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CSE123 - Lecture 4 S tructured 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

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slide2

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 operatorsin 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

slide3

Initialization

Initialization

Initialization

Input

Initialization

Input

Calculation 1

Calculation 2

Calculation 1

Calculation 3

Results

Results

Sequential and Structured Programming

Repeat the operation3 times…

slide4

The FOR loop

for loop_variable= start:step:finish

….

Statements

….

end

INCREMENTloop_variable

for

loop_variable= start:step: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.

loop_variable<=finish

Statements

slide5

The FOR loop

Example: testloop.m

% program to test a for loop

for i=1:10

disp(i)

end

>> testloop

1

2

3

.

.

10

% program to test a for loop

for i=1:10

disp(i*0.2)

end

>> testloop

0.2

0.4

0.6

2.0

% program to test a for loop

for i=1:0.1:2

disp(i*5)

end

>> testloop

5

5.5

6

..

9.5

10

>> disp(i)

2

>> disp(i)

10

>> disp(i)

10

slide6

The FOR loop

Example :

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

% program to test a for loop

sum=0;

for i=1:100

sum=sum+i;

end

disp(sum)

disp([‘result=‘,num2str(sum)])

  • 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=sum + next value

slide7

The FOR loop

Initialization

S=

Applications and uses of “for” loops

Use loop_variable as a counter

Example :

We want to calculate the …… 100 times

% program to test a for loop

for j=1:100

end

  • 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

S=pi;

S=sqrt(S)

Result variable: S

slide8

The FOR loop

Background tests and operations

Add step to loop_variable

for

loop_variable= start:step:finish

IF loop_variable>finish

IF loop_variable<=finish

Statements

INCREMENTloop_variable

Assume 1000values

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.

slide9

The WHILE loop

While

logical_expression

IF logical_expression FALSE

IF logical_expression TRUE

Statements

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.
slide10

The WHILE loop

Example: exloop1.m

% program test

for i=1:10

disp(i)

end

% program to test while loop

while i<10

i=i+1;

disp(i)

end

>> exloop1

1

2

3

4

5

6

7

8

9

10

>> disp(i)

10

i=0;

Had to create a

counter

slide11

The WHILE loop

Example: exloop2.m

>> 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

slide12

The WHILE loop

This is an Infinite loop !!!!

Need to stop the script manually !!!

CTRL C

Example: exloop3.m

>> exloop3

% BAD while loop

x=1;

while x>=0

x=x+0.5;

y=sin(x);

end

disp(‘END of program’)

% BAD while loop

x=1;

while x>=0

x=x+0.5;

y=sin(x);

end

disp(‘END of program’)

>>

slide13

The WHILE loop

Use while loop when the number of operation is unknown

% program to test a while loop

while

end

N=i-1

disp(S/N)

Example : exloop4.m

Calculate the sum of manually entered numbers. Entering 0 or a negative number stops the loop and display the average.

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

A>0

  • 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

variable: A

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

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

A>0

i=i+1

>> testloop

Value for A:1

Value for A:5

Value for A:8

Value for A:0

4.6667

>> testloop

Value for A:2

Value for A:2

Value for A:2

Value for A:2

Value for A:0

2

S=S+A

slide14

The WHILE loop

Use logical_expression for convergence limit

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)])

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

err>3e-6

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

  • 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

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

(err>3e-6)

i=i+1

>> Testloop

N=333333

S=S+ 1/i^2

slide15

The WHILE loop

Use logical_expression for convergence limit

% Convergence script

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

whileerr>3e-6

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

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

A(i)=err;

end

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

>> Testloop

N=333333

slide16

Nested loops and combination

Using nested loops

% 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)

Example: exloop6.m

Calculate 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

i

j

F=F*j

S=S+F

slide17

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

slide18

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

slide19

The FOR loop

Example :

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

slide20

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.

slide21

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