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 operation 3 times…

The FOR loop for loop_variable= start:step:finish …. Statements …. end Statements for loop_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. INCREMENT loop_variable

% program to test a for loop for i=1:10 disp(i*0.2) end >> testloop … 2.0 % program to test a for loop for i=1:10 disp(i*0.2) end >> testloop … 2.0 Example: testloop.m % program to test a for loop for i=1:10 disp(i) end >> testloop % program to test a for loop for i=1:10 disp(i) end >> testloop >> disp(i) 10 The FOR loop >> disp(i) 10 % program to test a for loop for i=1:0.1:2 disp(i*5) end >> testloop % program to test a for loop for i=1:0.1:2 disp(i*5) end >> testloop >> 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 1 st elements, obtain partial result, add 2 nd element, obtain partial result, etc…. Loop variable: i Result variable: sum % program to test a for loop sum=0; for i=1:100 sum=sum+i; end disp(sum) disp([‘result=‘,num2str(sum)]) % program to test a for loop 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 usage 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 % program to test a for loop for j=1:100 end % program to test a for loop for j=1:100 end S=sqrt(S) S=pi; Initialization S=

The FOR loop INCREMENT loop_variable Statements for loop_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 Background tests and operations

The WHILE loop Statements While logical_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 >> exloop >> disp(i) 10 % program test for i=1:10 disp(i) end i=0; Example: exloop1.m Had to create a counter

The WHILE loop >> exloop % 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.m Calculate 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 end N=i-1 disp(S/N) % program to test a while loop while end N=i-1 disp(S/N) Use while loop when the number of operation is unknown variable: A S=S+A i=i+1 >> testloop Value for A:2 Value for A:0 2 >> testloop Value for A:2 Value for A:0 2 A=input('Value for A:'); A>0 i=i+1; S=S+A; A=pi; S=0; i=0; >> testloop Value for A:1 Value for A:5 Value for A:8 Value for A: >> testloop Value for A:1 Value for A:5 Value for A:8 Value for A:

The WHILE loop Example 5: Series convergence: π 2 /6 Want to see how many terms you need to obtain an error of 3x % Convergence script while end disp(['N=',num2str(i)]) % Convergence script while end disp(['N=',num2str(i)]) Use logical_expression for convergence limit >> Testloop N= >> Testloop N= 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 % Convergence script 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 disp(['N=',num2str(i)]) % Convergence script 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 disp(['N=',num2str(i)]) Use logical_expression for convergence limit >> Testloop N= >> Testloop N=333333

Nested loops and combination 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 % 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 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>10000 break 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==0 continue end

The FOR loop Example : Write a Matlab script to calculate following expression for an entered x value

The FOR loop Example : Write a Matlab script to 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.

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; disp([num2str(ii),‘*’,num2str(jj),... ‘=’,num2str(product)]) end disp(‘end of inner loop’); end disp(‘end of outer loop’); 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