1. Basics (Variables, Assignments, I/O)
Chapter
Basics (Variables, Assignments, I/O)

2. Overview 2.1 Variables and Assignments 2.2 Input and Output Slide 2- 2

3. 2.1 Variables and Assignments 3
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4. Variables and Assignments
Variables are names places where we can store a value:
If we place another value in a variable, the previous value is gone:
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5. Variables

6. Variable names: Identifiers
Variables names are called identifiers
Choosing variable names is important!
Use meaningful names that represent data tobe stored
First character must be
a letter
the underscore character
Remaining characters must be
letters
numbers
underscore character
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7. Variable names: Identifiers
Variables names are called identifiers
Choosing variable names is important!
be stored
Bad
nxt
cliN lb xc
Good
nextTempSample
clientFirstName
LoanBalance
xCoordinate
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8. Identifier Name Rules
A valid identifier is a sequence of one or more letters, digits, or underscore characters (_).
Spaces, punctuation marks, and symbols cannot be part of an identifier.
In addition, identifiers shall always begin with a letter.
Which are okay:
temp4
4temp
Today&forever
Now,or,never
lessonsILearnedFromCalculus

9. Keywords Keywords (also called reserved words)
Are used by the C++ language
the programming language
Cannot be used as identifiers
Auto const double float int short struct
Unsigned break continue else for long
Signed switch void case default enum goto
register sizeof typedef volatile char do
extern if return static union while
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10. int i, j, k; char c, ch; float f, salary; double d;
Declaring Variables (Part 1)
Before use, variables must be declared
Tells the compiler the type of data to store, and the variable name
A type followed by a list of variables names
23, -5, 17, 20043
int i, j, k;
char c, ch;
float f, salary;
double d;
types
names
'd', 'A', '&', '('
3.45,
3.45, ,
Much larger . 10

11. Declaring Variables (Part 2)
Immediately prior to use
Generally is is considered good form to declare variables as close as possible to first use.
Two locations for variable declarations
int main() { …
int sum;
sum = score1 + score 2;
return 0; }
int main() {
int sum; …
sum = score1 + score2;
return 0; }
At the beginning
Right before used
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12. Variable being assigned
Assignment Statements
Direction
An assignment statement changes the value of a variable On the right of the assignment operator can be
Literals -- age = 21;
Variables -- my_cost = your_cost;
Expressions -- circumference = diameter * ;
total_weight = one_weight + number_of_bars;
Assignment operator
Variable being assigned
Source of value
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13. Assignment Statements and Algebra
The ‘=‘ operator in C++ is not an equal sign
The following statement cannot be true in algebra
is the previous value of number_of_bars plus 3
= is an operation, not a comparison
number_of_bars = number_of_bars + 3;
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14. Initializing Variables
Declaring a variable does not give it a value
Before a variable is given a value is value is unknown, and thus garbage!
Giving a variable its first value is initializing the variable
Declaration and initialization can be combined using two methods
Define
Initialize
Comment
mpg = 26.3; // initialize the variable
double mpg = 26.3, area = 0.0 , volume;
double mpg(26.3), area(0.0), volume;
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15. Section 2.1 Conclusion Can you
The variables are named feet and inches.
Both should be initialized to the appropriate form of 5.
Give good variable names for identifiers to store
the speed of an automobile?
an hourly pay rate?
the highest score on an exam?
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16. Variables Hands on Discussion: What did you get? Why? int i=13;
float x=1.5;
x = (x * i) + 8;
i = (x * i) + 8;
cout << “x is equal to:” << x << “\n”;
cout << “i is equal to:” << i << “\n”;

17. 2.2
Input and Output
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18. Input and Output A data stream is a sequence of data
Typically in the form of characters or numbers
An input stream is data for the program to use
Typically originates
at the keyboard
at a file
An output stream is the program’s output
Destination is typically
the monitor
a file
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19. Output using cout Example: This line sends two items to the monitor
The value of number_of_bars
The quoted string of characters " candy bars\n"
Notice the space before the ‘c’ in candy
The ‘\n’ causes a new line to be started following the ‘s’ in bars
cout << number_of_bars << " candy bars\n";
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20. Examples Using cout cout << " candy bars\n";
cout << "Total cost is $" << (price + tax);
Quoted strings are enclosed in double quotes ("Walter")
Don’t use two single quotes (')
in " candy bars\n"
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21. Include Directives
Include Directives add library files to our programs
#include <iostream>
Using Directives include a collection of defined names
using namespace std;
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22. Escape Sequences
Escape sequences tell the compiler to treat characters in a special way
'\' is the escape character
cout << endl;
\" a quote character
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23. Formatting Real Numbers
cout << "The price is $" << price << endl;
The price is $ e01
The price is $78.50
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24. Showing Decimal Places
cout includes tools to specify the output of type double
To specify fixed point notation
setf(ios::fixed)
To specify that the decimal point will always be shown
setf(ios::showpoint)
To specify that two decimal places will always be shown
precision(2)
<< price << endl;
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25. Input Using cin cin is an input stream bringing data from the keyboard
The extraction operator (>>) removes data to be used
cin >> one_weight;
This code prompts the user to enter data then reads two data items from cin
The first value read is stored in number_of_bars
The second value read is stored in one_weight
Data is separated by spaces when entered
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26. Reading Data From cin Multiple data items are separated by spaces
Data is not read until the enter key is pressed
Allows user to make corrections
cin >> v1 >> v2 >> v3;
Requires three space separated values to be entered
<enter key>
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27. Designing Input and Output
Prompt the user for input that is desired
cin >> age;
Notice the absence of a new line before using cin
Echo the input by displaying what was read
cout << age << " was entered." << endl;
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28. Section 2.2 Conclusion Can you
write an input statement to place a value in the variable the_number?
Write the output statement to prompt for the value to store in the_number?
Write an output statement that produces a newline?
Format output of rational numbers to show 4 decimal places?
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