CPS120: Introduction to Computer Science

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CPS120 Introduction to Computer Science

• Basic Structure

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Using Relational Operators

• Relational operators provide the tools with
  which programs make decisions with true and false
  evaluations

equal to NOTE this is two equals
symbols next to each other, not to be confused
with the assignment operator, gt greater
thanlt less thangt greater than or
equal tolt less than or equal to!
not equal to
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Using Logical Operators

• When complex decisions must be coded into an
  algorithm, it may be necessary to "chain
  together" a few relational expressions (that use
  relational operators)
• This is done with logical operators (also called
  Boolean operators.)

is the logical AND operator
is the logical OR operator ! is the
logical NOT operator
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Complete order of operations

• The complete order of operations including all of
  the arithmetic, relational, and logical operators
  including all of the basic arithmetic,
  relational, logical operators is
• , /,
• , -
• lt, gt, lt, gt, , !
• !

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Decision Making in C

1. if statement
2. switch statement
3. ? conditional operator statement
4. goto statement

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Selection

• Statements that include if statements and switch
  statements
• Selection statements are sometimes called
  conditional or decision statements.

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Structural Review Types of Statements

• sequence
• selection (if and switch structures)
• iteration (for and while loops)
• invocation (functions)

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Compare and Branch

• A program can instruct a computer to compare two
  items and do something based on a match or
  mismatch which, in turn, redirect the sequence of
  programming instructions.
• There are two forms
• IF-THEN
• IF-THEN-ELSE

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IF-THEN
Test condition p
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Use the IF structure

• Practically all computer languages have some sort
  of if structure. In C, the if structure is a
  one-way selection structure
• if (number 3)
• cout ltlt "The value of number is 3"
• IF structures use a control expression to
  determine if the code in the braces is to be
  executed or not

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Compound Conditionals

• You must use the AND operator () to form a
  compound relational expression
• if (0 lt number number lt 10)
• cout ltlt number ltlt " is greater than 0 but
  less than 10." ltlt endl

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IF-THEN-ELSE
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General Form

• if (test expression)
• True-block statements
• else
• False-block statements
• next statement

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The If Else statement

• Two-way selection structure since either the
  block of code after the "if" part will be
  executed or the block of code after the "else"
  part will be executed but not both
• The If" part is executed if the control
  expression is TRUE while the "else" part is
  executed if the "if" part is FALSE, guaranteeing
  one part of the expression to be executed or the
  other

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Nested If Statements

• If structures and if/else statements can be
  nested within one another in order to model
  complex decision structures.
• Use the braces and semicolons properly when
  coding such structures.

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General Form

• if (test condition 1)
• // true-block1 statements
• if (test condition 2)
• true-block2 statements
• else
• false-block2 statements
• else
• false-block1 statements

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Switch Structure

• The switch structure is a multiple-selection
  structure that allows even more complicated
  decision statements than a two-way if/else
  structure allows.
• It chooses one of the "cases" depending on the
  result of the control expression.
• Only variables with the INT or CHAR data types
  may be used in the control expressions (i.e.
  parentheses) of switch statements.
• Single quotes must be used around CHAR variables
• Single quotes are NOT used around the integer
  values

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Sample Switch Structure

• switch (character_entered)
• case A
• cout ltlt The character entered was A.\n
• break
• case B
• cout ltlt The character entered was B.\n
• default
• cout ltlt Illegal entry\n

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Grouping Cases

• switch (number)
• case 1
• case 3
• case 5
• case 7
• case 9
• cout ltlt number ltlt " is an even number." ltlt
  endl
• break
• case 2
• case 4
• case 6
• case 8
• cout ltlt number ltlt " is an odd number. " ltlt
  endl
• break
• default
• cout ltlt number ltlt " is not a value between
  or including 1 and 9." ltlt endl
• break

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Iterate

• A program loop is a form of iteration. A computer
  can be instructed to repeat instructions under
  certain conditions.

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Iteration

• Statements that allow the compiler to return to a
  point higher in the program in order to
  continuously repeat one or more statements
• All loops including while, do/while, and for
  loops are prime examples of iteration statements

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Looping Statements

• Count-controlled loops
• Repeat a specified number of times
• Use of a special variable called a loop control
  variable

Flow of control of while statement
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For Loops

• A for loop always executes a specific number of
  iterations.
• Use a for loop when you know exactly how many
  times a set of statements must be repeated
• A for loop is called a determinant or definite
  loop because the programmer knows exactly how
  many times it will iterate

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Syntax of a for Loop

• for (initializing expression control expression
  step expression) // one or more
  statements
• The initializing expression sets the counter
  variable for the loop to its initial value.
• The control expression ends the loop at the
  specified moment.
• The step expression changes the counter variable
• Semi-colons, not commas, divide the expressions

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Things to Remember About For Loops

• It is possible to have variable increase by a
  value other than one
• for (num 1 num lt 10 num num 3)
• It is possible to initialize the loop variable
  within the initializing expression. But I
  recommend against it
• Declare loop variables at the top of the function
  where they belong
• If an if statement only contains one body
  statement, the compiler doesn't require the use
  of curly braces

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Looping Statements

• The while statement is used to repeat a course of
  action
• Lets look at two distinct types of repetitions

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Iteration Control Structures

• Iteration control structures are looping
  mechanisms.
• Loops repeat an activity until stopped. The
  location of the stopping mechanism determines how
  the loop will work
• Leading decisions
• Trailing decisions

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Leading Decisions

• If the stop is at the beginning of the iteration,
  then the control is called a leading decision.
• The command DO WHILE performs the iteration and
  places the stop at the beginning.

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WHILE Loop
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While Loops

• A while loop does not necessarily iterate a
  specified number of times
• As long as its control expression is true, a
  while loop will continue to iterate
• An indeterminate or indefinite loop because only
  at run-time can it be determined how many times
  it will iterate
• While is considered a top-checking loop
• The control expression is located on the first
  line of code
• If the control expression initially evaluates to
  FALSE, the loop will not execute even once.

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While Loop Syntax

• while (control expression) // one or
  more statements
• The control expression must evaluate to TRUE in
  order for the while loop to iterate even once

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Trailing Decisions

• If the stop is at the end of the iteration, the
  control mechanism is called a trailing decision.
• The command DO UNTIL performs the iteration and
  puts the stop at the end of the loop.

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DO WHILE Loop
Loop statement a
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While Loops

• A while loop does not necessarily iterate a
  specified number of times
• As long as its control expression is true, a
  while loop will continue to iterate
• An indeterminate or indefinite loop because only
  at run-time can it be determined how many times
  it will iterate
• While is considered a top-checking loop
• The control expression is located on the first
  line of code
• If the control expression initially evaluates to
  FALSE, the loop will not execute even once.

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While Loop Syntax

• while (control expression) // one or
  more statements
• The control expression must evaluate to TRUE in
  order for the while loop to iterate even once

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Break and Continue Statements

• A break statement is used to stop the execution
  of a loop immediately and to continue by
  executing the statement that comes directly after
  the loop
• A continue statement is used to stop the
  execution of the statements in the loop's body on
  that particular iteration and to continue by
  starting the next iteration of the loop

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In Summary

• Loops
• goto loops -- simple ifthen structure
• for -- fixed number of loops
• while -- may never be run
• while (true) -- always true, needs break
• do while -- always run once
• continue causes while, do while, and for loops
  to start over
• break causes while, do while, for and switch
  statements to end

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Invocation

• Statements that allow you to place a block of
  statements that you wish to use several times
  during a program in one section
• You then have the ability to "call" those
  statements whenever you wish to execute them
  without having to retype the block of statements
  over and over again within the program
• In C, we use "function calls" as invocation
  statements

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Functions
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Function / Subprogram Statements

• We can give a section of code a name and use that
  name as a statement in another part of the
  program
• When the name is encountered, the processing in
  the other part of the program halts while the
  named code is executed

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Subprogram / Function Statements
Subprogram flow of control
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Subprogram Statements
Subprogram flow of control
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Functions

• Every C program must have a main function
• C may also have a number of user-defined
  functions
• Each of the smaller tasks (let's say, subtasks)
  can be coded as C functions that go together
  with the to make up a structured program.

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Library Functions

• There are many functions available to C
  programmers which were written by other
  programmers
• Use the include compiler directive at the top of
  your program to take advantage of these
  functions.
• To use the you do not even have to know exactly
  how they work
• You do have to know how many arguments to send to
  the functions and what datatypes to use for those
  functions.

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Function Statements

• There are times when the calling unit needs to
  give information to the function to use in its
  processing
• A parameter list is a list of the identifiers
  with which the subprogram is to work, along with
  the types of each identifier placed in
  parentheses beside the function name

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Describing a Function

• The first line of a function is called the
  function header
• Before the name of the function you must specify
  a "return type."
• The return type is the data type of the value
  that is returned by the function to the calling
  function
• If the function does not return any value, you
  must type the word void as the return type
• After the name of the function in the function
  header, you must include a parameter list.
• Immediately preceding each parameter, you must
  identify the data type of that parameter.

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Function Syntax

• Functions are given valid identifier names, just
  like variables and constants.
• A function name must be followed by parentheses
• Coded functions are placed at the bottom of a C
  program, after the main function.
• If the functions are listed after the main
  function, then function prototypes must be
  included at the top of the program, above the
  main function
• An error will definitely result if you call a
  function from within the main function that has
  not been prototyped.

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Structure of Functions in C

• function-name(argument list)argument
  declaration local variable declarations exec
  utable statement1 executable statement2 ------
  ---- ---------- return (expression)          
     

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An Example of A Function

• include ltiostream.hgt
• void printMyMessage(int numOfTimes) //
  PROTOTYPE and NAME
• int main( )
• int userInput 0
• cout ltlt "Enter a number between 1 and 10 (0 to
  Exit) "
• cin gtgt userInput
• if (userInput ! 0)
• printMyMessage (userInput) // CALL
  STATEMENT WITH ACTUAL PARAMETER
• else
• cout ltlt "Thanks, Bye!"
• return 0
• // end of main
• void printMyMessage(int numOfTimes) // FUNCTION
  HEADER WITH RETURN TYPE AND // ACTUAL
  PARAMETER

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Passing Data

• Data is passed to functions as arguments
• When a function is "called" by the main function
  one or more arguments are passed to the function
• On the receiving end, the function accepts these
  arguments
• The variable names of the arguments from the
  "calling" function do not have to be the same as
  the names in the "called" function.
• The data types of the arguments and the
  parameters should match exactly

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More About Passing Arguments

• There are technically three ways to pass data as
  arguments to functions
• passing by value is the preferred method. You
  simply use a variable name, an actual numeric
  literal, or an expression in the parentheses of
  the call statement
• passing by reference is to be used when you want
  the function to actually and permanently change
  the values of one or more variables
• You must use an ampersand () before the formal
  parameter names in the function header (the first
  line of the function definition) to denote
  passing by reference
• passing by address is technically what happens
  when you pass an array to a function

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Returning Values

• If the function is not a void function, there
  must be a return statement at the end of the
  function

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Another Look at Return Values

• Often, though, you want your function to return a
  computed value to the calling function
• It is not possible in C to execute two return
  statements within a function
• It is not possible to return two values in the
  same return statement

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Scope of Variables

• The scope of a variable is the area in which it
  can be legally referenced
• Variables are either global or local in nature
• Global variables are ones that are declared
  outside and above the main function
• They can be used in any function throughout the
  program.
• It is not wise to use global variables any more
  than you have to.
• Local variables are ones that are declared inside
  of a function, including main. They cannot be
  used or referred to in other functions