icp lecture 2

8/2/2019 ICP Lecture 2

1/37

Lets begin by discussing one

important point, which I should have

mentioned the last time Some of you are already aware of this acutely,

but for those who are not,

Let me remind you and make it clear to you thatsince you are computer science students, thiscourse is THEfundamentalcourse for you, not

basic English, business or physics etc, importantthough those courses are.

If you are going to be a computer scientist, thenyou better get serious about this course.


2/37

The first lecture was just an

introduction

From this lecture we will really get cooking. You willbe given at leastone programming task at the end ofeach lecture. And we will really get into core concepts.

I will also assign reading for each lecture that youshould study. If you dont, its your loss. Just mark mywords: These initial courses are what will build yourfoundation for your ENTIRE life as a computer

scientist. I say this from experience!!!

So without any further ado, lets begin


3/37

What do computer scientists do?

As I said in the last lecture, they map the

outside world to the world of computers.

Using, what?

Obviously


4/37

Computational models of the world

(programs?)

So just like other scientists use abstract models andtheories to map the world to their respective domains,

we computer scientists make computational models of

the world outside.

Whenever we see a problem in the world outside, ourfirst instinct as compute scientists should be to

consider how we would program or code thisproblem. How to make the computer do what we wantit to do? This question lies at the heart of computerscience


5/37

How to think like a computer scientist?

This is the goal of this course. (Suggested readingin the reference material. How to think like a

computer scientist)

Not to learn some one particular language

And please you cant depend on memorization.

This course is not for crammers. If you want to

cram, you are in the wrong field. And I say this

seriously. I am guessing there is still time, get

yourself transferred to the BBA course etc.


6/37

So what is computation??

Very simply, modeling the outside world in theworld of computers. And then using that modelto analyze and solve problems.

So what kind of knowledge is modeled in acomputational model?

There are two major types of knowledge in theworld.


7/37

Declarative and Imperative

Declarative: The list of prime numbers is:

2,3,5,7,11,13..

This does not tell us how we got these prime

numbers


8/37

Imperative: (or algorithmic if you like)

1. Create a list of consecutive integers from 2 to n: (2, 3,

4, ..., n).

2. Initially, let p equal 2, the first prime number.

3. Starting from p, count up in increments of p and

mark each of these numbers greater than p itself in

the list. These numbers will be 2p, 3p, 4p, etc.; note

that some of them may have already been marked.

4. Find the first number greater than p in the list that is

not marked; let p now equal this number (which is

the next prime).5. If there were no more unmarked numbers in the list,

stop. Otherwise, repeat from step 3.

This is basically a recipe, an algorithmwe discussedthis last time.


9/37

Or a simple recipe

Add sugar and milk

Beat egg yolk till the its smooth (or we can

say, while egg yolk is not smooth, keep

beating.

If sugar is not to taste, add more

And so on


10/37

How to capture this in a mechanical

process

One could build a circuit board?

Components to store data.

Wires to move data around. Something for multiplication

Something to check whether value is marked

or not.


11/37

This was an example of early

computers

Fixed Program Computers

Such as a calculator? Sure!

It is built for a specific purpose. You cannot do anythingelse on it.

Play Need For Speed on it? Powerpoint presentation?

No, it is meant for one purpose. So it is notprogrammable.


12/37

What if

We have a machine that can take an

algorithm, a sequence of steps and execute it.

So it can take any input and work accordingly.

Of course, there is the interpreter in the

modern computer that does this.

How does it work? A brief overview


13/37

Memory

Control Unit ALU

INSTRUCTION 1

INSTRUCTION 2

INSTRUCTION 3

And so on

Program

Counter

INPUT OUTPUT


14/37

So as discussed earlier

We need a programming language to code

these instructions

What is a programming language?


15/37

Programming language vs

programming skills and techniques

The main thing is to have a programmingmindset and to know the common

programming techniques across alllanguages.

So our goal is not to learn any onelanguage but rather equip ourselves insuch a manner that we can program in anylanguage that we are required to.


16/37

Therefore

Learning how to program, concepts, skill...

These are uniform across all languages.

What can be done in one language can be

done in another.


17/37

1. High level vs. Low level languages

What does high level mean? (I told you the

last time)

2. General purpose vs. targeted languages

3. Interpreted vs. Compiled Language


18/37

Syntax vs Semantics

Syntax: Rules governing the sequence of words

in a sentence.

Semantics: What sentence are meaningful?

Something that is correct syntactically might notbe meaningful semantically. E.g.

Colorless green ideas sleep furiously.


19/37

Tasks that we will undertake to delve

into the world of programming

1) Set up a programming environment.

2) I said last time, we shall be using C. Two

reasons. Major reason: It was written in thecourse outline that I was given

3) Minor reason: I was also taught C in myundergraduate. I think most teachers usuallyknow only C or at the most, C and C++

4) However, a bit of research has shown


20/37

So we shall be doing Python

Why?

Its simple,

Its fun

It lets you do a lot of powerful stuff simply

You learn all the tools/skills that you need tobecome a good programmer


21/37

Print hello world in three different

languages

C language requires this program to be wrapped in

a function and preceded by a preprocessor

directive:

#include

main()

{

printf("Hello World!);

}


22/37

In Java, the situation is terrible because our

code must be inside of a class:

public class helloWorld

{

public static void main(String [] args)

{

System.out.println("Hello World!");

}

}


23/37

And in Python?

print Hello World!

Simple isnt it?


24/37

Basic things in Python

Data types

Two major kinds of primitive data types

Numbers (example 2, 3, 6535, 0.001)

and

Strings (Rameez rahman, introduction tocomputer science and programming.


25/37

As discussed last time. Just like for C

A number can have many data types.

int (2, 3, 1000)

float (27.65, 5.6)

and others.


26/37

We can create custom data types as I

mentioned in the last lecture.

However, no matter how complex data types

we end up building, they will in the end be

made up of three basic data types Numbers (float, int etc)

Strings

Booleans (which we will discuss when we gothrough the code file)


27/37

Operators Associated with different data types are different

operators.

Operators do certain jobs.

E.g. + . So the operator + between 2 and 3, willgive the sum of 2 and 3

2+3 (this will give you 5)

Expressions are made up of operand andoperators

2+3 is a programming expression. So iscomputer + programming


28/37

Some examples

Example 2.5 + 2.6

Type conversion: str(10) + is a small number

works

Why 10 + number does not work.

Common sense: Python does type checking.


29/37

Weak vs strong typing

Different languages have different kinds oftype checking

No matter which language you are using, bestrict about types. So try not to do thefollowing:

student < 3

Even though this works in python, but it isnonsensical more or less and can give errors inother languages.


30/37

15/4

15%4

122 + 4 * 5

Operator precedence. Remember BODMAS


31/37

Variablesvalue assignment

We use variables to store values so that wecan use them later.

= < variable value>

firstVariableString = Cool

number = 1.01


32/37

How does assignment work deep

down

When we write for example x = 10

What we do is that in the memory somewhere isplaced the object 10, and we bind the variable xwith this object. So x points to 10 or is linked to10. we will come to this later.

y = x. so now y becomes 10 And later if we do x = 20

What does y become?


33/37

Whats the type of variables?

So for example what was the type of x?

In C, if you remember we would have written

something like,

int x; So we knew that the type of x is an integer.

But in python x simply inherits its type from the

value.

So if x = 10, then x is a number and if we write

x = computer then x is a string.

Some advice:


34/37

Some advice:

Dont change types of the same variable

arbitrarily in a program. It makes it harder to

debug

Try to give intuitive variable names

If a varialbe will store the humidity levels,

then

call it humidityLevel (or humid) rather than x.

Also, you can choose any variable name that

you want apart from keywords that python

uses.


35/37

Lets come to statements to get some

real programming

An expression is a legal command that python

can interpret.

We saw some earlier.

Print

Assignment

There are of course many other


36/37

What we have done until now is what is called

a straight line program.

We are simply executing one instruction after

another

This is not very interesting.

Lets go back to algorithms and recipes.


37/37

Branching programs

Something that can change the order of

instructions based on some test.

And that test is usually a value of a variable

Lets now go to our code file.

icp lecture 2

Documents