## Learning Objectives

By the end of today's class, you should know...
• What are the 3 logical operators?
• How do you test multiple conditions using logical operators?
• How do && and || operate when:
• both test conditions evaluate to true?
• both test conditions evaluate to false?
• one test condition evaluates to true and one to false?
• What are some of the mistakes you could make when you write your conditional statements involving logical operators?

## Announcements

• Return Midterms in the last 5 minutes of class - excellent results!
• As: 33
• Bs: 5
• Cs: 3
• Ds: 2
• Fs: 1
• De Anza Women in CS club meeting tomorrow at 12:30 in ATC 202!
• Next quiz on Thursday
• Numbers, Operators and Precision
• Logical Operators
• While loops (simple)

## Review 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:
double PI = 3.1415927;
• With a partner: Declare a constant variable called FEET_PER_YARD and assign it the value of 3.

## Logical Operators

• In certain situations, we may wish to use more than one test condition inside of an if statement.
• To do so, we will need to chain the test conditions together using logical operators (either && or ||)
• For example, recall our guessing game program:
cout << "I'm thinking of a number between 1 and 10.\n";
cout << "Can you guess it?\n\n";
cout << "Enter your guess: ";

cin >> guess;

if (guess < 1 || guess > 10) { //if guess is < 1 OR guess > 10
cout << "Invalid entry!";
} else if (guess < 7) {
cout << "Your guess is too low";
} else if (guess > 7) {
cout << "Your guess is too high";
} else {
cout << "*** Correct! ***";
}

### Review of Boolean Variables

• Sometime we need to evaluate a logical condition in one part of a program and use it elsewhere
• To store a condition that can only be true or false, we use a Boolean variable
• Boolean variables are named after George Boole (1815-1864), a pioneer in the study of logic
• We specify a Boolean variable using the `bool` type, which can hold just one of two values: `true` or `false`
```bool isCool = true;
bool lies = false; ```

### Test Conditions and Boolean Values

• Remember that test conditions always evaluate to `true` or `false`
```if (num > 0)
```
• Thus we can use a boolean variable as a test condition
```bool isPositive = (num >= 0);
if (isPositive)
```
• Note that we do not need to add a relational expression to a boolean variable, like:
`if (isPositive == true) // avoid!`
• Since the boolean variable already evaluates to `true` or `false`, adding the `== true` is redundant
• Likewise, we do not need to use:
`if (isPositive != false) // avoid!`
• If we want to reverse the test condition, we can use the not (`!`) operator
`if (!isPositive)`
• We can see the use of a boolean variable in the following example

#### Example Application Using a Boolean Variable

 ```1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 ``` ```#include using namespace std; int main() { double num;   cout << "Enter a number: "; cin >> num; bool isPositive = (num >= 0); cout << boolalpha << "The test evaluated to: " << isPositive << endl; if (isPositive) { cout << "The number was 0 or positive\n"; } else { cout << "The number was negative\n"; } } ```

### Introducing &&, || and !

Logical operators and search engines video

### Combining Test Conditions with Logical Operators

• logical operator, or boolean operator, is an operator that treats operands as boolean values (`true` or `false`)
• C++ has several logical operators, but we only need to use three to create any possible test condition
• These three operators are `and``or` and `not`, which are discussed below
• These logical operators are traditionally written as `&&` (`and`), `||` (`or`) and `! (not)`

#### Truth Tables for AND, OR and NOT

`&&` (`AND`) Operator Truth Table
expr1
expr2
expr1 `&&` expr2
ExampleResult
`true``true``true``5 < 10 && 5 > 2``true`
`true``false``false``5 < 10 && 5 < 2``false`
`false``true``false``5 > 10 && 5 > 2``false`
`false``false``false``5 > 10 && 5 < 2``false`
`||` (`OR`) Operator Truth Table
expr1
expr2
expr1 `||` expr2
ExampleResult
`true``true``true``5 < 10 || 5 > 2``true`
`true``false``true``5 < 10 || 5 < 2``true`
`false``true``true``5 > 10 || 5 > 2``true`
`false``false``false``5 > 10 || 5 < 2``false`
`not` (`!`) Operator Truth Table
If expr is...Then `!` expr is...ExampleResult
`true``false``!true``false`
`false``true``!(5 < 2)``true`

### Testing Multiple Conditions Using Logical Operators

• Sometimes we need to test for multiple conditions using a single if statement
• In this case, we will need to chain together the two conditions using a logical operator, either && or ||
• Note that one either side of the logical operator there must be a stand alone test condition

if (age >= 18 && age <= 25) //Correct!!!

if (age >= 18 && <= 25) //No! Incorrect!!

• For example, we want to test if an age is between 18 and 65
• We need to test that both parts of are true: that the age is >= 18 AND that the age is <= 65

int age = 0;
cout << "Enter your age: ";
cin >> age;
if (age >= 18 && age <= 65)
{
} else {
cout << "Child, Teen or Older Adult!\n";
}

• Another way to use logical operators to test the age is:
```int age = 0;
cout << "Enter your age: ";
cin >> age;
if (age < 18 || age > 65)
{
cout << "Child, Teen or Older Adult!\n";
} else {
}
```
• Many people confuse `&&` and `||` conditions, especially when learning about logical operators
• A value lies between 18 and 65 if the value is at least 18 and at most 65
• A value is outside that range if it is less than 18 or greater than 65
• There is no golden rule; we have to think carefully and test our conditions

#### Parenthesis

• Remember that a Boolean expression in an `if` statement must be enclosed in parenthesis
• Thus, an `if` statement with `&&` might look like:
`if ((guess != GUESS1) && (guess != GUESS2))`
• However, relational operators have a higher precedence than logical operators
• Thus, we can remove the inner parenthesis without affecting the meaning:
`if (guess != GUESS1 && guess != GUESS2)`
• However, if using parenthesis is easier to understand then use the extra parenthesis

## Activity 11.1: What's Your Generation? (10 pts)

• There are 6 generations living in America, side-by-side, today.
• Find a partner for pair programming and open up a new file and name it generation.cpp.
• To do so, you will need to take as input the year of his or her birth.
• Then, you will need a series of test conditions (think if - else if - else) to determine the generation of your user.
• You will also need to use logical operators (&&, ||, !).
• Below is a chart with the range of birth years for each generation.
• Note the double quotes around each generation's name. For full credit, you must include the " when you output the generation.
Years of Birth                    Generation
1900-1927                           "The Greatest Generation"
1928-1945                           "The Silents"
1946-1964                           "The Baby Boomers"
1965-1979                           "Generation X"
1980-1999                           "The Millennial Generation"
2000-2018                           "Generation Z"

• Your goal is to prompt the user for his or her date of birth and then print out a message about which generation he or she belongs to.
• To start, print the following message to the user:

• Then, you will need a variable to store the user's year of birth:
int year_of_birth;
• Next, prompt your user to enter his or her date of birth with a statement like the following:
• Subsequently, you will need 6 if and if else statements like the following:
if ( year_of_birth >= 1900 && year_of_birth < 1928)
{
cout << "You belong to the \"Greatest Generation\"" << endl;
}
else if ( year_of_birth >= 1928 && year_of_birth < 1946)
{
cout << "You belong to the \"The Silents\"" << endl;
}
• Important: Why are we using && here and not ||?
• Finally, you will need to do some error checking of the user input.
• If the user inputs a date that is either too high or too low, your program must print out the following message:
Invalid entry. Please enter a birth year in the range 1900 - 2018.
• The above should go in your else clause
• Make sure your output is identical to the sample output below before you submit.
• When you are finished, upload to Canvas

You belong to the "Greatest Generation".

Alternately,

Invalid entry! Please enter a birth year in the range 1900 - 2018.

## 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.

## 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 9:20am
• Lab 6 due Friday
• Quiz next class!

~See You Thursday!~