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CSE123 - Lecture 4 S tructured Programming - Loops

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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CSE123 - Lecture 4 S tructured Programming - Loops

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  1. CSE123 - Lecture 4Structured Programming-Loops

  2. 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

  3. Initialization Initialization Initialization Input Initialization Input Calculation 1 Calculation 2 Calculation 1 Calculation 3 Results Results Sequential and Structured Programming Repeat the operation3 times…

  4. 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

  5. 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

  6. 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

  7. 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

  8. 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.

  9. 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.

  10. 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

  11. 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

  12. 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’) >>

  13. 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

  14. 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

  15. 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

  16. 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

  17. 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

  18. 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

  19. The FOR loop Example : Write a Matlab script tp calculate following expression for an enetered x value

  20. 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.

  21. 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

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