cse123 - lecture 4 structured programming- loops

CSE123 - Lecture 4

Structured Programming-Loops

Sequential and Structured Programming

Common mistakes,

Do not use turkish letters in variable names

Do not use turkish letters and blanks in m-file names

Do not use two relational operators in one expression, divide the expression and then combine it using logical operators such as and, or or xor etc…

0<x<10 incorrect

x>0 & x<10 correct

Sequential and Structured Programming

Initialization

Calculation 1

Results

Input

Initialization

Calculation 2

Calculation 3

Initialization

Calculation 1

Results

Input

Initialization

Repeat the operation3 times…

The FOR loop

for loop_variable= start:step:finish …. Statements ….end

Statements

forloop_variable= start:step:finish

loop_variable<=finish

loop_variable>finish

Basic rules for “for” loop

• Default value for step: 1• The step can be negative • If start = finish, the loop is executed

once.• Usually NOT a good idea to change

the loop_variable inside the loop.

INCREMENTloop_variable

% program to test a for loop

for i=1:10 disp(i*0.2)end

>> testloop0.20.40.6…2.0


for i=1:10 disp(i*0.2)end

>> testloop0.20.40.6…2.0

Example: testloop.m


for i=1:10 disp(i)end

>> testloop123..10


for i=1:10 disp(i)end

>> testloop123..10

>> disp(i)

10

The FOR loop

>> disp(i)

10


for i=1:0.1:2 disp(i*5)end

>> testloop55.56..9.510


for i=1:0.1:2 disp(i*5)end

>> testloop55.56..9.510

>> disp(i)

2

The FOR loop

Example :Assume we have series of natural numbers up to 100 and want to find the summation of these numbers.

•Use a for loop, using the loop variable as the index

•At every step of the loop, we want to add the value corresponding to the index.

Summation is done by addition of 1st elements, obtain partial result, add 2nd element, obtain partial result, etc….

Loop variable: i

Result variable: sum


sum=0;

for i=1:100

sum=sum+i;

end

disp(sum)

disp([‘result=‘,num2str(sum)])


sum=0;

for i=1:100

sum=sum+i;

end

disp(sum)

disp([‘result=‘,num2str(sum)])

sum=sum + next value

The FOR loop

Applications and uses of “for” loops Use loop_variable as a counter

Example :We want to calculate the …… 100 times

•Use a for loop, using the loop variable as the index of the vector.

•At every step of the loop, we want to add the value corresponding to the index.

•Operation done by taking the square root of the preceding result, 100 times

Loop variable: j

Result variable: S


for j=1:100

end


for j=1:100

end

S=sqrt(S)

S=pi; Initialization

S= S

. .

The FOR loop

INCREMENTloop_variable

Statements

forloop_variable= start:step:finish

IF loop_variable<=finish

IF loop_variable>finish

Add step to loop_variable

Assume 1000 values

We want to stop the loop when we obtain enough preicison in the calculation

What will happen if the precision is obtained after 10 calculations?

The FOR loop will not stop until the 1000 .

Background tests and operations

The WHILE loop

Statements

Whilelogical_expression

IF logical_expression TRUE

IF logical_expression FALSE

while logical_expression

Statements

end

Basic rules for “while” loop

• Usually necessary to create your loop_ variable or counter.

• NECESSARY to change the loop_variable inside the loop.

The WHILE loop

% program to test while loop

while i<10 i=i+1; disp(i)

end

% program to test while loop

while i<10 i=i+1; disp(i)

end

>> exloop1 12345678910 >> disp(i) 10

% program test

for i=1:10

disp(i)

end

i=0;

Example: exloop1.m

Had to create acounter

The WHILE loop

>> exloop2 9.5000 2.2513 9.0000 2.1972 8.5000 2.1401 8.0000 2.0794 7.5000 2.0149 7.0000 1.9459 6.5000 1.8718 6.0000 1.7918 5.5000 1.7047 5.0000 1.6094 4.5000 1.5041 4.0000 1.3863 3.5000 1.2528 3.0000 1.0986 2.5000 0.9163 2.0000 0.6931 1.5000 0.4055 1 0

% program to test while loop x=10; while x>1

x=x-0.5; y=log(x); disp([x,y])

end

Example: exloop2.m

The WHILE loop

>> exloop3

% BAD while loop x=1; while x>=0

x=x+0.5; y=sin(x);

end disp(‘END of program’)

Example: exloop3.m

Need to stop the script manually !!!

CTRL C

This is an Infinite loop !!!!

>>

% BAD while loop x=1; while x>=0

x=x+0.5; y=sin(x);

end disp(‘END of program’)

The WHILE loop

Example : exloop4.mCalculate the sum of manually entered numbers. Entering 0 or a negative number stops the loop and display the average.

•Need input statement:

•Use the length of A in the logical_expression

•Inside the loop:•Increment a counter to count how many value we entered

•Add the value A to the sum S

A>0

% program to test a while loop

while

endN=i-1disp(S/N)

% program to test a while loop

while

endN=i-1disp(S/N)

Use while loop when the number of operation is unknown

variable: A

S=S+A

i=i+1

>> testloopValue for A:2Value for A:2Value for A:2Value for A:2Value for A:0 2

>> testloopValue for A:2Value for A:2Value for A:2Value for A:2Value for A:0 2

A=input('Value for A:');

A>0

i=i+1; S=S+A;

A=pi;S=0;i=0;

>> testloopValue for A:1Value for A:5Value for A:8Value for A:0 4.6667

>> testloopValue for A:1Value for A:5Value for A:8Value for A:0 4.6667

The WHILE loop

Example 5:Series convergence: π2/6

Want to see how many terms you need to obtain an error of 3x10-6.

% Convergence script

while

end

disp(['N=',num2str(i)])


while

end


Use logical_expression for convergence limit

N

1ii12

>> Testloop

N=333333

>> Testloop

N=333333

•Need to define and use an error variable.•Need to be part of the logical expression•Need to be updated during loop

•At every step of the loop, we want to•Verify if test is true

•Increment counter

•Add the new term to the sum

err

S=S+ 1/i^2

i=i+1

err>3e-6

err=abs(S-pi^2/6);

i=i+1;S=S+ 1/i^2;

S=0; i=0;err=10;

(err>3e-6)

The WHILE loop


S=0; i=0;err=10;

while err>3e-6 i=i+1; S=S+ 1/i^2; err=abs(S-pi^2/6); A(i)=err;end



S=0; i=0;err=10;

while err>3e-6 i=i+1; S=S+ 1/i^2; err=abs(S-pi^2/6); A(i)=err;end


Use logical_expression for convergence limit

>> Testloop

N=333333

>> Testloop

N=333333

Nested loops and combination

Example: exloop6.mCalculate the sum of factorials up to 20

•Need a for loop for sums

•Need a for loop for factorials

•Calculate the factorial of element j

•Do the sum on all elements

% program to test nested loops

S=0; for i=1:20 F=1; for j=1:i F=F*j; end S=S+F end disp(S)

% program to test nested loops

S=0; for i=1:20 F=1; for j=1:i F=F*j; end S=S+F end disp(S)

Using nested loops

S=S+F

j

i

!20....!4!3!2!1

F=F*j

The “BREAK” statement

BREAK

•Break terminates the execution of a for or while loop. Statements in the loop that appear after the break statement, are not executed.

•In nested loops, break exits only from the loop in which it occurs. Control passes to the statement that follows the end of that loop.

% BAD while loop

x=1;

while x>=0

x=x+0.5; y=sin(x);

end

if x>10000break

end

The “CONTINUE” statement

CONTINUE

• Continue passes control to the next iteration of the for or while loop in which it appears, skipping any remaining statements in the body of the loop.

% Problem of division by 0

x=1;

for i= -10:10

y=1/x;

end

if x==0continue

end

The FOR loop

Example :Write a Matlab script tp calculate following expression for an enetered x value

x1

1

11

11

11

The FOR loop

Example :Write a Matlab script tp calculate mean and standard deviation of an input data set containing an arbitrary number of input values. Check to see if there is enough input data (N>1) to eliminate division by zero.

N

iixN

x1

1

)1(

)(1 1

22

NN

xxNs

N

i

N

ii i

Break

Example :Run the following loops and report the result

for ii=1:3

for jj=1:3

if jj==3

break;

end

Product=ii*jj;

fprintf(‘%d * %d = %d \n’,ii,jj,product);

end

fprintf(‘end of inner loop\n’);

end

fprintf(‘end of outer loop\n’);

1 * 1 = 1

1 * 2 = 2

end of inner loop

2 * 1 = 2

2 * 2 = 4

end of inner loop

3 * 1 = 3

3 * 2 = 6

end of inner loop

end of outer loop

cse123 - lecture 4 structured programming- loops

Documents

loop increment loop

loop sum

loop x

loop dispi

loop example

loop statements

loop exloop3

loop exloop2