1. MatLab – Palm Chapter 4, Part 3 For and While Loops
Class 11.1 Palm Ch 4.5
4/5/2004
ENGR 111A - Spring 2004

2. RAT 11.1
As in INDIVIDUAL you have 2 minutes to answer the following question.
What is the FINAL value assigned to y in the following segment of MATLAB code?
x = 0;
while x < 5
y = sqrt(x);
x = x + 1;
end
When asked, you have 30-seconds to submit your paper.
Answer: y = 2
4/5/2004
ENGR 111A - Spring 2004

3. Learning Objectives Students should be able to:
Understand Matlab iteration structures: for … end Loops
while … end Loops
Identify Implied loops
Recognize these structures as repetition (or iteration) structures.
Develop flow (logic) diagrams for combinations of branching and looping structures
4/5/2004
ENGR 111A - Spring 2004

4. LOOPS A LOOP is a structure for repeating (iterating) a block of code.
MATLAB uses two types of loops.
for loop: number of passes known a priori (p. 210).
while loop: loop terminates when a specified condition is satisfied (p. 221).
4/5/2004
ENGR 111A - Spring 2004

5. for LOOPS SYNTAX: for k = start:step:stop block end
start = initial value
step = incremental value
stop = terminating value
See p. 212 for flowchart of for loop.
Indentation
Mandatory!
4/5/2004
ENGR 111A - Spring 2004

6. for LOOP FLOW CHART
Loop
structure
4/5/2004
ENGR 111A - Spring 2004

7. EXAMPLE
for k = 5:10:35
x = k^2
end
4/5/2004
ENGR 111A - Spring 2004

8. for … end Loops (p. 211) for k = 5:10:35 x = k^2 end Means:
starting with k = 5, execute the statements between for … end;
then increment k by 10 and execute the statements with k = 15; etc. until k > 35.
Then exit the loop (k=? at this point)
4/5/2004
ENGR 111A - Spring 2004

9. for … end Loops cont’d.
Step value may be negative (default is positive 1)
For step > 0, loop does not execute if start > stop
For step < 0, loop does not execute if start < stop
If start = stop, loop executes once
Non-integer step values give unpredictable results (don’t use them!)
4/5/2004
ENGR 111A - Spring 2004

10. INDIVIDUAL EXERCISE (5 min.)
Evaluate xn = sin (np/10) for n between 1 and 10 (in increments of 1).
Write a Matlab PROGRAM.
(Plot x versus n)
Save file on R:\ and name file after an historical figure.
4/5/2004
ENGR 111A - Spring 2004

11. SOLUTION for n = 1:10 x(n) = sin(n*pi/10); end plot(1:10,x) % or
title(‘In-class…
example’)
xlabel(‘n’)
ylabel(‘x(n)’)
4/5/2004
ENGR 111A - Spring 2004

12. for Loops for loops can be nested (one inside each other).
Each loop is closed with an end.
EXAMPLE: Proper indention is a MUST!!
N = 10;
for I = 1:N
for J = 1:N
A(I,J) = 1/(I + J – 1);
end
Why is I OK and i not OK?
4/5/2004
ENGR 111A - Spring 2004

13. for Loops for loops can be nested (one inside each other).
Each loop is closed with an end.
EXAMPLE: Proper indention is a MUST!!
N = 10;
for r = 1:N
for c = 1:N
A(r,c) = 1/(r + c – 1);
end
Using r (for row) and c (for col) is MUCH better!!!
4/5/2004
ENGR 111A - Spring 2004

14. In-class Assignment 11.1 (20 min.)
Work through the nested loop and if example on page 212.
Write an program file to solve T4.5-1 (p. 213)
Save your solutions on the R:\ drive
Name your files after an animal.
4/5/2004
ENGR 111A - Spring 2004

15. Solution % PROGRAM example1.m %
% Solution to T4.5-1 on p. 213 of the Palm
% Matlab book.
% S. Socolofsky
% ENGR 111A:
for r = 1:4
for c = 1:3
A(r,c) = 4 + (r-1)*6 + (c-1)*4;
end
4/5/2004
ENGR 111A - Spring 2004

16. The “break” Statement
You can EXIT out of any looping structure with a “break” (Both for… and while…)
y=0;
x=[ ];
for k = 1:length(x);
if x(k) < 0
break
end
y = y + sqrt(x(k));
% -- Jump to here on break
4/5/2004
ENGR 111A - Spring 2004

17. The “continue” Statement
You can skip “bad values” in any looping structure with a “continue”
y=0;
x=[ ];
for k = 1:length(x);
if x(k) < 0
continue
end
y = y + sqrt(x(k));
end % Jump to here on continue
4/5/2004
ENGR 111A - Spring 2004

18. Implied Loops (p. 216)
You can use built-in loops instead of for … end loops in most cases. Sometimes it is better, sometimes not.
Note the matrix example with simple logic
Note the for … end example with more complex logic
Note the find function example for a very normal situation that you once upon a time had to write code for every time.
4/5/2004
ENGR 111A - Spring 2004

19. Implied Loops (cont.) Many MatLab commands contain implied loops.
Example:
x = [0:5:100];
y = cos(x);
y = find(x>0);
See p. 216 for equivalent loops for these.
4/5/2004
ENGR 111A - Spring 2004

20. while LOOPS
The while loop terminates because a specified condition is satisfied.
The number of passes is NOT known a priori
SYNTAX:
while logical expression
statements
end
See p. 223 for flowchart of while loop.
4/5/2004
ENGR 111A - Spring 2004

21. while LOOP FLOW CHART
Loop
structure
4/5/2004
ENGR 111A - Spring 2004

22. while … end Loops Look at the examples on page 221. x = 5;
while x < 25
disp(x)
x = 2*x-1 %-make sure x changes
end
Means:
starting with x = 5, execute the statements between the while … end; this displays x and calculates a new value of x and loops back.
Then exit the loop when x exceeds or equals 25.
4/5/2004
ENGR 111A - Spring 2004

23. while … end Loops (p.222)
THE LOOP VARIABLE MUST HAVE A VALUE BEFORE THE while STATEMENT IS EXECUTED
THE LOOP VARIABLE MUST BE CHANGED SOMEHOW BY THE STATEMENTS
Note the usual mistakes toward the bottom of the page.
Make sure that you save your code before testing any while … end loop.
4/5/2004
ENGR 111A - Spring 2004

24. INDIVUDAL EXERCISE (5 min.)
Write an equivalent while loop for the implied loop shown:
x = [0:5:100];
y = sin(x);
disp (y)
Save on the R:\ drive
Name file after a city outside of Texas
4/5/2004
ENGR 111A - Spring 2004

25. SOLUTION x = 0; while x <= 100 y = sin(x); x = x+5; disp(y) end
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ENGR 111A - Spring 2004

26. NESTED while LOOPS while loops can also be nested.
Each loop must be closed with an end.
break statements may be used to jump out of loops.
4/5/2004
ENGR 111A - Spring 2004

27. EXAMPLE r = 1; N = 10; while r <= N c = 1; while c <= N
A(r,c) = 1/(r + c -1);
c = c + 1;
end
r = r + 1;
end %- note use of r,c instead of i,j.
4/5/2004
ENGR 111A - Spring 2004

28. Assignment 11.1 Individual assignment. Due: Nov 15, 2004 Palm MATLAB;
Chapter 4: #22 (use for-loop in part b), 28 (use a while-loop), T4.5-4 (p. 225)
Extra Problem (see assignments web page)!
Extra Credit (see assignments web page)!!
Read Palm section 4.7.
4/5/2004
ENGR 111A - Spring 2004