First C Program

1
Lecture 1

• Introduction
• First C Program
• Data Types and expressions
• Note Read Chapters 1 and 2 for this week

2
Architecture of a Computer
3
I/O Devices

• Communications with the outside world.
• Input devices
• keyboard
• mouse
• joystick
• Output devices
• screen
• printer
• computer speakers

4
Central Processing Unit (CPU)

• Has 2 components to execute program instructions
• Arithmetic/Logic Unit performs arithmetic
  operations, and makes logical comparisons.
• Control Unit controls the order in which your
  program instructions are executed.
• Uses one or more registers as scratch space for
  storing numbers between instructions.
• A typical CPU today can execute billions of
  arithmetic operations in a second.

5
Main Memory

• Sometimes called random access memory (RAM).
• Stores the numbers (data) that a program uses
  when it runs on the computer.
• Contains millions of circuits which are either
  off or on (0 or 1) ? Binary
• Also stores the instructions of the program that
  is running on the computer.
• Divided into fixed size units of memory called
  words.
• each word stores one number
• each word has its own address

Data
Program Instructions
6
Secondary Storage

• Permanent storage used to save data and programs
  when they are not running on the computer.
• Data and programs are organized into varying size
  units called files.
• Files are organized into directories that can
  contain subdirectories.
• Examples of secondary storage include hard disks,
  floppy disks, and CDs.

7
The Programming Process

• Use an editor (such as Notepad, Textpad or Visual
  Studio) to create a program file (source file).
• contains the text of the program written in some
  programming language (like C)
• Use a compiler/linker (such as gcc) to convert
  the source file into a machine code file (object
  file).
• converts from English to binary with machine
  operations
• includes other machine code that the program
  requires
• Run the executable file.

8
Creating a C Program

• A C program is a set of functions that
  collectively solve a given problem.
• each function is a sequence of C statements
  (instructions)
• execution always begins with a function named
  main
• one function calls another function to get it to
  execute its statements
• The C statements in a function are executed one
  after the other in sequential order as written.

9
First C Program Hello World!

• Anything between / and / is a comment intended
  to improve the readability of the program.
• include is used to tell the compiler and linker
  what library resources the program uses.
• the ltstdio.hgt library defines everything you need
  to display messages on the screen, including
  printf
• This program contains one function named main.
• The program outputs (prints to the screen) the
  words
• Hello, World!

• / Nate Preston
• Hello World Program
• prog01.c
• Jan 18, 2006
• /
• include ltstdio.hgt
• int main ()
• printf("Hello, World!\n")
• return 0

10
Using printf to Display Messages On The Screen

• To display a message on the computer screen use
  the printf statement.
• The back slash \ indicates an escape sequence.
  This means to take the alternate meaning of
  whatever immediately follows it.
• \n means to start a new line.
• \t is a tab
• \ is a quote
• Text strings must always be surrounded by double
  quotes.
• More examples

printf("Hello, World\n")
printf("Hello\nWorld!\n") printf("\tFirst part
of long message. ") printf("Second part of long
message.\n") printf("\"Hello\" World!\n")
11
Defining a C Function

• A C function has the following form
• The name of this function is main.
• The word int, means that this functions returns
  an integer number.
• usually 0 to indicate that the program ran
  correctly.
• this is what the return statement does at the end
  of the function
• The braces define the beginning and the end of
  the function.
• The first line of the function is called the
  function header.

int main ( ) sequence of statements
separated by semicolons . . . return
0
12
Example of Compiling and Running a C Program in
Cygwin
13
Standard Data Types in C

• A data type tells what type of data is stored in
  a given memory location. Standard C data types
  are broken into the following types
• Integral Types
• represent whole numbers and their negatives
• declared as int, short, or long
• Floating Types
• represent real numbers with a decimal point
• declared as float, or double, or long double
• Character Types
• represent single characters
• declared as char

14
C Variables

• Variables in C store data in memory so that the
  data could be accessed throughout the execution
  of a program
• A variable stores data corresponding to a
  specific type.
• Each variable that a C program uses must be
  declared at the beginning of the function before
  it can be used.
• specify the type of the variable
• numeric types int short long float
  double
• character type char
• specify a name for the variable
• any previously unused C identifier can be used
• Example int x
• float sum, product

15
What Does a Variable Declaration Do?
int ageOfDog float tax_rate_Y2K char
middleInitial
A declaration tells the compiler to allocate
enough memory to hold a value of this data type,
and to associate the identifier with this
location.
4 bytes for tax_rate_Y2K
1 byte for middleInitial
16
Variable and function Names In C

• Variables and function names in C are called
  identifiers.
• identifiers are used for many other things as
  well
• Rules for constructing valid identifiers in C
• can contain letters (upper and lower case),
  digits, and the underscore ( _ ) character
• cannot start with a digit
• cannot be a reserved word
• can be at most 256 characters long, though some
  compilers only look at first 32 characters
• are case sensitive

• Some reserved words in C

break case char const continue default do
double else enum float for goto if
int long return short signed static struct switc
h typedef union unsigned void while
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Identifiers

• VALID
• age_of_dog tax_rate_Y2K
• PrintHeading ageOfHorse_
• NOT VALID (Why?)
• age 2000TaxRate
  Age-Of-Cat day of week
• Using meaningful variable names is a good
  programming practice

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Giving a Value to a Variable
You can assign (give) a value to a variable by
using the assignment operator VARIABLE
DECLARATIONS char middleInitial char
letter int ageOfDog VALID
ASSIGNMENT STATEMENTS middleInitial X
letter middleInitial ageOfDog 12
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Assignment Statement

• An assignment statement is used to put a value
  into a variable.
• The previous value is replaced
• Syntax ltvariablegt ltexpressiongt
• ltvariablegt is any declared variable in the
  program
• ltexpressiongt is anything that produces a value of
  the appropriate type (more on this later)
• first, the expression on right is evaluated.
• then the resulting value is stored in the memory
  location of variable on left.
• Examples
• count 10
• count count 1
• area 3.14 radius radius
• NOTE An automatic type coercion occurs after
  evaluation but before the value is stored if the
  types differ for expression and variable

20
Arithmetic Expressions With Integers (int, long)

• Operators result is always an integer
• Symbol Name Example Value (x 10, y 3)
• addition x y 13
• subtraction x y 7
• multiplication x y 30
• / quotient x / y 3
• remainder x y 1
• unary minus x -10
• unary plus x 10
• You can string the operators together to build
  longer expressions.
• use parentheses to specify order of operations
• precedence (after sub-expressions in parentheses)
• first unary plus and minus from right to left
• second , / and from left to right
• third and from left to right

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Arithmetic Expression Example
int y 2 int z z -y 3 (4 5) 10 -
-3
-2 3 9 10 - -3
-2 3 9 10 3
-6 9 10 3
-54 10 3
-4 3
-1
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Arithmetic Exprs With Reals (float, double)

• Arithmetic expressions with real numbers (numbers
  with decimal points) work the same way as with
  integers, with a few exceptions
• there is no remainder operator ()
• the / operator means divide, computing the
  answer to many decimal places
• the result is a real value rather than an integer
  value
• Important Real values are approximate and may
  contain errors in the last few digits.
• about 7 digits of accuracy for type float
• about 14 digits of accuracy for type double

23
Mixed Mode Arithmetic Expressions

• Arithmetic expressions both integers and floats
  can get tricky.
• if both operands are integers, integer arithmetic
  is used
• if either operand is a float, float arithmetic is
  used
• an integer operand is converted to float for the
  operation
• Examples
• int a, x
• x 3.59 / x gets the value 3 (no rounding) /
• float y 3 / y gets the value 3.0 /
• a 12 / a gets the value 12 /
• float avg (a x) / 2 / avg gets the
• value 7.0 UH OH!!! /

24
Printing variables and constants to the screen
with printf

• Use the following conversion specifications
• d for an integer
• ld for a long int
• f float
• f double
• c for a character
• Example Output
• int sum 5
• float avg 12.2
• char ch A
• printf(The sum is d\n, sum) The sum is 5
• printf(avg f\n, avg) avg 12.2
• prinf(ch c\n, ch) ch A
• printf(d, f, c\n, sum, avg, ch) 5, 12.2, A
• printf(d\n, 5) 5
• Note see p. 152 in your textbook for a complete
  list of all datatypes

25
Reading data from the keyboard with scanf

• Used to assign a value typed on the keyboard to a
  variable.
• Used similarly to printf. You must use the
  following conversion specifications
• Data Type printf converstion spec. scanf
  conversion spec.
• int d d
• long ld ld
• float f f
• double f lf
• char c c
• character string s --
• The user must type a value followed by the Enter
  Key.
• Ex scanf(d, num)

26
Program to find the average of two numbers

• Note that you can declare more than one variable
  per line.
• We divide by 2.0 instead of 2 so that the right
  hand side is a float (i.e. no truncation takes
  place)
• scanf is used similarly to printf, except that
  you need to put an ampersand () before the
  variable name
• The first argument of the scanf function is the
  conversion specifier(s) in quotes, then the
  variable name(s)

• include ltstdio.hgt
• int main ()
• int num1, num2
• float avg
• / get two inputs /
• printf( "Enter the first integer )
• scanf( "d", num1 )
• printf( "Enter the second integer" )
• scanf( "d", num2 )
• / compute and print the avg /
• avg (num1 num2) / 2.0
• printf( "The average is f\n", avg )
• return 0

27
Note

• HW 1 is posted on the course web site, and is
  due at the beginning of next class. Please note
  the submission instructions.
• You may work on lab 1 for the rest of the class.
  When you're done, have one of the TAs or myself
  check you off.