CPS120 Introduction to Computer Science Exam Review Lecture 9.

CPS120 Introduction to Computer Science

Exam Review

Lecture 9

Compilers

• An engine that works on your behalf to process instructions and allow you to deal with various basic rules of the language– The compiler’s 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

Introduction to Programming

The Program Development Cycle

Get the program into machine-readable form

Test and debug the program

Translate the program

Understand the problem

Code the program

Plan the program's logic

Document the program

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

Terminator. Shows the starting and ending points of the program. A terminator has flow lines in only one direction, either in (a stop node) or out (a start node).

Data Input or Output. Allows the user to input data and results to be displayed.

Processing. Indicates an operation performed by the computer, such as a variable assignment or mathematical operation. With a heading – an internal subroutine

Decision. The diamond indicates a decision structure. A diamond always has two flow lines out. One flow lineout is labeled the “yes” branch and the other is labeled the “no” branch.

Predefined Process. One statement denotes a group of previously defined statements. Such as a function or a subroutine created externally

Connector. Connectors avoid crossing flow lines, making the flowchart easier to read. Connectors indicate where flow lines are connected. Connectors come in pairs, one witha flow line in and the other with a flow line out.

Off-page connector. Even fairly small programs can have flowcharts that extend severalpages. The off-page connector indicates the continuation of the flowchart on another page. Just like connectors, off-page connectors come in pairs.

Flow line. Flow lines connect the flowchart symbols and show the sequence of operations during the program execution.

Common Flowchart Symbols

Common Flowchart Symbols

Rules for Pseudocode1. 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. TERMINAE all routines with an END instruction

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 compiler’s rules. These errors are detected by the compiler– Also know as 'fatal compilation errors'

• A logic error is a mistake that complies with the rules of the compiler that causes the program to generate incorrect output

Compiling and Debugging

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

• Then the fun (with logic errors) begins

Warnings

• Even though an executable has been generated, you may not be done with syntax errors

• Compilers generate syntax warning messages which are not fatal errors but represent special error checking functions for certain common programming errors

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• Look for the name of the variable and see what lines

of code it occurs on using EDIT and FIND– LNIK2001: unresolved external – LNK1120: unresolved externals

Computer Mathematics

Representing Data

• The computer knows the type of data stored in a particular location from the context in which the data are being used; – i.e. individual bytes, a word, a longword, etc– 01100011 01100101 01000100 01000000

• Bytes: 99(10, 101 (10, 68 (10, 64(10

• Two byte words: 24,445 (10 and 17,472 (10

• Longword: 1,667,580,992 (10

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

• Extended Binary Coded Decimal Interchange Code (EBCDIC)

Decimal Equivalents

• Assuming the bits are unsigned, the decimal value represented by the bits of a byte can be calculated as follows:

1. Number the bits beginning on the right using superscripts beginning with 0 and increasing as you move left• Note: 20, by definition is 1

2. Use each superscript as an exponent of a power of 23. Multiply the value of each bit by its corresponding

power of 24. Add the products obtained

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

• 0110011110101010100111 becomes• 0001 1001 1110 1010 1010 0111

• Step 2: Replace each four-bit group by its hexadecimal equivalent– 19EAA7(16

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

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

Sign-Magnitude

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

• The remaining (N-1) bits are used for the magnitude

• The range of number represented in this convention is –2 N+1 to +2 N-1 -1

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

– If the result is outside the bounds of –2 n+1 to +2 n-1 –1, an overflow results

One’s 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

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

Two’s Complement Convention

• A positive number is represented using a procedure similar to sign-magnitude

• To express a negative number1. Express the absolute value of the number in binary2. Change all the zeros to ones and all the ones to zeros (called

“complementing the bits”)3. Add one to the number obtained in Step 2

– The range of negative numbers is one larger than the range of positive numbers

– Given a negative number, to find its positive counterpart, use steps 2 & 3 above

Two’s 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

Introduction to C++

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

A Simple C++ Program

Comments //Simple C++ Program // // Purpose: To demonstrate the // parts of a simple C++

programCompiler Directive #include <iostream.h>Main Function main ( )

Braces {Statements cout << "This is a simple program ";

return 0;}

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 */

Compiler Directives

• Instructions to the compiler rather than part of the C++ language– Most common directive is #include

• #include <iostream.h>– A .h file is a header file. It serves as a link between

program code and standard C++ code needed to make programs run

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

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

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

Variables

• Variables or identifiers are used to hold information– Usually mixed case with the first letters small

and the rest starting with a capital– e.g. theWeight

Literals

• Literals are system commands and other pieces of information that the compiler doesn’t understand, and therefore, takes your word for them

• In C++, literals are enclosed in straight double quotes " " which is the shift of the apostrophe

C++ Control Structures

1. "Sequence statements" are imperatives2. "Selection" is the "if then else" statement

– AND, OR, NOT and parentheses ( ) can be used for compound conditions

3. "Iteration" is satisfied by a number of statements– "while" – " do " – "for"

4. The case-type statement is satisfied by the "switch" statement.

– CASE statements are used for most non-trivial selection decisions

Variables

• Used to store values in virtually every computer program– Used for “remembering” things during

program execution– Variables have names, types and values

• Values can change during execution

Data Types - Whole Numbers

• To store whole numbers in a variable, we use a variable of the int data type. – An int variable uses 4 bytes of memory. – An int variable can store a number as low as

-2,147,483,648. – An int variable can store a number as high as

2,147,483,647.

Data Types - Decimal Numbers

• To store decimal numbers in a variable, we use a variable of the double data type– A double variable uses 8 bytes of memory– A double variable can store a number as low as

-1.7 x 10308– A double variable can store a number as high

as 1.7 x 10308– A double variable can store a number with up

to 15 digits of precision (significant digits)

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.

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– You can think of it as a data type for now

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

char grade; // a students semester grade

int numStudents; // number of students in our class

double price; // price of item

string userName; // user's name

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

Initializing Variables

• C++ does not automatically initialize all variables to the value 0

• If you do not initialize a variable, the variable will have an indeterminate value . Initialize your variables at the same time that you declare them.

Constants

• Sometimes you need to use the same value many times throughout a program. In this case, it is proper to use a constant rather than a variable

• Constants allow you to give a name to a value used several times in a program

• The value never changes

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

• For example:– sum = 3 + 1000;

• The variable sum ends up with the value 1003

– salary = 40000;– poundsPressure = 15 + 12;– sum = original + 300;– salary = salary + raise;

Common Arithmetic Operators

+ for addition

- for subtraction

* for multiplication

/ for division

% for modulus (like finding the remainder of a division problem)

Compound Operators

• Example 1: j += 1; // is the same as j = j + 1 • Example 2: total /= 2; // is the same as total = total / 2;

Increments and Decrement• The incrementing (++) and decrementing (--)

operators are useful at times if used carefully • counter++;

• is equivalent to counter = counter + 1; and counter += 1;

• counter--; • is equivalent to counter = counter - 1; and counter -=

1;

• Use the incrementing and decrementing operators with variables in statements such as these, that is with no other operators or code except the variable name that is being incremented or decremented.

Decision Making In Computers

• A circuit quite simply allows one out of two choices to be made depending on its inputs

• 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

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, => greater than< less than>= greater than or equal to<= less than or equal to!= not equal to

Order of Logical Operations

• Logical operators may be mixed within evaluation statements but the following order of preference must be respected:

1. NOT operator (!)

2. AND operator (&&)

3. OR operator (||)

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:*, /, %

+, -

<, >, <=, >=, ==, !=

!

&&

||

String Literals

• A string literal is a sequence of characters that is used in a C++ program between double quotes such as in the statementcout << "Hello world!";where "Hello world!" is the string literal

• Note: that the string literal, in this case, includes a space and an exclamation point

• Similar to a constant• Ends with an invisible null terminator (ASCII 0)

– Represented as \0

Character Arrays

• Used to store strings that change as the program runs– Unlike string literal

• 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)

Using Strings In Programs

• In order to use string variables within any C++program, you must use the compiler directive:

#include <string.h>• At the top of your program. You must also refer to

iostream.h header file as iostream

#include <iostream.h> • you must add the statement below your last

#include directive :

using namespace std;

String Functions

• You can determine the current length of a string with the use of a string's length function. For example:string name = “Paul";int lengthOfName = 0;

lengthOfName = name.length( );

cout << "His name is " << lengthOfName << " letters long.";

More cstring Functions

• strcpy– char [] strcpy (char[] dest, const char[] src);

• Copies the content of src to dest and returns dest

• strlen – Size_t strlen (const char[] string);

• Returns the length of string

Library functions manipulate strings

• length functionstringLength = userString.length( );

• To compare two stringsif (string1 = = string2)

• Test two strings alphabetically if (string1 < string2){ cout << "string1 comes before string2

alphabetically ";}

Input Operations• The operator >> is known as the input

operator. It is also known as the extraction operator

• You use the input operator in statements like,

cin >> numItems;

which would allow the user to input a value to be stored in the variable numItems.

Complexities of Word Input• Some things are done automatically with >>

– 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’);

Using setf and unsetf• Each stream has format options that can be

changed

OPTION DESCRIPTIONleft Left-justifies the output

right Right-justifies the output

showpoint Displays decimal point and trailing zeros for floats

uppercase Displays e in scientific as E

showpos Displays a leading plus sign

scientific Displays floating point number scientifically

fixed Displays floating-point in normal notation

Using Manipulators

• You must include the <iomanip.h> header file at the top of your program in order to use the setprecision, setw, and other manipulators. You must use place the following compiler directive at the top of your program.#include <iomanip.h>

• I/O manipulators are placed directly in the output statementcout << setprecision(2) << price << ‘\n’;

Decision Making in C++

1. if statement

2. switch statement

3. ? conditional operator statement

4. goto statement

IF-THEN

Test Test condition pcondition p

falsefalse truetrue

EntryEntry

ExitExitTrue True

statement astatement a

IF…ELSE

falsefalse truetrue

EntryEntry

ExitExit

Test Test condition pcondition p

““true” true” statement astatement a

““false” false” statement astatement a

General Form if (test expression)

{

True-block statements;

}

else

{

False-block statements;

}

next statement;

General Form if (test expression)

{

True-block statements;

}

else

{

False-block statements;

}

next statement;

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

Iterate

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

No

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

WHILE Loop

NoNo

YesYes

EntryEntry

ExitExit

Test Test condition pcondition p

Loop Loop statement astatement a

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

DO WHILE Loop

Loop Loop statement astatement a

NoNo YesYes

EntryEntry

Test Test condition pcondition p

ExitExit

Do While Syntax

do{ // body statements would be placed here}while (control expression);

• Don't forget to include the required semicolon after the control expression

In Summary

• Loops– goto loops -- simple if…then 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

An Example of A Function#include <iostream.h> void printMyMessage(int numOfTimes); // PROTOTYPE and NAME

int main( ) { int userInput = 0; cout << "Enter a number between 1 and 10 (0 to Exit): " ; cin >> userInput; if (userInput != 0) { printMyMessage (userInput); // CALL STATEMENT WITH ACTUAL PARAMETER } else cout << "Thanks, Bye!";

return 0;} // end of main

void printMyMessage(int numOfTimes) // FUNCTION HEADER WITH RETURN TYPE AND ACTUAL PARAMETER

{ int i=0; // LOCAL VARIABLE WITHIN THE FUNCTION for (i=0; i<= numOfTimes; i++) // BODY

{cout << "Let's Go State!!" << endl;} // OF THE} //end of printMyMessage // FUNCTION

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

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

Required Compiler Directives

• Any program that uses file pointers must include the fstream.h header file with the compiler directive,

#include <fstream.h>

at the top of the program

Preparing to Use Files• Opening a sequential-access file

ofstream outfile; – ofstream is a C++ keyword indicating the type of

pointer that you created – outfile is simply the programmer's chosen name

for the file pointer (and can be any valid name)

• Open the file "mydata.txt" that is stored on your PC's hard drive

outfile.open("mydata.txt", ios::out); or the shorter version

outfile.open("mydata.txt"); – output (out) is the default type of access for ofstream

objects

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", ios::in);

or the shorter version

infile.open("mydata.txt");

Preparing to Write Output

• It is wise to check to make sure that there wasn't an error actually opening the data file

• One can use an if statement like the following to protect the program from crashing.

if (outfile) // same as if (outfile != 0){ outfile << "John Doe" << endl;}else{ cout << "An error occurred while opening the file.\n";}

Writing Output

• To write data to a sequential-access data file you would use a statement like:

outfile << "John Doe" << endl;

to print that name to the next line in the data file pointed to by the file pointer, outfile.

Appending Data

• Adding data to the end of a sequential-access data file is called appending

• Open the file using the ios::app stream operation mode as in:

outfile.open("myfile.txt", ios::app);• where the app is short for append.

• If you accidentally open the file with the ios::out 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

Detecting the End of a File.

• Use the eof function to determine whether the end of a sequential-access file has been reached.

• This function returns a 1 (true) if an attempt has been made to read past the end of the file.

do{ infile >> x;

if ( !infile.eof( ) ) { cout << x << endl; }

} while ( !infile.eof( ) );

Pointer Use in C++.

• A pointer is a variable or constant that holds a memory address– a) Hexadecimal numbers are used for

representing memory locations

216793

216794 216801 iptr

…

216801 3 i

216802

Intializing Pointers

• Declare pointers before use, as with other variables.

• Each variable being declared as a pointer must be preceded by an asterisk (*).

• Initialize pointers before use to a 0, NULL or an address to prevent unexpected results

Pointer Operators

• & is the address operator which returns the address of its operand

• * is the indirection operator or dereferencing operator and returns the value to which the operand (pointer) points.

• sizeof - used to determine the size of an array during program compiliation

Structures

• Structures group variables together in order to make one's programming task more efficient. – Any combination of variables can be combined into

one structure. – This is a useful and efficient way to store data.

struct Student { string socSecNum; string lastName; string firstName; int pointsEarned; double gpa; };

CPS120: Introduction to Computer Science

Lecture 15

Arrays

Arrays: A Definition

• A list of variables accessed using a single identifier– May be of any data type

• Can be single or multi-dimensioned

• Vector classifications are object-oriented representations of arrays

Declaring an Array• To declare an array before it is used in the body of

your program, you must use a statement like:

int scores[10];• 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

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

Loops and Arrays

• Use loops in conjunction with arrays to allow the user to input data into an array

• Use loops with arrays to display data stored in an array

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 statement:

int studentGrades[3] [4];

where 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

Initializing a Multi-dimensional Array

• You can initialize the 2-dimensional array when you declare it by using commas and braces appropriately

int studentGrades[3] [4] = { { 1, 2, 3, 4}, { 5, 6, 7, 8}, { 9, 10, 11, 12} };

• Be careful though when assigning values to a specific position within a 2-dimensional array. – Just like one-dimensional arrays, the subscript positions with

regard to the rows and the columns begin at 0, not– In the example above the value of the variable

studentGrades[0] [2] is 3