CPS120 Introduction to Computer Science

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CPS120 Introduction to Computer Science

• Exam Review

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Compilers

• An engine that works on your behalf to process
  instructions and allow you to deal with various
  basic rules of the language
• The compilers job is to make sure you follow the
  rules, to require that you provide enough
  information that the compiler can translate you
  instructions into languages the components can
  understand

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Fundamental Programming Concepts

• One-after-another (Sequence)
• Decision-making (Selection)
• Making choices between 2 or more alternatives
• Repetition (Iteration)
• Concerned with repetitive tasks (and the
  termination conditions of loops)
• Invocation
• Delegation of sub-tasks to functions / procedures

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Common Flowchart Symbols
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Rules for Pseudocode

1. Make the pseudocode language-independent
2. Indent lines for readability
3. Make key words stick out by showing them
   capitalized, in a different color or a different
   font
4. Punctuation is optional
5. End every IF with ENDIF
6. Begin loop with LOOP and end with ENDLOOP
7. Show MAINLINE first all others follow
8. TERMINATE all routines with an END instruction

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Syntax Logic Errors

• A syntax error is simply the violation of the
  rules of a language misuse of structure and form
  in programming or a violation of the compilers
  rules. These errors are detected by the compiler
• A logic error is a mistake that complies with the
  rules of the compiler that causes the program to
  generate incorrect output

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Compiling and Debugging

• Executable code will not be created until you
  correct all of the syntax errors in your source
  code

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Linker Errors

• Not all syntax errors are detectable by the
  compiler
• These errors do not become apparent until files
  are put together to create an executable
• These errors are not linked to a specific line of
  code

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Alphanumeric Codes

• American Standard Code for Information
  Interchange (ASCII)
• 7-bit code
• Since the unit of storage is a bit, all ASCII
  codes are represented by 8 bits, with a zero in
  the most significant digit
• H e l l o W o r l d
• 48 65 6C 6C 6F 20 57 6F 72 6C 64

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Decimal Equivalents

• Assuming the bits are unsigned, the decimal value
  represented by the bits of a byte can be
  calculated as follows
• Number the bits beginning on the right using
  superscripts beginning with 0 and increasing as
  you move left
• Note 20, by definition is 1
• Use each superscript as an exponent of a power of
  2
• Multiply the value of each bit by its
  corresponding power of 2
• Add the products obtained

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Binary to Hex

• Step 1 Form four-bit groups beginning from the
  rightmost bit of the binary number
• If the last group (at the leftmost position) has
  less than four bits, add extra zeros to the left
  of the group to make it a four-bit group
• Step 2 Replace each four-bit group by its
  hexadecimal equivalent
• 19EAA7(16

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Converting Decimal to Other Bases

• Step 1 Divide the number by the base you are
  converting to (r)
• Step 2 Successively divide the quotients by (r)
  until a zero quotient is obtained
• Step 3 The decimal equivalent is obtained by
  writing the remainders of the successive division
  in the opposite order in which they were obtained
• Know as modulus arithmetic
• Step 4 Verify the result by multiplying it out

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Representing Negatives

• It is necessary to choose one of the bits of the
  basic unit as a sign bit
• Usually the leftmost bit
• By convention, 0 is positive and 1 is negative
• Positive values have the same representation in
  all conventions
• However, in order to interpret the content of any
  memory location correctly, it necessary to know
  the convention being used used for negative
  numbers

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Sign-Magnitude

• For a basic unit of N bits, the leftmost bit is
  used exclusively to represent the sign

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Sign-magnitude Operations

• Addition of two numbers in sign-magnitude is
  carried out using the usual conventions of binary
  arithmetic
• If both numbers are the same sign, we add their
  magnitude and copy the same sign
• If different signs, determine which number has
  the larger magnitude and subtract the other from
  it. The sign of the result is the sign of the
  operand with the larger magnitude

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Ones Complement

• Positive numbers are represented in the usual way
• For negatives
• STEP 1 Start with the binary representation of
  the absolute value
• STEP 2 Complement all of its bits

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One's Complement Operations

• Treat the sign bit as any other bit
• For addition, carry out of the leftmost bit is
  added to the rightmost bit end-around carry

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Twos Complement Convention

• A positive number is represented using a
  procedure similar to sign-magnitude
• To express a negative number
• Express the absolute value of the number in
  binary
• Change all the zeros to ones and all the ones to
  zeros (called complementing the bits)
• Add one to the number obtained in Step 2

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Twos Complement Operations

• Addition
• Treat the numbers as unsigned integers
• The sign bit is treated as any other number
• Ignore any carry on the leftmost position
• Subtraction
• Treat the numbers as unsigned integers
• If a "borrow" is necessary in the leftmost place,
  borrow as if there were another invisible
  one-bit to the left of the minuend

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C Usages Conventions

• C is absolutely case sensitive
• For Instance A is 97 in ASCII and a is 65
• Remember in ASCII , , and ( are not equivalent
• No keywords in ANSI standard are even partially
  uppercase
• While is not a keyword, while is
• Be careful if you define new keywords

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Comments

• Document what is happening, why it is happening
  and other issues
• Commentary is ignored by the compiler
• C has inline, block and documentary comments
• Inline comments are within line of code
• Use the // symbols
• Block comments are long comments delimited with
  / and /

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Compiler Directives

• Instructions to the compiler rather than part of
  the C language
• Most common directive is include
• include ltiostream.hgt
• A .h file is a header file. It serves as a link
  between program code and standard C code needed
  to make programs run

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Functions

• A function is a block of code that carries out a
  specific task
• Every C program has a main function that
  executes when a program initiates
• Includes open parenthesis to designate a function
• Ends with a return 0 statement

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Scope Delimiters

• A symbol or pair of symbols used to define a
  region or area which is considered a locale
• In programming, many structures need to have
  their scope defined because they should not
  affect the entire program
• In C, the symbols and are used

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Semicolons

• There must be a semicolon after every statement
• To tell the compiler that the statement is
  complete
• Function definitions and compiler directives are
  exempt

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C Control Structures

• "Sequence statements" are imperatives
• "Selection" is the "if then else" statement
• AND, OR, NOT and parentheses ( ) can be used for
  compound conditions
• "Iteration" is satisfied by a number of
  statements
• "while"
• " do "
• "for"
• The case-type statement is satisfied by the
  "switch" statement.
• CASE statements are used for most non-trivial
  selection decisions

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Data Types - Characters

• To store a letter or a single character (such as
  , , , etc.), we use a variable of the char
  data type.
• A char variable only uses 1 byte of memory.
• A char variable can only hold one letter, digit,
  or character.

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Data Types Words / Phrases

• To store a word or phrase (string value), we use
  a variable that is a string
• Technically string is not a data type but a class

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Using Variables in C

• Variables must be declared before they are used
  in C. Get into the habit of doing this at the
  top of your functions

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Variable Names

• Choose your own variable names but you must be
  careful to use valid ones
• do not use keywords that are defined in the
  programming language (Reserved Words)
• do not include spaces or other disallowed
  characters
• do not use more than 255 characters
• do begin the identifier with a letter

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Initializing Variables

• C does not automatically initialize all
  variables

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The Assignment Operator

• The assignment operator is the equal symbol ()
• The assignment operator changes the value of the
  variable to its left after evaluating the
  expression on its right

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Decision Making In Computers

• When decisions are made in a computer program,
  they are simply the result of a computation in
  which the final result is either TRUE or FALSE
• The value zero (0) is considered to be FALSE by
  C. Any positive or negative value is considered
  to be TRUE

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Using Relational Operators

• Relational operators provide the tools with
  which programs make decisions

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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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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Character Arrays

• An array is a group of variables of the same data
  type that appear together in memory
• In this case each variable holds a character and
  the last variable in the string holds the null
  terminator (/0)

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Input Operations

• The operator gtgt is known as the input operator.
  It is also known as the extraction operator
• You use the input operator in statements like,
• cin gtgt numItems
• which would allow the user to input a value to
  be stored in the variable numItems.

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Complexities of Word Input

• Some things are done automatically with gtgt
• get does not skip over line breaks and spaces
• If the user enters a string longer than the
  length specified in the call to the get function,
  the remaining characters are left in the input
  stream
• Get always ignores the new line character (\n)
  and leaves it in the stream
• Use the ignore function to flush the contents of
  the input stream
• cin.ignore(80, \n)

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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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General Form

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

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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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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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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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Do While Syntax

• do // body statements would be placed
  herewhile (control expression)
• Don't forget to include the required semicolon
  after the control expression

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Break and Continue

• 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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An Example of A Function

• include ltiostream.hgt
• void printMyMessage(int numOfTimes) //
  PROTOTYPE and NAME
• int main( )
• printMyMessage (userInput) // CALL
  STATEMENT WITH ACTUAL PARAMETER
• return 0
• // end of main
• void printMyMessage(int numOfTimes) // FUNCTION
  HEADER
• int i0 // LOCAL VARIABLE WITHIN THE FUNCTION
• for (i0 ilt numOfTimes i) // BODY
• cout ltlt "Let's Go State!!" ltlt endl // OF THE
• //end of printMyMessage // FUNCTION

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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

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Reading From a File

• Declare a file pointer as an ifstream object
  with
• ifstream infile
• ifstream is a keyword and infile is the name for
  the file pointer.
• Open the actual file for reading with
• infile.open("mydata.txt", iosin) infile.
  open("mydata.txt")

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Writing Output

• To write data to a sequential-access data file
  you would use a statement like
• outfile ltlt "John Doe" ltlt endl
• to print that name to the next line in the data
  file pointed to by the file pointer, outfile.

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Appending Data

• Adding data to the end of a sequential-access
  data file is called appending
• Open the file using the iosapp stream operation
  mode as in
• outfile.open("myfile.txt", iosapp)
• where the app is short for append.
• If you accidentally open the file with the
  iosout mode, you will end up overwriting data
  in the file because C will write the first
  piece of outputted data at the beginning of the
  sequential-access data file

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Declaring an Array

• To declare an array before it is used in the body
  of your program, you must use a statement like
• int scores10
• This would declare an array of integers, named
  "scores".
• In this case, scores can store up to 10 different
  integer values.
• The positions of the array are identified by
  their index positions which run from 0 to 9 (not
  1 to 10.)
• Each one of the 10 variables in scores is called
  an element

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Initializing an Array

• If you wish to initialize each element of an
  array to a specific value, you can use the
  statement,int scores 65, 76, 45, 83, 99
• You don't even have to specify a size of the
  array in this case since the initialization
  statementwould cause the compiler to declare an
  array of size 5 since there are five values in
  the set of curly braces

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Declaring a Multi-dimensional Array

• To declare an array of integers called
  studentGrades to be a 2-dimensional array with 3
  rows and 4 columns, you would use the
  statementint studentGrades3 4where the
  first integer value is used to specify the number
  of rows in the array and the second value
  specifies the number of columns
• Think of remote control