your guess is too low.\n";
    } else if (guess > 7) {
        cout << "Your guess is too high.\n";
    } else {
        cout << "*** Correct! ***\n";
    }

    return 0;
}

Programming Style: Indentation of if-else-if Statements

Note the alignment of the nested statements below:

if (guess < 7)
{
    cout << "Your guess is too low.\n";
}
else
{
    if (guess > 7)
    {
        cout << "Your guess is too high.\n";
    }
    else
    {
        cout << "*** Correct! ***\n";
    }
}

The above style is WRONG

Instead, we use:

if (guess < 7)
{
    cout << "Your guess is too low.\n";
}
else if (guess > 7)
{
    cout << "Your guess is too high.\n";
}
else
{
    cout << "*** Correct! ***\n";
}

This shows more clearly that we are making a single choice among multiple alternatives
Also, it prevents indentations from cascading to the right as we add more selections

Multiple Alternatives

By using collections of if-else statements, a program can distinguish between multiple alternatives

Choosing Between Alternatives

This program has five alternatives to choose from:
1. "penny"
2. "nickel"
3. "dime"
4. "quarter"
5. erroneous input
Note that the order that the alternatives are checked is unimportant
We can follow the alternatives in the flowchart shown below

Flowchart of Multiple Alternative for Converting Coins to Values

For example, consider the following example where a user enters the name of a coin, and the program displays the value:

Program Converting Coin Names to Values

#include <iostream>
using namespace std;

int main() {
    cout << "Enter coin name: ";
    string name;
    cin >> name;
    double value = 0;

    if (name == "penny") {
        value = 0.01;
    } else if (name == "nickel") {
        value = 0.05;
    } else if (name == "dime") {
        value = 0.10;
    } else if (name == "quarter") {
      value = 0.25;
    } else {
        cout << name << " is not a valid coin name\n";
    }
    cout << "Value = " << value << "\n";

    return 0;
}

Activity 9.1: Grades (10 pts)

We want to write a program to calculate a student's letter grade according to the following table:

Numerical Grade Letter Grade
greater than or equal to 90 A
less than 90 but greater than or equal to 80 B
less than 80 but greater than or equal to 70 C
less than 70 but greater than or equal to 60 D
less than 60 F

Find a partner for pair programming.
Copy the following program into a text editor, save it as grader.cpp. Change the comments, then compile and run the starter program to make sure you copied it correctly.

/*

* Name 1

* Name 2

* Section

*/

#include <iostream>
using namespace std;

int main() {
    // Enter your code here

    return 0;
}

Add code to get user input into a variable named score of type double. When you run the program after adding this code, the output should look like:
```
Enter a score: 95.7
```
Make sure you declare the variable with a compatible data type. Note that the underlined numbers above shows what theuser enters and is not part of your code.
First we will look at a series of if statements and see that if statements alone are not enough to solve this problem. Copy the following into your program after the input statements:
Compile and run your modified program. There is a logic problem with this code. Each test condition needs to work over a range of values rather than with a single value.
One way to correct the problem is to nest an if statement inside of another if statement. To see how this works, modify your code to add nested if statements as shown below:
To test the range, the outer if statement tests the lower condition and the inner if statement tests the upper condition.

string grade;

if (score >= 90)
{

grade = "A";

}

if (score >= 80)
{

if (score < 90)

grade = "B";

}

if (score >= 70)
{

if (score < 80)

grade = "C";

}

if (score >= 60)
{

if (score < 70)

grade = "D";

}

if (score < 60)
{

grade = "F";

}

cout << grade << endl;

What is another way to fix this problem that we have already seen?
Perhaps the most elegant solution is to nest an if statement in the else clause of the preceding if. Modify the series of ifstatements to include an else clause as shown below:

string grade;

if (score > = 90)
{

grade = "A";

}

else if (score >= 80)
{

grade = "B";

}

//you fill in the statements here

else
{

grade = "F";
}

cout << grade << endl;

We are nesting if statements in the else clause. Nesting in the else clause makes each test condition of the if statement exclusive of the others because each test condition eliminates all the preceding conditions.

Compile and run your modified program to make sure you made the changes correctly. When you run the program, the output should look like:
```
Enter a score: 80
B
```
Run your program a few times with different score to verify that any score displays the correct letter grade.
When finished, upload your program to Catalyst.

When Order Matters

In some cases, the order of the tests is important
For example, look at the following program from the textbook that displays a description of the likely impact of an earthquake based on its magnitude on the Richter scale

Program Showing Multiple Alternatives Where Order Matters

#include <iostream>
using namespace std;

int main() {
    cout << "Enter a magnitude on the Richter scale: ";
    double richter;
    cin >> richter;

    if (richter >= 8.0) {
        cout << "Most structures fall\n";
    } else if (richter >= 7.0) {
        cout << "Many buildings destroyed\n";
    } else if (richter >= 6.0) {
        cout << "Many buildings considerably damaged, "
             << "some collapse\n";
    } else if (richter >= 4.5) {
        cout << "Damage to poorly constructed buildings\n";
    } else if (richter >= 3.5) {
        cout << "Felt by many people, no destruction\n";
    } else if (richter >= 0) {
        cout << "Generally not felt by people\n";
    } else {
        cout << "Negative numbers are not valid\n";
    }

    return 0;
}

Order Is Important

Note that the order of the tests is important to ensure that the right results are printed
If we rearranged the order of the if-else statements, we would get the wrong results

For example, if we reversed the order of the tests:

if (richter >= 0) { // tests in wrong order
    cout << "Generally not felt by people\n";
} else if (richter >= 3.5) {
    cout << "Felt by many people, no destruction\n";
} else if (richter >= 4.5) {
    cout << "Damage to poorly constructed buildings\n";
} else if (richter >= 6.0) {
    cout << "Many buildings considerably damaged, "
         << "some collapse\n";
} else if (richter >= 7.0) {
    cout << "Many buildings destroyed\n";
} else if (richter >= 8.0) {
    cout << "Most structures fall\n";
} else {
    cout << "Negative numbers are not valid\n";
}

This does not work because all values meet the first condition
Every other test will never be attempted

Importance of Using `if-else-if` Structure

Note that we cannot remove the else portion of the structure like shown below:

if (richter >= 8.0) { // Does not use else
    cout << "Most structures fall\n";
}
if (richter >= 7.0) {
    cout << "Many buildings destroyed\n";
}
if (richter >= 6.0) {
    cout << "Many buildings considerably damaged, "
         << "some collapse\n";
}
if (richter >= 4.5) {
    cout << "Damage to poorly constructed buildings\n";
}
if (richter >= 3.5) {
    cout << "Felt by many people, no destruction\n";
}
if (richter >= 0) {
    cout << "Generally not felt by people\n";
}
if (richter < 0) {
    cout << "Negative numbers are not valid\n";
}

The conditions must be exclusive and we need the else-if conditions to ensure exclusivity
Independent if statements may cause a single input to print several messages

Wrap up

Review today's learning objectives

Midterm 1

Upcoming Assignments

Assignment 9 due Thursday at 3:20pm
Lab 5 due this Friday

~See You Thursday!~

Example Showing a Nested if Statement

Nesting in the else Clause

Example Showing a Nested else if Statement