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Variables, Types,
Assignments, Input, and
Output
EECS 280
Chuck Severance / Jim Eng

Variables
• A variable is a location where some
value is stored - “the upper left drawer”
• A value might be the current checkbook
balance
• A variable has a name - “Which drawer?”
• A variable has a “type” - What can be
stored in the drawer?
X
Y

Rules for Variable Names
• Variable names start with a letter or an
underscore and consist of numbers, letters,
or underscores
• Good names: x1 i abc123
• Bad names: x+1 52pickup t&e
• C++ is case sensitive
– X1 and x1 are different variables
• Variable names can be any length

Variable Types
• Each variable in C++ must have a type
• The type determines what can be legally
stored in a variable
• The type determines how much storage a
variable occupies
• C++ has some built-in types
• Later we will define our own new types and
then declare variables our types
X
J
1123 3.1415

Some Built-In Types
C++ Type Examples Legal Values
short 1, -43, 1024 -/+ 32,767
int 1, -43, 1024 -/+ 2,147,483,647
long 1, -43, 1024 -/+ 2,147,483,647
char ‘a’ ‘b’ ‘0’ All single character values
float 0.001 3.14 -10.2 10-38
to 1038
Seven digits of accuracy
double 1.56 -0.1 46.3 10-308
to 10308
Fifteen digits of accuracy
Note: On some systems the int type has a range of +/- 32,767

Declaring Variables
• At the beginning of a block of code, we
declare variables for that block of code.
• We give each variable a type and a
name
main()
{
int i;
float bankbal;
char initial;
}

Assignment Statements
• Set a variable to a value
• A variable can only hold one value
• The next assignment
replaces the variable
contents
X
3.1415
X = 3.1415
100.0
X = 100.0
3.1415
X

This is not Algebra
• In Algebra “x=5” means “For the
purposes of this problem, x is 5”
• In programming, “x=5” means:
X=5
“Go find the memory location called
x, blast whatever is in it, and set it to
5. And I mean NOW!”
178829
X
5

Still More
main () {
int ed,mary;
ed = 17;
mary = 2 * ( 14 + ed );
ed = ed + 3;
}
• The right-hand side
can be an
expression instead
of a constant
• The expression is
computed BEFORE
the assignment
occurs
ed
mary

In the Beginning...
• What values are in
variables right at the
beginning of the
program?
main () {
int ed,mary;
ed = mary;
}
??????
ed

In the Beginning...
• What values are in
variables right at the
beginning of the
program?
• Garbage, bad stuff,
smelly leftovers
• Behavior is officially
“undefined”
• Don’t assume zero
main () {
int ed,mary;
ed = mary;
}
??????
ed

Defining the Undefined
• There is a simple solution - we specify
the initial value as part of the
declaration of the variable.
• The syntax looks like an assignment
• Ed does not “stay” zero
• Ed just starts out
containing the value zero*
main () {
int ed = 0;
int mary=17;
ed = mary;
}
*Apologies to anyone named “Ed” - it is just an example

A Program
main () {
float celsius;
float faren;
celsius = 33.2;
faren = ( (celsius * 9 ) / 5) + 32.0;
}
celsius
faren

Language Assignment Syntax
APL X 20
Pascal X := 20
COBOL MOVE 20 TO X
BASIC LET X = 20
FORTRAN X = 20
C and C++ X = 20
The History of Assignment
• There is disagreement among computer
languages as to how to represent this
act of moving data into a variable

Running A Program
main () {
float celsius;
float faren;
celsius = 33.2;
faren = ( (celsius * 9 ) / 5) + 32.0;
}
Output:
% g++ samp1.c
% a.out
%
Run the G++ Compiler
Run the compiled program

Expressions
• Any place a value can be used, an
expression can also be used - the value
is computed from the expression
celsius = 33.2;
faren = ( (celsius * 9 ) / 5) + 32.0;

Operators
C/C++ Operator Operation
+ - Addition Subtraction
* / Multiplication Division
% Remainder (Integer division)
Expression Value Notes
3 + 4 7
5 - 1 + 2 6 5 - 1 is done first (left to right)
5 * 2 + 4 14 All multiplication and division is
done before add and subtract
5 * ( 2 + 4 ) 30 Parenthesis rule!
32 % 6 2 The remainder of 32/6 is 2

Expression Gymnastics
celsius = 100;
faren = ( (celsius * 9 ) / 5) + 32.0;
900.0
100.0
180.0
900.0

Mixing Types
• When expressions mix types, C++
converts data according to rules
char -> int -> long -> float -> double
• These rules apply as each sub-expression
is evaluated
• For an assignment, the type of the right-
hand-side is converted at the last minute
just before the value is assigned.

Type Gymnastics
celsius = 100;
faren = ( (celsius * 9 ) / 5) + 32.0;
900.0
100.0
180.0
900.0

Input and Output
• Programs must interact with something
– Keyboard, screen, joystick, speaker,
temperature sensor, keypad (like on a cell
phone)
• When a program takes data from
something we call it “input”
• When a program sends its data to
something, we call it “output”

Streams
• When we start programming, we keep
things simple - we read from the
keyboard and write to a text screen
• In C++ this is done using “streams”
– cin - The input stream
– cout - The output stream

Ouput Stream
• To display information on the screen we
use the insertion operator with cout
#include <iostream.h>
main () {
float celsius;
float faren;
celsius = 33.2;
faren = ( (celsius * 9 ) / 5) + 32.0;
cout << faren;
}
Output:
% g++ samp2.c
% a.out
91.76%

Your Responsibility
Computers are getting so fast and so capable
that it will not be long until computers will do
what you want them to do instead of what you
tell them to do.
Gerard P. Weeg (paraphrased ca. 1963)

Output Streams
• When using cout, we need to manage
the end of lines and white space
• End of line
• White space
cout << endl; or cout << “n”;
cout << “ “; NewLine character

main () {
float celsius;
float faren;
celsius = 33.2;
faren = ( (celsius * 9 ) / 5) + 32.0;
cout << faren;
}
Output:
% g++ samp2.c
% a.out
91.76%
main () {
float celsius;
float faren;
celsius = 33.2;
faren = ( (celsius * 9 ) / 5) + 32.0;
cout << faren;
cout << endl; // Add A NewLine
}
Output:
% g++ samp3.c
% a.out
91.76
%
We Said We Got
We Wanted
We Needed to Say

More cout
• You can “send” multiple things to cout with
multiple “<<“ operators
• Most people think of the “<<“ operator as
“send to”
means “send the value contained in the
variable faren to the screen”
cout << “Farenheight “ << faren << endl;
cout << faren;

Input
• The most basic form of input is from the
keyboard. For this we use the “cin” input
stream and the “>>” extraction operator.
• Based on the type of the variable
(celsius), a value is read from the
keyboard and stored in the variable
cin >> celsius;

A “Real” Program
main () {
float celsius;
float faren;
cout << “Enter a Celsius Value ”;
cin >> celsius;
faren = ( (celsius * 9 ) / 5) + 32.0;
cout << “Converted value “;
cout << faren << endl;
}
Output:
% g++ samp4.c
% a.out
Enter a Celsius Value 100
Converted value 212
%
Note Spaces

The History of Input/Output
FORTRAN:
READ *,X
PRINT *,X
BASIC:
READ X
PRINT X
C:
scanf(“%f”,&x);
printf(“%f”,x);
C++:
cin >> x;
cout << x;
FORTRAN (Formatted):
READ 100,X
100 FORMAT(F8.2)
PRINT 200,X
200 FORMAT(1X,F8.2)

A Pattern in Programs
Processing
(Calculations)
Input Output
y = x + 1;
cin >> x; cout << y;

Prompting for Input
• Another common pattern for interactive
programs is to issue a “prompt” before
reading input so the user knows what to
type.
….
cout << “Enter a Celsius Value ”;
cin >> celsius;
faren = ( (celsius * 9 ) / 5) + 32.0;
….
Enter a Celsius Value 100
Converted value 212

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