CS 101 Computer Programming and Utilizationcs101/2019.1/lectures/... · 2019-08-08 · Autumn 2019 CS101@CSE IIT Bombay discount calculator •write a program to read total cost of

Autumn 2019 CS101@CSE IIT Bombay

CS 101Computer Programming and

Utilization

Puruwith

CS101 TAs and CSE Staff

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

Lecture 4: Conditional Execution and Loops

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


recap• turtleSim, repeat, forward, left, right, penUp, penDown• char, int, float, double, bool

• indentation, variable naming, comments• operators, operator precedence• variables, assignment, re-assignment, expressions• repeat, nested repeat• input, output from screen

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surprise ungraded quiz1. int n=12345678, d0, d1, d2;

d0 = n % 10;d1 = (n / 10) % 10;d2 = d1 % 3;cout << d0 << “ “ << d1 << “ “ << d2;

2. char c = ‘a’; // ASCII value of a is 98cout << c << “ “ << ‘c’;cout << ‘c’ + 1 << “ “ << c+1;

3. Write a program to convert a character from lower case to upper case. i.e., a to A.

Get input from screen in lowercase.All ASCII mappings are sequential.

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surprise ungraded quiz1. int n=12345678, d0, d1, d2;

d0 = n % 10;d1 = (n / 10) % 10;d2 = d1 % 3;cout << d0 << “ “ << d1 << “ “ << d2;

2. char c = ‘a’; // ASCII value of a is 98cout << c << “ “ << ‘c’;cout << ‘c’ + 1 << “ “ << c+1;

3.

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shadowing and scopemain_program{int x=5;cout << x << endl; // prints 5repeat (3) {cout << x << endl; // prints 5int x = 10;x *= 2;cout << x << endl; // prints ____

}cout << x << endl; // prints ____

}

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conditional execution and loops

• basic if statement• if-else statement

• general if statement form

• basic while loop

• switch statement

• general while loop, do-while loop and for loop• computing logical expressions

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discount calculator

• write a program to read total cost of items in shopping cart and compute discount, using following rules

• If total ≤ 1000, then discount = 0• If total is between 1000 and 2000 then discount = 10% of total• If income is between 2000 and 5000, then discount = 20% of

total• If income > 5000, then discount = 500 + 30% of total

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an even simpler problem

• using the rules given earlier, read in the total print a message indicating whether any discount will be offered

• what is required?

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an even simpler problem


• what is required?• not a single sequential execution of a program

• only some parts of program/actions on certain inputs/conditions• conditional execution

• the if, if-else statements

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basic if statement

• Form:if (condition) consequent

• condition: boolean expression • boolean : variable/expression that evaluates to true or false• consequent: C++ statement, statements, e.g. assignment

• If condition evaluates to true, then the consequent is executed• If condition evaluates to false, then consequent is ignored


conditions

if (condition) consequent

• Simple condition: exp1 relop exp2

• relop : relational operator

< <= == > >= !=

• less than, less than or equal, equal, greater than, greater than or equal, not equal

• condition is true if exp1 relates to exp2 as per the specified

relational operator


flowchart

• pictorial representation of a program

• statements put inside boxes

• If box C will possibly be executed after box B, then put an arrow from B to C

• specially convenient for showing conditional execution, because there can be more than one next statements

• diamond shaped boxes are used for condition checks


flowchart of the if statement

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Condition

Previous Statement

Consequent

Next Statement

True

False


discount calculator• write a program to read total cost of items in shopping cart and

compute discount, using following rules

• If total less than 1000, then discount = 0

• If total is between 1000 and 2000 then discount = 10% of total

• If income is between 2000 and 5000, then discount = 20% of total

• If income >= 5000, then discount = 500 + 30% of total


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examples for conditions#include <simplecpp>

main_program {int a=1, b=2, c=3, d=4;

if(a < b) cout << “a is less than b” ;

if(a > b) cout << “a is greater than b” ;

if (b) cout << “b is b”;

if ((b+c) < d) cout << “d is not big enough”;}

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the discount program

main_program {double total, discount;

cin >> total;

if (total < 1000)cout << �No discount� << endl;

if (total >= 1000)cout <<�Discount coming your way!”<< endl;

}


the discount programmain_program {

double total, discount;

cin >> total;

if (total <= 1000)cout << �No discount� << endl;

if (total > 1000)cout <<�Discount coming your way!”<< endl;

}• // Always checks both conditions• // If the first condition is true,• // then you know second must be false,• // and vice versa. Cannot be avoided• // using just the basic if statement


the slightly improved discount program

main_program {double total, discount;

cin >> total;

if (total < 1000)cout << �No discount� << endl;

elsecout <<�Discount coming your way!”<< endl;

}


flowchart of the if-else statement

Condition

Previous statement

AlternateConsequent

True False

Next statement

if (condition) consequent else alternate


Most General Form of the IF-ELSE Statement

if (condition_1) consequent_1else if (condition_2) consequent_2

…else if (condition_n) consequent_n

else alternate

• Evaluate conditions in order• Some condition true: execute the corresponding consequent.

Do not evaluate subsequent conditions• All conditions false: execute alternate


flowchart of the general if-else statement

Next Statement

Condition 2

Condition 3

Consequent 1

Consequent 2

Consequent 3 Alternate

True

True

False

False

Previous Statement

Condition 1True False


the discount programmain_program {

double total, discount;

cin >> total; // get input

if (total < 1000) discount = 0;else if (total < 2000)

discount = 0.1 * total;else if (total < 5000)

discount = 0.2 * total;else discount = 500 + 0.3 * total;

cout << “Total discount = “ << discount << endl;}


the discount + points program• along with discount,

• points to calculated– if total less than 1000, points = 10– if total between 1000 to 2000 , points = 20– if total between 2000 to 5000 , points = 30– if total greater than equal to 5000 , points = 50

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the discount + points programmain_program {

double total, discount; int points;

cin >> total; // get input

if (total <= 1000) discount = 0; points= 10;else if (total <= 2000)

discount = 0.1 * total; points = 20;else if (total <= 5000)

discount = 0.2 * total; points = 30;else discount = 500 + 0.3 * total; points = 50;

cout << “Total discount = “ << discount << endl;}


the discount + points programdouble total, discount; int points;cin >> total; // get input

if (total <= 1000) { discount = 0; points= 10;

}else if (total <= 2000) {

discount = 0.1 * total; points = 20;

}else if (total <= 5000) {

discount = 0.2 * total; points = 30;

}else { discount = 500 + 0.3 * total; points = 50; }

cout << “Total discount = “ << discount << endl;


another example (homework)• rules

– for each cricket match• Win gets 3 points• Loss no points• Draw one point• Update number of wins, losses and draws of a team for each

match

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combining conditions

condition1 && condition2 : true only if both true Boolean AND

condition1 || condition2 : true only if at least one is true Boolean OR

!condition : true if only if condition is falseBoolean negation


examples for combined conditionsif ((total < 1000) || (nitems == 1)) discount = 10;

if ((discount > 500) && (nitems == 1)) discount = 50;

write without combined conditions …

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bool flag = (2>3);if (!flag) cout << “ 2 is really less than 3 \n”;

if (!flag || flag) cout << “why flag? \n”;


homework

1. Based on user input, draw a circle or a square or a rectangle

2. Determine if a number is prime– program should take as input a number

– expect number to be an integer > 1

– Output: Number is prime if it is, or number is not prime if it is not

– Hints:• prime numbers are only divisible by 1 and the number itself

• program will need a loop and conditional execution

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need of a more general loop

Read marks of students from the keyboard and print the average

• number of students not given explicitly

• if a negative number is entered as marks, then it is a signal that all

marks have been entered

• Examples

− Input: 98 96 -1, Output: 97

− Input: 90 80 70 60 -1, Output: 75

•The repeat statement repeats a fixed number of times. Not useful

• We need a more general statement

• while, do while, or for


the while statement

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while (condition) body

next_statement

1. Evaluate the conditionIf true, execute body. body can be a single statement or a block, in which case all the statements in the block will be executed

2. Go back and execute from step 13. If false, execution of while statement

ends and control goes to the next statement


the while statement

while (condition)

body

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• The condition must eventually become false, otherwise the program will never halt.Not halting can be tricky!Infinite loop.

• If the condition is true originally, then the value of some variable used in condition must change in the execution of body, so that eventually condition becomes false

• Each execution of the body = iteration


the while statement flowchart

Condition

Body

Previous statement in the program

Next statement in the Program

False

True


a program that does not halt

// infinite loop!

main_program{

int x=10;

while(x > 0){

cout << “Iterating” << endl;

}

}

// Will endlessly keep printing

// Not a good program


a program that halts

main_program{int x=3;while(x > 0){

cout << “Iterating” << endl;x--; // Same as x = x – 1;

}}// Will print “Iterating.” 3 times // Good program (if that is what// you want)!


while vs. repeat

repeat can be expressed using while?

while can be expressed as repeat?

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while vs. repeat

while can be expressed as repeat?

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the break statement

• the break keyword is a statement by itself

• break the loop!• when it is encountered in execution, the

execution of the innermost while statement which contains it is terminated, and the execution continues from the next statement following the while statement

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the continue statement

• the continue keyword is a statement by itself

• (skip rest of loop) continue the loop from start!• when it is encountered in execution, the

execution of the innermost while statement which contains it is skipped, and the execution continues from start of the loop

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example for break/continue

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main_program{float nextmark, sum = 0;

while (true){cin >> nextmark;if(nextmark > 100)

continue;if(nextmark < 0)

break;sum = sum + nextmark;

}cout << sum << endl;

}

Documents

CS 101 Computer Programming and Utilizationcs101/2019.1/lectures/... · 2019-08-08 · Autumn 2019 CS101@CSE IIT Bombay discount calculator •write a program to read total cost of