Unit 1: Introduction - McGill School Of Computer Science · 2011-09-01 · COMP-202 - Introduction...

transcript

COMP-202Unit 1: Introduction

CONTENTS:What Is Programming?How a Computer WorksProgramming LanguagesJava Basics

Announcements• Did you miss the first lecture? Come talk to me

after class or email me• dpomer@cs.mcgill.ca

Last Class

• Administrative stuff•

• Solving many little problems to avoid thinking about too much at once!

• A tiny bit about java

• What is programming?•

• How computers think•

• A tiny bit more about java: The structure of a program

Part 1: What Is Programming?

COMP-202 - Introduction 6

Programming and Computers•In order to understand what programming is, we need to know what a computer is

Programming and Computers•In order to understand what programming is, we need to know what a computer is•A computer is a machine that executes lists of instructions–We feed a list of instructions to the computer and the computer executes them–The computer may apply the instructions on additional information fed to the computer (the input)–The computer may produce information as a result of executing this list of instructions (the output)

Programming and Computers Programming a computer involves two things:

1) Designing lists of instructions that will make the computer solve specific problems (a.k.a. writing your program)

2) Having the computer execute the instructions (a.k.a. running your program)

Why do we want to program?Before we go any further, it's important to answer the

question:

“What's the benefit of programming?”

Why do we want to program?

Benefits of programming• Programming computers allows us to sleep much more

easily knowing that nothing ever could go wrong.• Our programs will always work perfectly and will never

crash or make any sort of mistakes.

Actually....maybe not

Anyone used Windows Vista?

Benefits of programmingAll jokes aside, generally we program computers because

we either can't or don't want to do the task ourselves.

Some reasons:

-very repetitive task-arithmetic of large numbers-searching through millions of documents

-having someone “help” us without having to bother a real human

-playing a game against a computer-researching information

Giving a computer instructionsA very good way to think about giving a computer

instructions is the following:

1)First figure out what is given in the problem and where it is you are trying to go.

2)Figure out a broad picture of how you want to get from your start to the goal. When you do this, you should figure out, at each step, what you are given, and what you want to produce. Now you have several, smaller problems.

You can solve the smaller problems the same way.

Example: Planning Dinner

Start: Goal:

We need to construct a path from the start to the goal

Example: Planning Dinner

Start: Goal:

Giving a computer instructionsAt each partial step, we need to make sure we have very

precisely determined what we have at the moment and what we need to produce in that step.

For example:Step 1)Shopping:

We have : a recipea burning desire to eat foodmoney

We want to produce:raw ingredients

Giving a computer instructions

Here we can also notice that one of the things we have is not really needed to solve this partial problem.

For example:Step 1)Shopping:

We have : a recipea burning desire to eat foodmoney

We want to produce:raw ingredients

Step 2)Chopping:

We have : raw ingredients

We want to produce:chopped ingredients

What is very important about this?

Giving a computer instructionsStep 2)Chopping:

We have : raw ingredients

We want to produce:chopped ingredients

The input of step 2 is part of the output of step 1. This means that we can use it.

If step 2 required something else (for example, if we also needed to buy knives), then this wouldn't be a complete solution.

Step 3)Cooking

We have : chopped ingredients

We want to produce:cooked food

And we've found a path from one to the other.

Giving a computer instructionsWhen you are giving instructions to a computer, most of

the time, you will repeat this divide and conquer process.

For example:Shopping could be split into:1)Pick up onions2)Pick up bread3)Pick up meat4)Wait in line at the cash register5)Pay the cashieretc.

Input vs Output : InputThe input to a program is what goes into it. It is

whatever is given to the program or problem.

This is anything that is necessary to solve it.

Input vs Output : OutputThe output from a program is what comes from it.

This is anything that is produced as a result of the program running

Input/OutputFor example, what is the input and output of the problem

“cooking scrambled eggs”

Mmmmm.......

Cooking Scrambled Eggs (1)Input (things we need to follow the instructions):–Two eggs–A tablespoon of oil or butter–A pan–A stove–Salt / pepper-maybe milk-DEFINITELY NOT KETCHUP

Output (things that result from following the instructions):–Scrambled eggs

Cooking Scrambled Eggs (2)Once we have the “input” and the “output” of the problem, we can work on figuring out a path from start to finish:

Instructions:–Add oil to pan–Heat pan on stove–Crack eggs into pan–Mix until light and flaky

Example: f(x) = 2xThe input to this mathematical function is “a number”

The output to this function is “a number”

Note: Input is similar to “domain” in math. Output is similar to “range”

We have to judge by context whether this function is being used on integers, real numbers, or imaginary numbers. (Or something completely different!)

In Java, we will explicitly write what the domain and range are.

Example: f(x,y) = 2x + 3yWhat is the input and output?

Example: f(x,y) = 2x + 3yThe input to this mathematical function is “two numbers”

-- x and y

The output is 1 number. The sum of 2x and 3y

Giving the instructions to a computer

Once you have mapped out a solution to a problem in small enough instructions, you can program a computer to execute the instructions

Two things to remember about computers

1)They require very specific instructions:

-You can't tell a computer to cook dinner. It has to be much more detailed!

Two things to remember about computers

2)They “interpret” instructions very literally.

-No sense of idioms as we do in human languages.

Amelia Bedilia

“Draw the drapes”

“Make a sponge cake”

“Pitch the tent”

Programs (1)•

A computer program is essentially a list of instructions telling a computer what to do

The computer is “stupid” in that it is just following the instructions without knowing what it is doing.

Thus you must be very precise and omit no details.

Programs (2)•

Computer problems will have an “input” and an “output” as well.

If you omit the proper instructions or include the wrong instructions, generally 4 things can happen:

1)You get incredibly lucky and on that particular input it works anyway

2)The program gives the incorrect output3)The program crashes4)The program goes on forever and ever•

Case 1Sometimes you will give the computer the wrong

instructions, but it will work anyway on a particular input.

For example, suppose I teach a computer to compute x-squared, and define x^2 as being equal to x + x. Of course this is wrong, but sometimes it will give the right answer.

When would this still lead to the “right” answer?

Case 1If x = 0 or x = 2 then x*x = x+x

It's always very important when you test any program you write to test it on as many possible inputs as possible!

Case 2•

The program gives the wrong output

This would happen in the previous example on any input other than x = 0 or x=2

Case 3Sometimes the program will crash.

This could happen if we defined x^2 as being 1 / x and the input was x = 0

The computer might not know how to divide a number by zero.

Case 4The program goes on forever and ever

Suppose I defined x^2 in terms of x^2. Sometimes (amazingly!) that will actually work, but let's say I told the computer

“You should computer x-squared by computing x-squared”

Giving a computer your instructions

This step consists of translating your human words into a language the computer understands

Kind of like learning a foreign language. But there are some key differences.

Sentences in human languages can often have many interpretations.

Human and Computer Languages

Consider the following English sentence:"The lady hit the man with a baby"

Does this mean 1)A lady hit a man who had a baby? (Dude, what a jerk!)2)A lady used a baby to hit a man? (Good lord!)3)A lady and a baby ganged up on a man and hit him. (Kids today!)

Computer statements, on the other hand, always have only one possible interpretation—although sometimes no one knows how a computer will interpret something!

Human and Computer Languages

One of the challenges is to learn the different interpretations the computer will give to commands.

The computer will not normally tell you how it is interpreting things. It is up to you to figure it out, both by looking at your code and observing the output.

Solving Complex TasksSometimes, we want the computer to perform complex tasks

Writing a detailed list of precise instructions for this complex task would be very difficultThere are just too many instructions for humans to manage

Remember, we only want to think about 4 or 5 (and at absolute most 7) instructions at a time.

Solving Complex TasksSolution:–Break down the complex task into a series of simpler tasks and figure out the input and output of each step; if the simpler tasks are still too complex, break them down into even simpler tasks.–Write a list of instructions for each of the simpler tasks–The list of instructions for each of the simpler tasks can be used as a single instruction in the list of instructions that performs the original task

While considering each subtask, don't worry about how you will solve the other parts.

Summary so far•It's critical to only be thinking about a few things at a time.•When we are trying to solve a problem on a computer, we need to figure out the input and output first.•Then we should construct a path from the input to output, with various “checkpoints” in between.

Java!Now, let's go over our very first program!

Java Program: Hello Worldpublic class HelloWorld { public static void main(String[] args) { System.out.println("Hello World!"); }}

One thing to notice is all these { and }

These are used to denote “blocks” of code.

The purpose of a block is mainly to help the programmer keep track of what parts of code are related.

{'s always mark the beginning of a “block” (or segment) of code.

Each { always has a corresponding } to mark the end.You can tell which corresponds to which because the first

{ opened, will correspond with the last }

Try at home: What happens if you remove the final }, so that there are not matching braces? What happens if you add an extra } at the end so that there are 3 }s?

Most (but not all) of the time, right before a { will be some code specifying what kind of block of code something is.

For the first {, the code is “public class HelloWorld”

class in Javapublic class HelloWorld { public static void main(String[] args) { System.out.println("Hello World!"); }}

One of the key units of organizing things in Java is called a class. A class can have many different things in it which we'll see throughout the term.

class in Javapublic class HelloWorld { public static void main(String[] args) { System.out.println("Hello World!"); }}

Here we are saying: “I want to create a class called HelloWorld and I want it to be public”

The first { signifies the start of the definition of the classThe final } signifies the end of the definition of the class

On the second line we have another {

In this case, the code before is defining a method

methods in Javapublic class HelloWorld { public static void main(String[] args) { System.out.println("Hello World!"); }}

A method is also a key unit of organization in Java.

Here we are saying: “I want to create a method called main

There are several other words (public, static, void, String[], args) that we'll talk about later.

The second { signifies the start of the method called main.

The first } signifies the end of the definition of the method called main.

Since the definition of the method called main is inside the definition of the class called HelloWorld, it means that the method main is a part of the class HelloWorld.

Methods and classesAlmost every line of code you write in Java will be inside

a class. This helps keep things nicely organized.

Almost every line of code you write in Java will also be inside of a method. This also helps keep things nicely organized.

Every method you ever write, will be part of a class.

main methodThe main method is a very particular method.

When you run a Java program, you actually run a Java class

When you do this, the execution of your program will always start at the beginning of the method called main inside whatever class you run.

Inside of a method, you can put many statements or commands.

The highlighted line is an example of a statement.

All statements in Java end in a semi-colon.

The command System.out.println is a way to print something to the screen.

It will print everything between (“ and “) as long as it's on 1 line.

Summary:-Important to break problems down into many pieces-Input / ouput-Classes and methods are just 2 of the ways that Java

helps us to organize things. These are important because they'll help make sure we don't need to think about too much at once.

Part 2: How a Computer Works

Hardware and Software (1)•A computer system consists of both hardware components and software•Hardware consists of the physical, tangible parts of a computer–Cases, monitors, keyboard, mouse, chips–Rule of thumb: If you can take it in your hands and it is part of a computer system, then it is hardware–It is the hardware which executes the instructions•Software: Programs and data that they use•A computer requires both hardware and software–Software cannot run without hardware; instructions are useless unless they are performed by someone / something

Hardware and Software (2)–Hardware will not do anything without software telling it what to do–Therefore, each is essentially useless without the other

An (old) Personal ComputerMonitor /screen(output)

Speakers(output)

Keyboard (input)Mouse(input)

•Case;•contains:•CPU•Memory•Disk drives•...

Central Processing Unit (CPU)•The "operation/action" part of the computer's brain–Basically controls the information / data in a computer•Perform instructions–Arithmetic operations–Logic operations–Decisions•The instructions it understands are much simpler and fine-grained than those we have seen in previous examples

MemoryMemory holds the data–Think of a filing cabinetMain memory: Most of it is called RAM, which stands for Random-Access Memory–Data has to be in main memory so that the CPU can access it–Volatile: its contents are lost when the program terminates or when the computer shuts downSecondary storage: Hard drive / CD / DVD / Blu-Ray disc / USB memory stick–Persistent: its contents will not be lost when the computer shuts down–This is where you keep the data for long-term storage–Secondary storage has much slower access times than main memory

taking notes; packing a box

remembering what Dan said 2 minutesago

remembering your name

taking notes; packing a box(CPU)

remembering what Dan said 2 minutesago (RAM)

remembering your name (secondarystorage)

How do we store things?• Computer memory is electronic. It's just a bunch of

wires!•

• All it can recognize is “on” (current goes through) and “off” (no current goes through)

• Using many of these on/off “switches” together, we can encode many things.

• For example, we can store whether it is morning or afternoon using the following encoding:

• “if the 1st switch is on, then it must be PM. If the 1st switch is off, then it must be AM”

Storing whether it is afternoon or morning

• Pick one electrical switch in memory.• Whenever it is “on” it is AM• Whenever it is “off” it is PM•

• A computer stores billions or trillions of these “switches” By combining many of these, we can control many things.

• (Note: This is just an example. Your computer probably stores this in an entirely different way.)

Encoding the day of the week

• How could we encode the day of the week?•

• If we just use 1 switch, there will not be enough information.

• How many “switches” will we need?

Storage is Exponential

• In general, if there are n possible values to store, we can encode it using

• log2(n) “switches”•

• Of course, there is no such thing as a fraction of a switch, so we will always have to round up.

• Put another way, if we have n switches, we can store 2n values

Bits = Switch

• 1 “bit” is the same thing as a “switch”• It has one or two values “on” or “off”• For simplicity of notation, we will often just refer to

these as 1 (on) and 0 (off)•

• If you like, you could call them “true/false,” “yes/no,” “oui/non,” or “cats/dogs”

Byte = 8 bits

• A byte is simply 8 bits•

• Question: How many possible values can we store in a byte?

Other Memory Units

• A kilobyte is 210 bytes (1024 bytes)• A megabyte is 210 kilobytes (1024 bytes)• Strangely, a gigabyte is just 1,000,000,000 bytes•

Practice Exercises

1) How many bits does it take to encode the day of the month?

2) How could you encode the letters of the alphabet?3) How could you encode a 3 letter word?4) How would you encode a sentence?

Main Memory Organization

92789279928092819282928392849285

address cell

Main memory is divided into many memory locations (or cells)

Each memory cell has a numeric address which uniquely identifies it

Each cell contains a data value (for example, 22)

Bits and Bytes•In a computer, data values are stored as sequences of bits•1 bit: most basic unit of memory•1 byte = 8 bits–1 byte can therefore represent 28 = 256 different values•In memory, one cell contains one byte, not one bit

92789279

0001011000000111

Number systems(decimal vs. binary)

The way we usually count (1, 2, 3, 4, 5, 6...) corresponds to a decimal number system. Each number position (digit) represents a power of 10 multiplied by an integer between 0 and 9.• 6 = 6 x 100

• 31 = 3 x 101 +1 x 100 • 0.5 = 0 x 100 + 5 x 10-1

In the binary number system, each number position (bit) represents a power of 2 multiplied by an integer between 0 and 1. • 6 = 1 x 22 + 1 x 21 + 0 x 20 = 110• 31 = ?

Devices•Devices are hardware components which the CPU can access and control–Input devices are used to feed information to the computer–Output devices is what the computer uses to give information back to the user–Some devices are both input and output devices•Examples of devices:–Cards (Video card, sound card, network card, ...)–Optical (CD / DVD / Blu-Ray) drives–USB ports–...

Motherboard•Acts as a bridge between the CPU, memory, disks, and other devices•Data transfers taking place between the hardware components of a computer take place via the motherboard

Program Execution and Hardware

MonitorKeyboard

MainMemory

READ DISP

LOAD STORE

17→18

Hardware Interaction

Monitor

Keyboard

CentralProcessing

Unit(CPU)

MainMemory

HardDrive

Program Execution (fetch-execute cycle)

•A program tells the CPU how to manipulate and/or move information•The CPU repeatedly performs the three following operations:–Reads the next instruction in the program–Figures out what the instruction means•add two values?•load some value from memory?•store some value in memory?•compare two numbers?•...–Performs the instruction•This is called the fetch-execute cycle

Program Execution (2)•Suppose you want to write a program that reads a number from the keyboard, adds 1 to it, and displays the new value to the screen•This program might consist of the following instructions:–READ a value from the keyboard and store it in memory location x–LOAD the value stored in memory location x into the CPU–ADD 1 to the value stored in the CPU–STORE the value currently in the CPU back into memory location x–DISPLAY the value stored in memory location x to the screen

Program Execution and Hardware

MonitorKeyboard

MainMemory

READ DISP

LOAD STORE

17→18

Part 3: Programming Languages

Programming Languages (1)•We need to expresses our ideas in a form that a computer can understand: a program•A programming language specifies the words and symbols that we can use to write a program

– e.g. “red” belongs to English; “rouge” belongs to French

•A programming language employs a set of rules that dictate how the words and symbols can be put together to form valid program statements–e.g. “Banana red and” in not a valid statement in English.

Programming Languages (2)Computers are very intolerant of incorrect programming language statements–Humans are much more tolerant of incorrect natural language statements

• You understand “The kiten is cute” even though kitten is mispelled.

Syntax and Semantics•The syntax rules of a language define what words and symbols are valid in this language, and how they can be combined to make a valid program

– “The kiten is cute” is not syntactically correct.

•The semantics of a program statement define what those words, symbols, and statements mean (their purposes or roles in a program)

• “Banana red and.” is not semantically correct.

Machine Language•Each instruction that a CPU understands is represented as a different series of bits

• The set of all instructions that a CPU understands directly forms the machine language for that CPU

•Each CPU type understands a different machine language–In other words, for each different model of CPU, a given series of bits could mean a different instruction

• For example, on an x86-compatible CPU (Intel, AMD), the series of bits 10101010 could mean ADD, while on a PowerPC CPU (old Macs, PlayStation 3) it could mean LOAD

Machine Language Example•Here are the first 20 bytes of a machine language program that:–asks the user to enter an integer value using the keyboard–reads this value from the keyboard–adds one to this value, and–displays the new value to the screen

01111111 01000101 01001100 01000110 0000000100000001 00000001 00000000 00000000 0000000000000000 00000000 00000000 00000000 0000000000000000 00000010 00000000 00000011 00000000

More the 6500 bytes in total!

Do you think it's fun or easy to write a binary program?

Machine Language Disadvantages

•Very tedious and confusing: machine language is extremely difficult for humans to read•Error-prone–If you change one bit from 1 to 0 (or vice-versa), or forget a bit, your program's behavior will likely be not even close to what you expected–Moreover, errors are hard to find and correct•Programs are not portable–Running the program on a different processor or CPU requires a complete rewrite of the program

High-Level Languages (1)•To make programming more convenient for humans, high-level languages were developed•No CPU understands high-level languages directly–Programs written in these languages must all be translated in machine language before a computer can run them (that's what a compiler is for)•Basic idea:–Develop a language that looks like a mix of English and mathematical notation to make it easier for humans to read, understand, and write it–For each CPU type, develop a program that translates a program in high-level language to the corresponding machine language instructions (a compiler)

High-Level Language Example #include<stdio.h>

int main(void) { int v; printf("Enter an integer value: "); scanf("%i", &v);

v = v + 1;

printf("New value (old value + 1): %i\n", v); return 0;}

Compilers vs. Interpreters

Compilers

CPU 1 CPU 2

Source code(high-level)

Compiler(to CPU 1)

Compiler(to CPU 2)

Binary code(CPU 1)

Binary code(CPU 2)

Interpreters (1)•An interpreter is another kind of program. It takes source code and translates it into a target language–However, the target language instructions it produces are executed immediately–No executable file is created

Interpreters (2)

CPU 1 CPU 2

Source code(high-level)

Interpreter(for CPU 1)

Interpreter(for CPU 2)

Java combines a compiler with an interpreter

• Java compiler (javac, included in JDK 6) takes source and translates it into bytecode

foo.java(Java)

foo.class(bytecode)

foo.class can than be executed using an interpreter, the Java Virtual Machine (JVM)

Programming Errors•A program can have three types of errors•Compile-time errors: the compiler finds problems with syntax and other basic issues•Run-time errors: a problem occurs during program execution, and causes the program to terminate abnormally (or crash)–Division by 0•Logical errors: the program runs, but produces incorrect results

• celcius = (5.0 / 9.0) * fahrenheit - 32; // Incorrect equation; should be // (5.0 / 9.0) * (fahrenheit – 32)

Compiling and Running Programs

•Type your program using a text editor (not a word processor) or an IDE (Integrated Development Environment)•Save your program: e.g. foo.java•Compile the program–If there are syntax errors, the compiler will not generate the target language program; it will report the errors instead–In this case, fix the program, save it, and try again•Run the program and observe the results–If there are logical errors or run-time errors, they will be detectable in this step–In this case, fix the program, save it, recompile it, and try again

Development Life Cycle

Runprogram0 errors

Syntaxerrors

Logic andrun-timeerrors

Compileprogram

Write program

•Errors may take a long time to debug!–Important Note: When you compile for the first time and see 150 errors, do not despair. Only the first 1 or 2 errors are relevant. Fix those and compile again. There should be fewer errors (like 50). Repeat until there are no more errors.

Summary•Computers can only process information in binary (electrical switches)•Binary = hard to read and write•High level languages were designed to make this easier•Compilers vs Interpreters

Next Class •Java!•How to write things to the screen•How to get text from a user•How to perform arithmetic in Java

Unit 1: Introduction - McGill School Of Computer Science · 2011-09-01 · COMP-202 - Introduction...

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