Dr Deepak B Phatak - it.iitb.ac.in€¦ · Dr. Deepak B Phatak 3 IIT BOMBAY Some short-cuts in C++ Session 5, iterative algorithms • C++ provides for a few variants of some instructions

CS 101Computer Programming and utilization

Dr Deepak B PhatakSubrao Nilekani Chair Professor

Department of CSE, Kanwal Rekhi BuildingIIT Bombay

Session 5, Iterative algorithmsTuesday August 9, and Wednesday, August 10, 2011

IIT BOMBAY

Session 5, iterative algorithms

Dr. Deepak B Phatak 2

IIT BOMBAY


Overview

• Increment and decrement operators– Precedence table for operators

• Need for iterative solutions– Identification of a repetitive block– Flow chart of basic iteration– Instructions required to specify iteration

• Announcements – Clicker devices, Discussion sessions– Labs next week– Special lectures


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Some short-cuts in C++


• C++ provides for a few variants of some instructions– Make these simpler to write– Should be used with care

• When an assignment operation changes the value of one objectx = x + 3.75 * y;k = k * (a + b)/c;m = m - 1;

• Such assignments can be written asx += 3.75 * y;k *= (a + b) /c;m -= 1;


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Increment and decrement operators

• If we wish to increment the current value of m by 1m = m+1;

• Special increment and decrement operators ++ and --++ m; is same as m = m+1;-- n; is same as n = n-1;

• We can also use these operators in a ‘post’ formi++; is same as i=i+1;j--; is same as j=j-1;

• What is the difference between ‘pre’ and ‘post’ operations?– As stand-alone statements, both forms achieve the same

result, namely increment or decrement the variable value


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Pre and Post increment

• When such usage appears in a larger expression– it is the original value which participates in the evaluation of

expression in case of post increment– It is the modified value which participates in the evaluation

of expression in case of pre increment• Example 1

int m, n, j; j = 5; n = 2;m = j++ * n; // This same as m = j * n; j = j + 1;

– Effect on j will occur after the expression is evaluated• Example 2

int m, n, j; j = 5; n = 2;m = n*++j; // This is same as j = j + 1; m = j * n;

– Effect on j will occur before the expression is evaluated

• Should be avoided as a good programming practice


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Precedence and associativity rules

1. ++ -- (post increment and decrement) left to right2. ! (not) ++ -- (pre ... ) + (unary) – (unary) right to left3. * / % left to right4. + (addition) – (subtraction) left to right5. < <= > >= (comparison) left to right6. == != (comparison) left to right7. && (logical and) left to right8. || (logical or) left to right9. ?: right to left10 = += -= *= /= %= (assignment) left to rightParentheses () has the highest precedence

Check http://www.cppreference.com/wiki/operator_precedence


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Finding maximum of five given numbers


int a, b, c, d, e, max;cin >> a >> b >> c >> d >> e;max = a;if (b > max) {max = b;}if (c > max) {max = c;}if (d > max) {max = d;}if (e > max) {max = e;}cout << max;


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Need to specify repetition

• Many problems will deal with a large number of values, performing similar operations on each value

• In this case, we are required to write a separate instruction for each comparison which takes place with a different value

• For 10 values, we will have to write 9 comparison instructions• What if the number of values are 100, or 1000, or more?

– The number of instructions will become very large• Large Number of Lines of Code (LoC) should usually indicate the

complexity of the solution, not just the size of code


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Need to specify repetition


• We would like to define a ‘pattern’ or ‘block’ of instructions, and somehow instruct C++ to execute that block repeatedly– Each execution of the block should handle a different value– The instructions in the block have to be same

• Same locations may have different values upon each execution


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Identifying repeatable actions


• We first rewrite the program to input a value when needed

int a, b, c, d, e, max;cin >> a ; max = a;cin >> b; if (b > max) {max = b;}cin >> c; if (c > max) {max = c;}cin >> d; if (d > max) {max = d;}cin >> e; if (e > max) {max = e;}cout << max;


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• We are interested in only finding the maximum– We do not need different names for these values

int a, max;cin >> a ; max = a;cin >> a; if (a > max) {max = a;}cin >> a; if (a > max) {max = a;}cin >> a; if (a > max) {max = a;}cin >> a; if (a > max) {max = a;}cout << max;


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Execution with some sample values


• Consider that input values are 3, 9, 2, 17, and 5– Let us trace what happens during execution

a maxint a, max;cin >> a ; max = a;cin >> a; if (a > max) {max = a;}cin >> a; if (a > max) {max = a;}cin >> a; if (a > max) {max = a;}cin >> a; if (a > max) {max = a;}cout << max;


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Extended program


• We will include an output instruction after each new value is input and after it is compared with max

i a, max;cin >> a ; max = a;cin >> a; if (a > max) {max = a;} cout << max;cin >> a; if (a > max) {max = a;} cout << max;cin >> a; if (a > max) {max = a;} cout << max;cin >> a; if (a > max) {max = a;} cout << max;cout << “Maximum of all 5 numbers is: ” << max << endl;


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int a, b, max;cin >> a; max = a;

cin >> a ;if (a > max) {

max = a;}cout << max;


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Identifying repeatable actions ...

int a, max;cin >> a; max = a;




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Setting up an ‘infinite’ loop




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How long to continue looping?



condition true


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While statement

While statement of c++ sets up an iterative loop

statement-before-the-block;while (condition) {

block-statement1;block-statement2;-- -

}next-statement;

• Condition is evaluated at the beginning of each iteration. • The block is executed if the value is true

• else, program exits the loop and goes to next-statement


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How to set a condition


• We need to define with care, the condition to be tested by our repetitive structure – It should evaluate to ‘true’ for continuation of the loop– Should evaluate to ‘false’ to get out, or exit from the loop

• Some value in the condition must change during each iteration


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Effect of the ‘continuation’ condition


• If components of the condition do not change in any iteration• it will always evaluate to either true or false

– The control will either keep repeating the loop infinitely– Or control will come out without executing the body even once


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Example of use of While statement ...

int a, max;max = 0;while (1) {



}return 0;


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While statement ...

int a, max;

while (1) {cin >> a ;if (a > max) {


}return 0;


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Iterative solution

#include<iostream>using namespace std;int main(){

int a, max;max =0;while(1){

cout<<"enter the number:";cin>>a;if(a > max)

{max = a;}cout<<"max is "<<max<<endl;

} return 0;

}


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Need for instructions to specify repetition


Modified Problem– Finding the maximum in a set of positive integers, given as

input one after another. Stop when value 0 is given. After reading each value, output the current maximum value


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Iterative solution for finding maximum


int a, max;cin >> a; max =a;while(a !=0){

if(a > max){max = a;}

cout<<"max is "<< max << endl; cout<<"enter the next number:";cin>>a;

} return 0;

}


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Finding sum of given numbers


• Find the sum of a set of positive integers, given as input one after another. Stop when value 0 is given.


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Iterative solution for finding sum


int a, sum = 0;cin >> a;while(a !=0){

sum = sum + a;cout<<"enter the next number:";cin>>a;

} cout << “Sum is “ << sum << endl;return 0;

}


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The ‘ do while’ construct


do {------

} while (condition)


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Looping for a given number of times


• Find the sum of N numbers given as input


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Find factorial (N)


• N! = 1 x 2 x 3 x 4 … x N• Essentially, we have to find the product of first N natural numbers


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Announcement

• Clicker devices– Will be used for attendance and for conducting quizzes– A user manual handout will be given on Friday– We will do some simple testing on that day

• Make up lab and Discussion sessions on Saturday and Sunday• Lab sessions

– Lab sessions: Please bring a lab notebook with you

Documents

Dr Deepak B Phatak - it.iitb.ac.in€¦ · Dr. Deepak B Phatak 3 IIT BOMBAY Some short-cuts in C++ Session 5, iterative algorithms • C++ provides for a few variants of some instructions