Transcript Basic Computations

Introduction to Programming
Using C
Basic Computations
Contents
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How do programs work?
Out first C program
Variables
Data types
Reading
Evaluating expressions
How do Programs Work ?
INPUT
COMPUTATION
OUTPUT
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Program
How do Programs Work ?
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The average program
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Reads some input
Performs some calculations on the input
Writes some output
Our First C Program
The function main is
always the first function
called
This is the start
of a function
called main
main()
{
printf(“Hello, world!\n”);
}
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The braces show
the start and end of
the function
The function
body contains
one or more
statements.
Printf prints a
string or other
data
Our First C Program
Your program
must have a
main, but can
contain other
functions
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A parameter list inside
parentheses can be used
to pass data to a function
main()
{
printf(“Hello, world!\n”);
}
The braces can be used
to delimit the start and
end of many things
Strings are
enclosed in
double quotes
Every statement
is terminated by
a semicolon
\n causes it to
start printing on
a new line
Program Structure
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A program contains
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One or more functions
Every program must have a function called main
main is the first function called
A function contains
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One or more statements
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A statement reads input, performs computations, or writes
output
A statement is terminated by a semicolon
Strings
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A string is simply a collection of characters
It is one way we can print text to a screen or
printer
Strings are enclosed in double quotes to
mark the beginning and end
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“This is a string”
Control Characters
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Strings can contain special control characters
These do not print, but have some effect on the
printer or computer
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\n
\a
\b
\f
\t
\\
newline
sounds the beep or alarm
backspace
form feed
tab
a backslash character
Computer Memory
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Our first program simply printed a string
which never changed
Most program want to do something more
exciting
This means they want to read input which
might change and produce output based on
the input
Computer Memory
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After the program reads data, it
performs calculations
It needs to store
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The data it read
Temporary results
These are stored in memory
Memory is used like a
scratchpad
Variables
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Memory is a big place
Modern computers have several billion
locations in memory
How do you find anything?
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Use the address, like a house number
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Give the address a name
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Can be hard to remember
Fred’s house is easier than34487291 Elm St.
Variables
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When we name a location in memory, it is
called a variable
addresses
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continues
A long way …
A named location “X”
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Variables
address
A variable can contain
only one value
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Variable name
X
If another value is assigned, the
original value is overwritten, like a
whiteboard
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Data Types
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Computers are dumb
Not only do you have to name the memory
locations, you have to state what type of data
you will store there
This makes sure that the computer knows
how to treat the data stored in the variable
Data Types
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The simplest data types store numbers
We make a major distinction between numbers
which can have fractions and those which cannot
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Integers
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Floating point
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Cannot have a fraction
2, 457, -87, 0, 1, 99999, -456
Can have a fraction
1.5, -2.98, 3.14159, -11.74
Data Types
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The integer types we use are
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your average number
numbers less that 256
really big numbers
The floating point types include
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int
short
long
double your average high-precision number
float
a less precise fractional number
Declaring Variables
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We must declare a variable before using it
This tells the computer
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To create an int variable called age we would
write
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To find some free memory for us to use
What type will be stored in the memory
What name to use for that memory
int age;
Variable Names
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Variable names must meet some rules
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May contain letters (a-z, A-Z), digits (0-9), and
underscore (_)
The first character must not be a digit
Some compilers limit a variable name to 31
characters
Cannot be the same as a keyword in the C
language
Variable Names
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Valid names
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Invalid names
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X
First_number
R2d2
Day of week
1st_number
int
(spaces not allowed)
(can’t start with a digit)
(language keyword)
Using a Variable
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Let’s double the value of a variable and print the result
main()
{
int number;
number = 18;
number = number * 2;
printf(“The result is %d\n”, number);
}
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Using a Variable
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First we declare the variable number
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Then, we assign it a value
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number = 18;
Then we replace the value with the result of a
calculation
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int number;
number = number * 2;
This multiplies the old value of number by 2 and makes tha
the new value of number.
Using a Variable
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Finally, we print the value, using a formatted
print statement
%d to format next parameter
as a decimal number
printf(“The result is %d\n”, number);
Value to be formatted by the
format string %d
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Reading Data
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If a program could only work with data you
typed into the program while writing it, the
program would not be very useful
A better approach is to allow the program to
read data from
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The keyboard
A file
The internet
Reading Data
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We start with reading data from the keyboard
Reading data is the reverse of printing
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scanf is the reverse of printf
It uses a format string to tell it what type of data to
read
It provides a list of variables into which to store
the data being read
Reading Data
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Now, let’s rewrite our program to double any number
main()
{
int number;
scanf(“%d”, &number);
number = number * 2;
printf(“The result is %d\n”, number);
}
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Assignment Operator
The equals sign “=“ is the assignment
operator
 It assigns the value on the right to the
variable on the left
number = number * 2;
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This calculates the right hand side
It then assigns the result to the variable on the left
hand side
Reading Data
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The difference lies in using scanf …
Variable where data
should be stored
scanf(“%d”, &number);
Format to read a
decimal number
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Address operator
which returns
address of variable
Reading Data
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When we reference a variable by its name
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number
We get the value stored there
However, when we want to store a value at
that location, we need the address, not the
value
Therefore, we use the address of operator,
&, to get the address of a variable
Arithmetic Operators
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So far we have seen the multiply operator, *
There are several arithmetic operators
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+
*
/
addition
subtraction
multiplication
division
Expression Evaluation
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Arithmetic expressions are evaluated
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Left to right
Things in parentheses are evaluated before
things outside parentheses
Mutiplication and division is performed first
Addition and subtraction is performed second
Expression Evaluation
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Expression
Result
2*3+4
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2*(3+4)
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2+3*4+1
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(2+3)*(4+1)
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(6-4)*5/2
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((2+1)*2) / (3 – 1)
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Mixed Data Types
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All of out expressions have involved integers
What happens when we combine an integer with a
floating point number?
The result is a floating point number
Otherwise, we might lose the fraction
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2 * 3.4 is 6.8
Here we multiply integer 2 times floating point 3.4 to yield
another floating point, 6.8
Writing a number without a fraction indicates it is an integer
Division
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The division operator works differently
depending on whether it is used with integers
or floating point numbers
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2.0 / 3.0
2/3
2 / 3.0
 1.5
1
 1.5
The result is an integer and the fraction is
discarded only if both numbers in the
expression are integers
A Cash Register Example
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Now, let’s write a program to act as a simple
cash register
The program will
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Read a price for some product
Read the number of product purchased
Print the total, including taxes
A Cash Register Example
main()
{
int quantity;
double unit_price, tax, total;
printf(“Enter product cost\n”);
scanf(“%lf”, &unit_price);
printf(“Enter quantity purchased\n”);
scanf(“%d”, &quantity);
total = quantity * unit_price;
tax = total * 0.14;
total = total + tax;
printf(“The amount due is %d\n”, total);
}
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