CS 101 Computer Programming and Utilizationcs101/2019.1/lectures/Lecture2.pdf · Autumn 2019...

Autumn 2019 CS101@CSE IIT Bombay

CS 101Computer Programming and Utilization

Autumn 2019https://www.cse.iitb.ac.in/~cs101

Puru

Lecture 2: Variables, Data Types, and Expressions

https://www.cse.iitb.ac.in/~cs101


Recap

02/08/19 2


programming is like riding a cycle

• try• fall / fail• try again• get hurt / program crashes• get help from friends• try again• try• try • try• ... fly

02/08/19 3


keywords from last class• logic, syntax, sequence• simplecpp, c++• codeblocks• program• programming• main_program, repeat, curly braces, functions

02/08/19 4


how to draw an octagon?• commands seem quite repetitive?

• there is a better way!

502/08/19

#include <simplecpp>main_program{

turtleSim();forward(100); right(45);forward(100); right(45);forward(100); right(45);forward(100); right(45);forward(100); right(45);forward(100); right(45);forward(100); right(45);forward(100); right(45);

wait(10);}


a better way #include <simplecpp>

main_program {turtleSim(); repeat(8) {

forward(100); right(45);

}}

repeat (n) {

some commands

}

The instructions within {...} are repeated n times

Each round of execution is called an iteration

repeating a set of actions is a common action/requirementas part of logic to do some work

02/08/19 6


how to draw a polygon?

02/08/19 7

#include <simplecpp>

main_program{

turtleSim();

repeat(8){

forward(100);

right(45);

}

}



802/08/19


main_program{turtleSim();

repeat(num_sides){forward(10); right(360.0/num_sides);

} }



repeat(8) {forward(10); right(45);

} }



902/08/19



int num_sides;

repeat(num_sides){forward(10); right(360.0/num_sides);

} }

We need some magic so that num_sides can have the right value

Tell the computer: Reserve space in your memory where I can store an integer (int). I will refer to it by the name num_sides

Divide the number 360 by the number stored in the space named num_sides and pass the result as an argument to this command


explanation#include <simplecpp>


int num_sides;

cout << �No. of sides?�;cin >> num_sides;

repeat(num_sides) {forward(200);right(360.0/num_sides);

}}

Print the sentence within the quotes on the screencout è “Console out” (display)

Read the number that the user types and store it into the space in memory named num_sides cin ç “Console in” (keyboard)

Use the integer stored in the space in memory which is namednum_sides

02/08/19 10


can we improve the program further?

02/08/19 11


can we improve the program further?

Both values for a polygon,

number of sides andside length

are user inputs.



int num_sides; int side_length;double exterior_angle;double sum_exterior = 360;


cout << �Side length?�;cin >> side_length;

exterior_angle = sum_exterior/num_sides;

repeat(num_sides) {forward(side_length);right(exterior_angle);

}}

02/08/19 12


formatting: indentation, grouping, naming#include <simplecpp>

main_program{turtleSim();int num_sides;


repeat(num_sides) {forward(200);right(360.0/num_sides);

}}

#include <simplecpp> main_program{turtleSim(); cout << �No. of sides?�;int n; cin >> n; repeat(n) {forward(200); right(360.0/n);} }

You will lose marks and your sleep for bad formatting02/08/19 13


language syntax• syntax = grammatical rules indicating how

commands must be written• syntax of programming languages is very strict,

e.g.– right(90);cannot be written as right(90)penUp() cannot be written as penup()or even penUp, i.e., without parentheses

– all statements, instructions, keywords of a language have a format and have to adhered to

1402/08/19


what does the following program do?


cout << �a�;repeat(5){

cout << �b�;repeat(2){ cout << �c�; }cout << �d�;

}}

02/08/19 15


what does the following program do?


cout << �a�;repeat(5){

cout << �b�;repeat(2){ cout << �c�; }cout << �d�;

}}

abccdbccdbccdbccdbccd02/08/19 16


curly braces group statementsrepeat(4){

forward(50); right(90);wait(2);

}

repeat(4){}

forward(50);

right(90);

wait(2);

repeat(4){

forward(50);}right(90);wait(2);

02/08/19 17


more commands/functions• sqrt(x) : square root of x

• trigonometric functions,

• x is in radian: sin(x), cos(x), tan(x)

• x is in degree sine(x), cosine(x), tangent(x)

• also for arcsine, arccosine, arctangent etc.

02/08/19 18


running/executing the program• Compiling a program:

– translating it into a form that a computer can understand– high level language to machine instructions

• the result of compilation: an executable file

• compiler used by us (for simplecpp code) is called s++

• g++ compiler for vanilla C++ programs

02/08/19 19


running a program on computer

• Type in an editor (say, gedit)• Save the file (say prog.cpp)

• Compile (s++ prog.cpp)

• It generates a binary file a.out• Execute (./a.out )

• Note that in case compilation fails with some error, existing a.out file is untouched

2002/08/19


homework/practicedraw a square with dashed lines

2102/08/19

repeat(4){

…

}


Outline

• How to store data in the memory of a computer

• How to perform arithmetic

• How to read numbers into the memory from the keyboard

• How to print numbers on the screen

• Many programs based on all this

02/08/19 22


variable declaration

a general statement of the form

data_type_name variable_name;

creates and declares variables (named space in memory to store data)

earlier example …

int sides;

int : name of the data type. Short form for integer. Says reserve space for storing integer values

sides : name given to the reserved space, or the variable created

02/08/19 23


variable declaration0123456789

.......

32 bits

int sides;Results in a memory location being reserved for this variable. The program will refer to it by the name sides

02/08/19 24


variable names: identifiers• sequence of letters, digits & the underscore �_� character

• case matters. ABC, Abc and abc are distinct identifiers

• should not begin with a digit

• some words such as int cannot be used as variable names. reservedby C++ for its own use

• Which of these are valid variable names:sides, #sides, 3rd_cousin, third cousin, tele_number, x, third_cousin,

02/08/19 25


naming practices

• use meaningful names, describing the purpose for which the variable will be used

exterior_angle = sum_exterior/num_sides;

• never use single letter variable name unless appropriate in the context. Example:

u*t+1/2*a*t*t

e = s/n

02/08/19 26


the assignment operator• assignment operator =

exterior_angle = sum_exterior/num_sides;used to store results of computation into a variableForm: variable_name = expression;

• expressiona constant (a number) ora variable name ora formula involving constants or variables ora function call

02/08/19 27


initialization - an INITIAL value is assigned to the variable

the value stored in the variable at the time of its creation

variables i, vx, vy are declared and are initialized

2.0e5 is how we write 2.0*105

‘f’ is a character constant

int i=0;double vx=1.0;double vy=2.0e5;

char value = ‘f’;

variable initialization examples

02/08/19 28


int x=2, y=3, p=4, q=5, r, s, t;

x = rt;

x = r*s;

r = x*y + p*q;

s = x*(y+p)*q;

t = x – y + p – q;

more examples

02/08/19 29


int x=2, y=3, p=4, q=5, r, s, t;

x = rt; // syntax error!

x = r*s; // disaster! r, s undefined

r = x*y + p*q; // r becomes 2*3 + 4*5 = 26

s = x*(y+p)*q; // s becomes 2*(3+4)*5 = 70

t = x – y + p – q; // equal precedence, // so evaluated left to right,// t becomes (((2-3)+4)-5 = -2

more examples

02/08/19 30


reading values into variables

• can read value of a variable from the terminal• can read into several variables one after another• int, char etc. can be read

• user expected to type in values consistent

with the type of the variable into which it is

to be read

• whitespaces (i.e. space characters, tabs, newlines)

typed by the user are ignored

• newline/enter key must be pressed after values are typed

cin >> num_sides;

cin >> vx >> vy;

char command;

cin >> command;

02/08/19 31


printing variables on the screen• general form: cout << variable;

• many values can be printed one after another

• need to put “ “ if you want to have gap between successive values

• to print newline, use endl

• text can be printed by enclosing it in quotes

cout << x;

cout << x << y;cout << x << �, � << y;

cout <<�Position: " << x << �, � << y << endl;

char var = �G�;cout << var;

02/08/19 32


int x=2, y=3, z, w;double q=3.1, r, s;r = x; // representation changed

// 2 stored as a double in r "2.0"z = q; // store with truncation

// z takes integer value 3s = x*q; // convert to same type,

// then multiply// Which type?

cout << (360/240) << “ ” << (360.0/240) << “ ” << (360/240.0);

arithmetic between different types allowed

02/08/19 33


• e.g. x*q

if varA, varB have the same data type: the result will have same

data type

• if varA, varB have different data types: the result will have more

expressive data type

• int/short/unsigned int are less expressive than float/double

• shorter types are less expressive than longer types

• we will only use int and double in this course

evaluating varA op varB

02/08/19 34


int x=2, y=4, p=3;

double u;

u = x/p + y/p;

cout << u << (x+y)/p;

• x/p : both are int. So truncation. Hence 0

• y/p : 4/3 after truncation will be 1

• So the output is “1 2”

Integer Division, Evaluation Order

02/08/19 35


int num_sides=100;int i_angle1, i_angle2;double d_angle1, d_angle2;

d_angle1 = 360/num_sides + 0.45; // 3.45i_angle1 = 360/num_sides + 0.45; // 3

d_angle2 = 360.0/num_sides + 0.45 // 4.05i_angle2 = 360.0/num_sides + 0.45; // 4

more examples

02/08/19 36


main_program{

double centigrade, fahrenheit;

cout << “Give temperature in Centigrade: ”;

cin >> centigrade;

fahrenheit = centigrade * 9.0 / 5.0 + 32;

cout << “In Fahrenheit: ” << fahrenheit

<< endl; // newline

}

a sample program

02/08/19 37


int p=3, q=4, r;r = p + q; // 7 stored into rcout << r << endl; // 7 printed

r = p * q; // 12 stored into rcout << r << endl; // 12 printed

• Same variable can be assigned a value again• When a variable appears in a statement, its value at the

time of the execution of the statement gets used

Re-Assignment

02/08/19 38


int p=12;p = p+1;

See it as: p p+1; // Let p become p+1

rule for evaluation: • FIRST evaluate the RHS and THEN store the result into the

LHS variable• 1 is added to 12 (the value of p)• the result, 13, is then stored in p• thus, p finally becomes 13

p = p + 1 is false in mathematics “=” in C++ is different from “=” in mathematics

in C++ `=‘ is assignment, not equal’

02/08/19 39


main_program{int i=0;

repeat(10){i = i + 1;cout << i << endl;

}}

repeat and re-assignment

02/08/19 40


main_program{int term, s = 0;

repeat(10){cin >> term; s = s + term;

}cout << s << endl;

}

• values read are summed into s• we could have used other binary operators too, say

multiplication, or gcd or max or min

another re-assignment example

02/08/19 41


sequence generation

• can you make i take values 1, 3, 5, 7, …?

• can you make i take values 1, 2, 4, 8, 16, …?

• both can be done by making slight modifications to

above program

fundamental idiom

02/08/19 42

main_program{int i=0;repeat(10){i = i + 1;

cout << i << endl;}

}


Write a program to calculate n!

5! = 1 x 2 x 3 x 4 x 5

composing two idioms

02/08/19 43


Write a program to calculate n! given n.

main_program{int n, nfac=1, i=1;cin >> n;repeat(n){

nfac = nfac * i;i = i + 1;

}cout << nfac << endl;

}

Accummulation idiom

Sequence idiom

composing two idioms

02/08/19 44


finding remainder

• x % y computes the remainder of dividing x by y• Bbth x and y must be integer expressions• Example

•

•• d0 will equal 8 (the least significant digit of n)• d1 will equal 7 (the second least significant digit of n)

int n=12345678, d0, d1;d0 = n % 10; // 8d1 = (n / 10) % 10; // 7

02/08/19 45


concluding Remarks

• variables are regions of memory which can store values• variables have a type, as decided at the time of creation

• choose variable names to fit the purpose for which the variable is defined

• the name of the variable may refer to the region of memory (if the name appears on the left hand side of an assignment), or its value (if the name appears on the right hand side of an assignment)

• operators have an ordering (* / before + -)

02/08/19 46


more practice• What is the result of evaluating the expression (3+2)/4?

• What is printed by this code snippet …float f=6.022E23; float r=f+2-f; cout<<r;

• What is printed by this code snippetint t=10; repeat(2){t=t-1.2;} cout<<t;

• What is printed by this code:int i=2, j=3, k=4; i=j; j=k; k=i; cout << (i*j*k);

02/08/19 47


endosaurus

02/08/19 48


we have labs on 3 days of the week --- Tue, Wed, Thu 8 pm to 11 pm

check for student to lab section mapping• three lab sections L1, L2, L3• students from both sections will be mixed across sections• labs for sections will be held on different days of the week• you will be assigned a fixed seat

for lab related logistics, please visit the web page, every week … Lab section to day of the week may change

02/08/19 49


more nested repeat statementsIt will draw a square with dashed lines

5002/08/19

repeat(4){

repeat(3){

forward(50); penUp();

forward(50); penDown();

}

right(90);

}

CS 101 Computer Programming and Utilizationcs101/2019.1/lectures/Lecture2.pdf · Autumn 2019...

Documents

Transcript of CS 101 Computer Programming and Utilizationcs101/2019.1/lectures/Lecture2.pdf · Autumn 2019...