subrao nilekani chair professor department of cse, kanwal ...€¦ · cs101 computer programming...

CS101

Computer programming and utilization

Dr Deepak B PhatakSubrao Nilekani Chair Professor

Department of CSE, Kanwal Rekhi Building

IIT Bombay

Lecture 1, Introduction to programming

Tuesday, 26 July 2011

And Wednesday, 27 July 2011

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Overview

• Programmable machines

• Mr Dumbo: Model of Computing for a program

• Dumbo‟s memory and its interaction with us

• Dumbo‟s computing

• Instructions and their execution

• Sample c++ programs

• Announcements

Dr Deepak B Phata 2Lectue 1 Introduction to programming

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Machines in our life

• Refrigerator, washing machine

• Bicycle, car, railway engine, airplane

• Calculator

• Computer

• Internet, web


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How do machines work?

• All machines carry out defined tasks

• These react to given „instructions‟

• Example of „instructions‟

• „Switch-on‟ a light (the bulb lights)

• Push the bicycle pedal (the bicycle moves ahead)

• Rotate the car steering wheel (Car turns)


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Example of a calculator

• You wish to add 23 and 156 using a calculator

• Using the keys available on the calculator, we press:

• Keys 2 and 3

• „+‟ sign

• Keys 1, 5, and 6

• „=‟ sign

• The result value 179 is displayed in the window

• The above „instructions‟ are given interactively

• Calculator responds to an instruction as it is given


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Digital Computer

• A computer requires all instructions to be

Pre-written in advance

• Such a pre-written set of instructions is called a

program

• Computer is capable of doing the following:

• First „read‟ the entire program and „Understand‟

all instructions („Compilation‟)

• Carry out instructions, one by one, in the

stipulated order („Execution‟)


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Basic capabilities of a computer

• Ability to handle numerical values

• 257, -78, 4.675, etc.

• Large and small values (1.4E18, 0.356E-9)

• Ability to carry out numerical operations

• Add („+‟), Subtract („-‟),

• Multiply („*‟), divide („/‟), ...

• Ability to collect values from as input, and to give

back to us the calculated results, as output

• Ability to store these values temporarily

• Notion of a „memory‟ location

• Ability to refer to locations by symbolic names


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A sample program in English

1. Get a number from me, and store it in a location.

Call this location A.

2. Get another number from me, and store it in

another location. Call this location B.

3. Multiply the numbers in locations A and B, and

store the resulting value in yet another location.

Call this location P.

4. Give me the value stored in location P.


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Compilation process

1. Get a number from me, and store it in a location. Call this location A.

2. Get another number from me, and store it in another location. Call

this location B.

3. Multiply the numbers in locations A and B, and store the resulting

value in yet another location. Call this location P.



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Execution process

1. Get a number from me, and store it in a location. Call this location A.

2. Get another number from me, and store it in another location. Call

this location B.

3. Multiply the numbers in locations A and B, and store the resulting

value in yet another location. Call this location P.



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Live experience with a program

• Next slide shows a test program. Instructions in that

program can be easily carried out by a human

• Compile and execute that program. Try to work as

correctly, and as quickly, as a computer can!!

Good luck


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Compile and Execute this program

1. Raise one of your hands

2. Put down your raised hand

3. Loudly say „Ha Ha Ha‟

4. Ask your neighbor‟s name. Write down that name in

your notebook

5. Write the value of Pi ( π ) correct to 3 decimal places

6. Speak loudly the name of your mother tongue

7. Calculate the product of 25 and 4 and speak the value

loudly

8. Clap three times

9. While executing this program, ignore all earlier

instructions and raise both hands


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Back to Computers

• We will rewrite our earlier program to add numbers

1. Get a number from me, and store it in a

location. Call this location A.

2. Get another number from me, and store it in

another location. Call this location B.

3. Add the numbers in locations A and B, and store

the resulting value in yet another location. Call

this location C.

4. Give me the value stored in location C.


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Back to the computers

• We will use a simple robot like creature called

Buddhu Ram or Mr Dumbo, to understand how the

computer works

• Forming a „conceptual model‟ in our minds

• We will provide Mr Dumbo with facilities similar to

that of a computer

• Memory locations

• Computational abilities

• Input-output capability


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Buddhu Ram Computer - Mr Dumbo


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Buddhu Ram Computer - Mr Dumbo

• Dumbo cannot do anything on his own

• but can execute given instructions correctly

• He is an obedient servant

• Can do arithmetic, logical comparisons, string manipulation, etc.

• Essentially, can manipulate values, and produce resultant values

• He can Compute, So He is a computer


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Dumbo has poor memory

• Dumbo cannot remember anything in its head

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Dumbo has poor memory

• Dumbo uses a cupboard or a chest of drawers as memory

Dumbo’s memory

A

B

C

D

F

E


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Dumbo can store values in its memory

• Each drawer contains some value, for example

• A number: 437

• A name (string): “Ranade”

Location B: 437

Location F: “Ranade”

B

F

437

Ranade

Dumbo’s memory


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Some Peculiarities of memory

• Each location can contain only one value

• multiple values cannot exist in any location

• How does a value get into a location?

• We supply an input value which Dumbo stores there

• Dumbo stores a value assigned through calculations

• Some values may already exist in a location (How?)

• Such values are „unknown‟ or „undefined‟ as these are

neither supplied by us, nor computed by Dumbo while

executing any of the instructions in our program


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Exchanging values with Dumbo

• Dumbo uses a cart („Data Bus‟) to move values in and out

• He can put a value in a location as instructed or can copy a value

from a location and give it to us

A

B

C

D

F

E

58


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Exchanging values with Dumbo …

• Dumbo does anything only if instructed

• The instruction to Dumbo to collect an input value from us is:

Input B;

• Dumbo will collect a value given by us, take it to its memory cupboard, and will store it in location B

• The value already present in that location is overwritten


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Exchanging values with Dumbo …

• Dumbo can output values from locations to us

• The instruction to do so is

Output C;

• Dumbo will copy value from memory location and present it to us

• Value in named location remains unchanged


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Dumbo Computing

• If two numbers are stored in drawers A and B, say 35 and 12,

Dumbo can add these and store the result in a drawer, say, C,

B

C

12

?

A [35]


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Dumbo Computing

• We instruct him by saying:

C = A + B;

B

C

12

?

A [35]


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Dumbo Computing …

• How will Dumbo do this?

• Dumbo uses rough sheets or „Registers‟

• Access the location A and copy the value (35) into a register

• Access the location B and copy the value (12) into another register

• Add the two values to get (47)

• Store the result value back in location C

Location C will now contain value 47


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Dumbo Computing

• Dumbo uses a set of local registers

• Working Sheets on a table

B

C

12

?

A 35 + 12 = 47[35]


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Result of Dumbo Computing

• A and B continue to hold the existing values

• Location C will now contain 47

B

C

12

47

A [35]


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How do we give instructions

• Most machines take one instruction at a time, and

execute it

• Dumbo is special

• We have to write all our instructions in advance, and

give the entire lot to him

• This collection is called a program

• Dumbo has a „stored program‟ capability


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Where does Dumbo keep the program?

• In the same memory!

• Dumbo uses some space for storing data values, and

another separate space in the same cupboard to store

instructions of our program

• Instruction space

• Data Space


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Animated Dumbo

Just to consolidate the Dumbo model of computing,

we will see very short animations describing:

• An Introduction to Dumbo

• Showing Dumbo, its memory drawers

and a typical input operation

• Dumbo performing computations

• Showing addition of two numbers


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Dumbo‟s Programming Language

• We have seen some instructions already, which can

perform input, computations, and output

• Input A.

• C = A + B.

• Output C.

• If there are multiple values being given as input

• We may not remember which value is to be given

when!

• An additional facility to output a “messgae”

• Output “Give value of X”.


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Dumbo‟s Programming Language ...

• We may wish to explicitly inform Dumbo, the

different symbolic names for different locations,

which we propose to use in our programs

Integer location A.

• We can also list multiple locations in single

instruction

Integer locations A, B, C.


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Dumbo‟s Programming Language ...

• Since we will also be reading such programs, we

may like to write some simple explanatory

comments in plain English

• We will write single line comments by starting

the line with two slashes (“//”)

• We will write multi-line comments by enclosing

all lines in symbols “/*” and “*/”

// Program to calculate sum of two numbers

/* Program written by Deeepak B Phatak as an

illustration for CS101 course. Written in July 2011

IIT Bombay */


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A sample program for Dumbo

// Program AddNumbers.

/* program to calculate sum of two numbers */

Integer locations A, B, C.

output “Give value for A”.

Input A.

Output “Give value for B.

Input B.

C = A + B.

Output “The value in location C is”.

Output C.


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A C++ program

// file add_two_numbers.cpp

#include <iostream>

using namespace std;

// this program reads two integer numbers and calculates the sum

int main() {

int A, B, C;

cout << “Give two numbers”;

cin >> A >> B;

C = A + B;

cout << “Sum is” << C;

return 0;

}


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Convert temperature from 0C to 0F

// convertCtoF.cpp

#include <iostream>

using namespace std;

// To convert temperature from Centigrade into Fahrenheit

int main() {

float C; float F;

cout << “Give Temperature in degree Centigrades";

cin >> C;

F=C*9.0/5.0+32.0;

cout << F;

return 0;

}


subrao nilekani chair professor department of cse, kanwal ...€¦ · cs101 computer programming...

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