CIS105 Class 2 Phill Conrad, Fall 2003

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CIS105 Class 2Phill Conrad, Fall 2003

• A journey of a thousand miles must begin with a
  single step. - Lao Tzu

Thanks to Phil Viscito for some of the material
in these slides used by permission
2
Compiling a program(simplified view)
Note this simplified viewomits the linking
stepand the role of the .o file Well talk
about that in afuture lecture.
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Introduction to the C Programming Language
(This will definitely take more than one lecture!)
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What is C?

• C is a high-level language.
• A C compiler takes a C source code file as input
  and produces an executable file, after compiling,
  assembling, and linking.
• C is the industry standard high-level programming
  language.

5
An Example C Program
/ Converts distances from miles to kilometers
/ include ltstdio.hgt define KMS_PER_MILE
1.609 int main(void) double miles, /
distance in miles / kms / equiv. distance in
kms / / Get the distance in miles
/ printf(Enter the distance in
milesgt) scanf (lf, miles) / Convert
the distance to kilometers / kms KMS_PER_MILE
miles / Display the distance in kilometers
/ printf (That equals f kilometers.\n,kms)
return(0)
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Comments

• Comments are text provided by the programmer to
  allow others (and the programmer too) to better
  understand what the program is doing
• Any text between a / and a / is considered
  to be a comment and is ignored by the C
  preprocessor and the compiler itself.
• Comments can appear anywhere in a program and can
  be multiline.

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Comments
/ Converts distances from miles to kilometers
/ include ltstdio.hgt define KMS_PER_MILE
1.609 int main(void) double miles, /
distance in miles / kms / equiv. distance in
kms / / Get the distance in miles
/ printf(Enter the distance in
milesgt) scanf (lf, miles) / Convert
the distance to kilometers / kms KMS_PER_MILE
miles / Display the distance in kilometers
/ printf (That equals f kilometers.\n,kms)
return(0)
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Preprocessor Directives

• A preprocessor directive is a command given to
  the C preprocessor, which is a part of the
  compilation process that modifies a C source code
  file before it is compiled.
• Preprocessor directives always begin with a
  character.
• In the example program, there are two
  preprocessor directives used, include and
  define.

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include

• The include directive tells the C preprocessor
  to give a program access to a library. By
  putting a include in a program, the preprocessor
  loads a header file, which tells the compiler
  what functions and other information is provided
  in the library.
• In this program, include ltstdio.hgt indicates
  that this program uses the stdio library which
  contains functions such as printf().

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define

• The define directive specifies a constant macro.
• This tells the preprocessor that every time it
  encounters the first text, it should replace it
  with the second text.
• In this program, define KMS_PER_MILE 1.609 tells
  the preprocessor to replace KMS_PER_MILE with
  1.609 every place KMS_PER_MILE appears in the
  program.

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Preprocessor Directives
/ Converts distances from miles to kilometers
/ include ltstdio.hgt define KMS_PER_MILE
1.609 int main(void) double miles, /
distance in miles / kms / equiv. distance in
kms / / Get the distance in miles
/ printf(Enter the distance in
milesgt) scanf (lf, miles) / Convert
the distance to kilometers / kms KMS_PER_MILE
miles / Display the distance in kilometers
/ printf (That equals f kilometers.\n,kms)
return(0)
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Reserved Words

• Reserved words are words that have special
  meaning in the C language and cannot be used for
  other purposes.
• In this program, examples include int, void,
  double, and return.

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Reserved Words
/ Converts distances from miles to kilometers
/ include ltstdio.hgt define KMS_PER_MILE
1.609 int main(void) double miles, /
distance in miles / kms / equiv. distance in
kms / / Get the distance in miles
/ printf(Enter the distance in
milesgt) scanf (lf, miles) / Convert
the distance to kilometers / kms KMS_PER_MILE
miles / Display the distance in kilometers
/ printf (That equals f kilometers.\n,kms)
return(0)
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Identifiers

• Identifiers identify memory cells that hold data
  or operations (and functions).
• Standard identifiers have special meaning in C,
  however they can be redefined by the programmer.
• Examples of standard identifiers in the program
  are printf and scanf.
• User-defined identifiers are memory cells that
  hold data, operations, and functions that the
  programmer defines.

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Identifiers
/ Converts distances from miles to kilometers
/ include ltstdio.hgt define KMS_PER_MILE
1.609 int main(void) double miles, /
distance in miles / kms / equiv. distance in
kms / / Get the distance in miles
/ printf(Enter the distance in
milesgt) scanf (lf, miles) / Convert
the distance to kilometers / kms KMS_PER_MILE
miles / Display the distance in kilometers
/ printf (That equals f kilometers.\n,kms)
return(0)
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What is a valid identifier?

• Identifiers can only be composed of letters,
  digits, and underscores.
• Identifiers cannot begin with a digit.
• Reserved words cannot be identifiers.
• Identifiers can be as long as you want.
• Upper and lower case letters are different (i.e.
  kms and Kms are not considered to be the same
  identifier).

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Review

• Which of the following are valid identifiers?
  For each that is not valid, why is it not?

This_is_a_long_identifier_but_is_it_valid?
8timesTheRadius
miles
Phils variable
kilometers_per_hour
x
radius
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Variables

• A variable is a memory cell that is used to hold
  data acted upon by the program.
• A variable declaration tells the C compiler the
  name and type of a variable used in a program.
• A variable declaration consists of a data type
  and an identifier which is the name of that
  variable.
• Every variable that will be used in a program
  must be declared.

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Variable Declarations
double mile int counter

• The first line declares a variable named mile of
  the double data type.
• The second line declares a variable named counter
  of the int data type.

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Data Types

• There are a large number of data types. These
  are some of the most popular ones.
• int An integer (a whole number). This data
  type can represent an integer in a specific
  range, at least 32767 through 32767.
• On strauss, -2,147,483,648 up to 2,147,483,647
• Why?
• Because strauss computes numbers 32 bits at a
  time
• Because 231 2,147,483,648(Just remember, a
  little more than two billion)
• Later, well talk about different kinds of
  integers (short, long, etc.)
• (data types continued on next slide)

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Data Types

• char A character. One letter, digit, or
  symbol. This is enclosed in single quotes.
• float A real number (an integer part and a
  decimal part).
• double A real number. Note that this data type
  is a memory cell double the size of a float data
  type. This allows a bigger number to be
  represented, or a specific number to be
  represented more precisely. This is referred to
  as a double-precision floating point number.
• void this keyword means no data type.

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Bigger or More Precise?

• Why can we represent either bigger numbers or
  more precise numbers?
• Because floating points numbers are stored in
  scientific notation, consisting of a number and
  an exponent, i.e.

1.2555 x 106
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Review of Variables

• Write a define preprocessor declaration for a
  constant macro of STUDENTS_PER_SECTION to 22 and
  variable declarations of num_students as an
  integer, GPA and class_GPA as double-precision
  floating point numbers, and letter_grade as a
  character data type.

define STUDENTS_PER_SECTION 22 int
num_students double GPA, class_GPA char
letter_grade
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Assignment Statements

• An assignment statement is one type of executable
  statement.
• An assignment statement uses the operator,
  and follows the form

variable expression

• This statement first evaluates the expression on
  the right and stores the result in the variable
  on the left.

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Assignment Statements

• Here are some examples of assignment statements
• x 12
• negative_x -x
• x y 12 z 5
• result y
• Note that any variables in the right-side
  expression are not modified by an assignment
  statement.

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Function Calls

• A function is a piece of code which performs a
  specific task.
• Functions can be created by programmers or
  supplied as part of the C compiler toolset.
• A function is called, which causes it to execute.
• A function call is composed of the function name,
  an open paren, a set of function arguments
  separated by commas, and a close paren.

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Function Calls

• A function call looks like this

function_name(argument1, argument2, argument3)
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The printf Function

• The C function for displaying output on the
  screen to the user is printf.
• printf is called in the following manner

printf(The final GPA of student number d is
f.\n,student_num, GPA)
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The Format Stringand Print List

• The format string is the text that is to be
  displayed on the screen.
• The characters are called placeholders. They
  indicate the display position for variables whose
  values are to be displayed.
• The variable names to be displayed are specified
  in the print list and appear in the same order as
  their placeholders.

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Placeholders
printf(The final GPA of student number d is
f.\n,student_num, GPA)
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Placeholders

• All placeholders begin with a .
• The text after the symbol indicates how to
  format the output, i.e. what kind of variable it
  is.
• d decimal number (int)
• f floating-point number (float or double)
• c character (char)

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Escape Sequences

• All escape-sequences begin with a backslash, \.
• A letter after the \ character denotes an
  escape sequence, which has special meaning.
• The \n sequence indicates a new-line character,
  which cause any following text to appear on the
  next line on the display.
• In order to display a \, the format string must
  contain a \\.

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Placeholders and theNewline Escape Sequence
printf(The final GPA of student number d is
f.\n,student_num, GPA)
Therefore, if the student_num variable was set
to 10 and the GPA variable was set to 3.03 this
printf function call would cause The final GPA
of student number 10 is 3.03. to be displayed on
the screen and any subsequent output to begin on
the next line.
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The scanf Function

• The C function for reading input from the user is
  scanf.

scanf(d f,student_num, GPA)
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The Format String

• The format string is the set of placeholders
  which specify what type of data is being input.
• The same placeholders are used as for printf,
  except for when inputting a floating-point
  number. A float type still uses the f,
  however the double type uses the lf
  placeholder.

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The Input List

• The variables to store the inputted data are
  specified in the input list. They must be in the
  same order as their corresponding placeholders.
• Notice that each variable name is preceded by a
  .
• The is an operator which means the address
  of.
• Therefore, student_num tells the scanf
  function to store what it reads from the user at
  the memory address of student_num.

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Placeholders and the Input List
scanf(d f,student_num, GPA)
Therefore, if the input into this scanf function
call was 9 3.560 ltENTERgt the student_num
variable would be set to 9 and the GPA variable
would be set to 3.560
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Review of printf and scanf

• What happens when the following code fragment is
  run?

int student_num float GPA printf(Enter the
student numbergt) scanf(d,student_num) print
f(Enter student number ds GPAgt,student_num) s
canf(f,GPA) printf(Student number d now
has a f GPA.\n,student_num, GPA)
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Review of printf and scanf

• What is the displayed output when the following
  code fragment is run and the inputs are 8 and 12?

int x, y printf(My name is) printf( Phil
Viscito) printf(\nEnter two integersgt
) scanf(dd,x, y) x x 2 y x
y printf(Thanks! The answer is d.\nBye
now!,y)
My name is Phil Viscito. Enter two integersgt 8
12 Thanks! The answer is 22. Bye now!
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Customizing Integer Output

• The d placeholder, used to display an integer
  variable, can be altered to format how the number
  is displayed.
• Instead of d, use a Xd where the X is an
  integer that is the field width, the number of
  digits to display.
• For example, 4d displays four digits of the
  result. The negative sign (for negative
  integers) is also considered a digit here.

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Customizing Integer Output

• When there are more places (the field width) than
  digits to be displayed, the output is
  right-justified.
• When there are more digits than places, the field
  width is ignored, and the entire integer is
  displayed.

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Customizing Integer Output

• As an example

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Customizing Floating-Point Output

• Floating point output (float and double) can be
  formatted in the same manner, using X.Yf).
• Here, X is the total number of digits to display
  (the field width) and Y is the number of digits
  to display to the right of the decimal point.
• The same rules for field width apply as for
  integer formatting.
• The specified number of decimal digits is always
  displayed.

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CustomizingFloating-Point Output

• As an example

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The return Function

• The return function terminates a running function
  and returns control to the function that called
  the function.
• In the case of main(), the return function
  returns control back to the computers operating
  system and ends the program.
• This function has one argument. The main()
  function must return an integer, normally just
  set to 0 (zero).

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Arithmetic Expressions

• Arithmetic expressions are executable statements
  that manipulate data.
• Arithmetic expressions operate on both integer
  (int) and floating-point (float and double)
  numbers.
• Arithmetic operators can operate on mixed types
  (i.e. one int and one float). The resulting type
  of such an expression is the highest data type
  present.

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Resulting Data Types

• The highest data type is always considered to
  be a floating point number, with double-precision
  floating point numbers taking precedence over
  single-precision floating point numbers.

int int int int float float int
double double float double
double
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Arithmetic Expressions

• All of the common arithmetic operators are
  present in C
• (addition)
• - (subtraction)
• (multiplication)
• / (division)
• (modulus or remainder)

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Note Integer Division

• If two integer values are divided, the resulting
  data type is also an integer, as previously
  described.
• Therefore, only the integer portion of the actual
  result will be the returned result.
• For example,

9 / 4 2 9 / 10 0
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The Modulus Operator

• The operator is a modulus operator, which
  also means the remainder of the division.
• For example,

9 3 0 10 6 4 90 8 2
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Review of BasicArithmetic Expressions

• What is the resulting output of this program
  segment?

int x, y, z float a x 9 0.5 a 9
0.5 y 15 15 z 15 2 printf(xd\n,
af\n, yd\n, zd\n,x,a,y,z)
x4 a4.5 y0 z1
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More ComplexArithmetic Expressions

• C evaluates arithmetic expressions in the same
  order rules as normal mathematics.
• Parentheses take priority over everything else.
• Then, multiplication, division, and modulus
  operations from left to right.
• Then, addition and subtraction from left to right.

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More ComplexArithmetic Expressions
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Review

• What is the resulting output of this program
  segment?

define PI 3.14159 int x, y, z float a, b x
5 y 10 z x (4 y y y) PI a x
(4 y y y) PI b (x / y)
a printf(xd, yd, zd\na7.2f,
bf\n,x,y,z,a,b)
x5, y10, z4008 a4008.14, b4008.14159
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Special CaseOperations on a Variable

• Often, one wishes to perform an operation on a
  variable, such as adding or subtracting a value.
• C has a special syntax for an operation performed
  on a variable (i.e. the variable is used on the
  right side of the assignment statement and is the
  target of the assignment as well.)

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Special CaseOperations on a Variable

• This syntax follows the form

variable (operation) expression

• This statement first computes the operation on
  the right side of the equals sign. Then, it
  performs the (operation) on the variable and the
  expression.

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Special CaseOperations on a Variable

• For example,

x 19

• This statement adds 19 to the value contained in
  x.
• This statement is exactly the same as

x x 19
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Another Special CaseIncrement and Decrement

• Incrementing (adding one) and decrementing
  (subtracting one) are two very common operations.
• C has a special syntax for increment and
  decrement operations that follows this form

x
x
--x
x--
OR
OR
Increment
Decrement
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Another Special CaseIncrement and Decrement

• Notice that the increment operator expression x
  is the same as saying x 1
• So, why are there two forms of each operator?
• The answer lies in when the increment (or
  decrement) operation is actually performed.
• This distinction only occurs when the (or --)
  operator is used on a variable in the same
  expression in which the value of the variable is
  used.

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Another Special CaseIncrement and Decrement

• For example,

int x 20, y y x printf(xd,
yd\n,x,y)
int x 20, y y x printf(xd,
yd\n,x,y)
x21, y20
x21, y21
y x x 1 printf(xd, yd\n,x,y)
x 1 y x printf(xd, yd\n,x,y)
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Another Special CaseIncrement and Decrement

• The difference lies in when the expression (x
  or x) gets evaluated in relation to when the
  operator () gets performed.
• The x expression is equal to the value of x and
  the operator is performed after the evaluation
  is over.
• The x expression indicates that the operator
  is performed first (before x is evaluated) and
  thus, is equal to the value of x 1.

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Review

• What is the output of the following program
  fragment?

int x, y, z x 5 y 10 z x y 2 z
x printf(xd, yd, zd\n,x,y,z)
x7, y10, z55