lesson4 - De Anza College CIS

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Announcements

Quiz 1 after the break
Reminder: Lab 2 due Friday

Learning Objectives

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

What kinds of data the computer can store in its memory
The four most common data types in C++ and what kind of data they represent:

int - whole number or integer data
bool - true or false values
double - numbers with a decimal point
char - character data

How computers store data in memory.
What is a variable.
How to name variables.
How to declare variables.
How to assign values to variables.

Review Activity
1. True or False? When pair programming...

Two people will be working on one computer
One person is called the driver and one is called the helper
You can work alone up to 25% of the time
In special circumstances, 3 people can work together

2. With a partner, write a complete program to print the message Greetings, earthlings! to the console window

Introducing Data Types

What is a data type?

What is data?

Information produced or stored by the computer.

What is a type?

The kind of data.
For example, numbers vs letters.

Data types are important because the compiler treats different types of data differently:

What kind of operations can be performed. Ex: We can't add and subtract letters.
How much storage to allot. Numbers and letters are stored differently in memory.
We can think of a data type as a child's game where we match shapes to hole.
C++ does not allow square data pegs in round memory holes.

There are four basic categories into which each data type falls:

Category	Explanation	Example
Integer	A whole number. No decimal point.	106
Floating Point Number	A number with a decimal point.	3.14
Character	Individual letters, digits and symbols	'A', '%', '9'
Boolean	True/False, 0/1, On/Off	true, false

C++ Has a Variety of Data Types:

Type	Bytes	Use
`bool`	1	A `true` or `false` value.
`char`	1	All ASCII characters.
`short`	2	Short integers from -32,768 to 32,767.
`int`	4	Integers from -2,147,483,647 to 2,147,483,647.
`long`	4	Integers from -2,147,483,647 to 2,147,483,647.
`unsigned`	4	Integers from 0 to 4,294,967,295.
`float`	4	Single-precision, floating-point numbers from about E-45 to E38 with 6 or 7 significant digits.
`double`	8	Double-precision, floating-point numbers from E-308 to E308 with from 14 to 15 significant digits.

Note that the number of bytes used for storage depends on the system and compiler

Group Activity: Name That Data Type!

Which of the 4 most common data types are the values below. Select int, double, bool or char.

'A' is a(n) _____________________
-213.555 is a(n) ________________
14 is a(n) _____________________
14.0 is a(n) ____________________
false is a ______________________
'$' is a ________________________
-2147483647 is a ______________
true is a _______________________

Most Common Data Types:

Type Memory Used Example Size Range

int 4 bytes 10 -2,147,483,648 to 2,147,483,647

double 8 bytes 10.5 10e-38 to 10e38

char 1 byte 'c' not applicable

bool 1 byte true not applicable

Storing Data In the Computer's Memory

Partner Activity: Remembering Things

Just like in real life, it is hard to do anything without being able to remember things.
For example, we cannot even do something as simple as adding two numbers together without using our memories.
I need a volunteer to come up to the front!
Imagine a conversation between Hal and Grace
Turn to a partner and read this short dialogue together.
One person should read Hal's part and one person should play Grace.
Hal: Hey Grace, I just learned to add two numbers together.
Grace: w00t!
Hal: Give me a number.
Grace: 2
Hal: OK, give me a second number.
Grace: 3
Hal: OK, the answer for 2 + 3 is 5
What just happened here?
After Grace says, "2", Hal has to store the number in his memory
The same things happens with the number, "3"
Even if the numbers were given in the same sentence, Hal would have to store the numbers somewhere in his memory
After adding the two numbers together, Hal has to store the result of the addition, at least temporarily, so he can state the answer.
If we were to write a program to add two numbers together, we would have to use memory just like Hal did.

Displaying sum of two integers using two variables only in C programming

Computers Need To Remember Things, Too!

Recall How Computer Memory Works:

Each memory location has eight bits, which are known as a byte
The memory address is the number that identifies a memory location
Some data is too large for a single byte
For instance, most numbers are too large for one byte
In these cases, the address refers to the first byte
The next few consecutive bytes store the additional bits for larger data

Variables

When we want to instruct a computer to store or retrieve data from its memory, we don't want to have to refer to the memory location in 1s and 0s.
Wouldn't it be nice if we could use a name or label for a location in memory?

Most people can remember a name better than a number.
We would rather remember a name than a memory location in 1s and 0s.

Fortunately, there is a way to give labels to the contents of memory addresses.
Variables are labels used to reference locations in computer memory.

We don't want to have to remember memory addresses like this:

Instead we would rather remember memory addresses like this:

In the above diagram, num1 is a variable that refers to the contents of memory address 1652, num2 is a variable that refers to the contents of memory address 2548, and total is a variable that refers to the contents of memory address 45.
When we use variables, we let the computer remember the address and size of the memory, we just remember the name of the variable.

Variables as Labeled Boxes Analogy

You can think of computer memory like a series of labeled boxes located in a storage unit.
When we put something inside a box, we need to write a label on the outside to recall what is inside the box.

Think of computer memory as a series of boxes in a storage unit.

We need to put a label on each box to remember what is inside.

Image source.

Bigger items will require a bigger box or multiple boxes and smaller items will require smaller boxes.
Later, when we go back to search for a particular item in one of the boxes, we look for the label on the outside to help us find the item we are looking for.
If we remove an item from a box, we can put the same kind of item back in the box without having to change the name.

e.g. In a box labeled "television," you can take out the old television and put a new one inside without having to cross out the label on the box.

The following image illustrates how you would store the integer value 22 in computer memory, inside of a variable named x.

Variable Declarations

To create a new variable, you need to tell the compiler what kind of data to expect.

Different data types require different numbers of bytes to store

e.g. integers are stored in 2 bytes while doubles are stored in 4 bytes.

Different operations can be performed on different types of data

e.g. You can't add characters together, but you can add integers together.

You also need to give your variable a name.
Each variable declaration (statement) needs to be followed by a semi colon ;
Let's look at some examples

double pi;

int numberStudentsClass;

bool is_answer_correct;

char grade;

bool tooCool4U;

All C++ program variables must be declared before using them
A declaration statement both names a variable and specifies the type of data it can store

General syntax:

dataType VariableName1, VariableName2, ...;

Where:
- dataType: one of the C++ data types
- VariableNameX: the name of the variable
When we declare a variable, the compiler sets aside memory space
However, the contents of the storage space is undefined until we assign a value
For instance, after we declare the following variable, what is its value?

int my_height;

Naming a Variable

A variable name is a sequence of letters, numbers, or underscore characters
However, C++ has some limitations on variable names
Specifically, the first character must be either a letter or an underscore character ( _ )
- Cannot be a number
- A $ is allowed but its use is discouraged
Also, the variable name cannot be one of the C++ reserved words (keywords)
For a list of reserved words, see: C/C++ Keywords
Note that we cannot have spaces in a variable name
A space is NOT a letter, number or underscore character
Also, variable names are cAsE sEnSiTiVe

id, ID, iD and Id are all valid but different names

Programming Style: Variable Naming Conventions

Use meaningful names that are easy to remember
- Well-chosen identifiers make it easier for other programmers (like the instructor) to understand your program
- For example, the following is syntactically correct:
```
a = b * c;
```
- However, it hides the intent or meaning
- Contrast this with:
```
weeklyPay = hoursWorked * payRate;
```
- This tells other programmers the intent and meaning of the code
Use a consistent naming style, one of the following two commonly-used styles:
1. Start with a lower-case letter and use uppercase letters as separators. Do not use underbars ('_'). This style is called CamelCase.
```
int myVarName;
```
2. Use all lower case letters and use underbars ('_') as separators.
```
int my_var_name;
```
You must be consistent and only use one style in a program.

Group Activity: Variable Declarations

Open up CodeBlocks and create a new program called practice.cpp. Copy and paste the starter code into your program.

/**
* Name

* Section Information

*/

#include <iostream>
using namespace std;

int main()
{
//declare your variables here
}

Inside this program create 5 new variables as follows:

A variable to store distance from your house to De Anza college
A variable to store your De Anza student id number
A variable to store whether or not you are the coolest person on the planet
A variable to store the first letter of your first name
A variable to store the number pi

Be ready to share your answers with the class.

Assigning Values to Variables

Once we have specified the name and data type for a variable, we want to give it a value.
The value should match the data type.
We give a variable a value by using the = sign.
The variable name goes on the left side of the = sign.
The variable value goes on the right side of the = sign.
A semi colon ; always goes at the end of the statement.

variableName = expression;

where expression is the value we assign to the variable.
Below is an example of how to declare and assign a variable.

int numHorses;

numHorses = 12;

We can also combine the two statements above into one statement.

int numHorses = 12;

Either method is valid though the second option is preferred.

Group Activity Continued:

Let's practice by giving values to the variables that we declared in our last group exercise.
Be ready to write your examples on the board under the correct column: int, double, bool, char

Assigning Literals and Complex Expressions to Variables

The simplest expression is a literal value:
```
length = 25;
width = 17.5;
```
Numbers like 25 and 17.5 are called literal numbers, or just literals, because we literally write them in the usual way
In each statement above, the value on right is assigned to the variable on the left
We can assign results of more complex expressions to a variable as well
```
total = num1 + num2;
slope = (y2 - y1) / (x2 - x1);
```
The expression on the right is evaluated (computed) before assignment to the variable on the left
Values placed into a variable replace (overwrite) previous values
Reading variables from memory does not change them

Initializing Variables

Initial values may or may not be assigned when variables are declared:

// These are not initialized when declared
// and have unknown values
int sum, number1, number2;

// These are initialized when declared
int sum = 0;
int number1 = 5, number2 = 10;

Good programming practice: initialize variables when declared
Variables can also change their values as the program progresses

Why are they called variables? Their content varies.

int myAge = 28;

//later in program... after one year passes

myAge = 29;

Activity 4.1: Area of a Football Field (10 pts)

Find a partner for pair programming.
Open up CodeBlocks.
Create a new file called football.cpp.
Put a block comment at the start of your program with your names and section information
Add the library and namespace information
Create a main function
Create a variable called length and assign it the value 360 (pro football fields are 360 feet in length)
Create a variable called width and assign it the value 160 (pro football fields are 160 feet in length)
Create a variable called area and assign it the value of length * width;
Run the program and verify that there are no errors.
Is this program satisfactory?
Now, alter the program to print out the value of the area

For example, you could print out the area like this:

The area of a professional football field is 57600 ft2

Submit your program to Catalyst. Note that each partner should submit.

Review:

With a Partner, review our learning objectives.
Answer the following question:

What Gets Printed to the Screen?

//assume below statement are part of a main function

int oranges = 5;

int apples = 8;

int numFruit = apples + oranges;

cout << "Total Fruit: " << numFruit << endl;

oranges = oranges + 1;

apples = oranges;

cout << "Apples: " << apples << endl;

cout << "Oranges: " << oranges << endl;

Homework:

Lab 2 due this Friday at midnight
Assignment 4 due Tuesday at 3:20pm

~Have a Good Weekend!~