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MatLab – Palm Chapter 4, Part 3For and While Loops

Class 11.1 Palm Ch 4.5

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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

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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

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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).

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for LOOPSSYNTAX:for k = start:step:stop blockendstart = initial valuestep = incremental valuestop = terminating valueSee p. 212 for flowchart of for loop.

Indentation Mandatory!

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for LOOP FLOW CHART

Loop structure

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EXAMPLE

for k = 5:10:35x = k^2

end

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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)

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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!)

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INDIVIDUAL EXERCISE (5 min.)

Evaluate xn = sin (n/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.

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SOLUTIONfor n = 1:10 x(n) = sin(n*pi/10);endplot(1:10,x) % orplot(x)title(‘In-class… example’)xlabel(‘n’)ylabel(‘x(n)’)

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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); endendWhy is I OK and i not OK?

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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); endendUsing r (for row) and c (for col) is MUCH better!!!

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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.

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Solution% PROGRAM example1.m %% Solution to T4.5-1 on p. 213 of the Palm% Matlab book.%% S. Socolofsky% ENGR 111A: 501-503

for r = 1:4 for c = 1:3 A(r,c) = 4 + (r-1)*6 + (c-1)*4; endend

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The “break” Statement You can EXIT out of any looping structure

with a “break” (Both for… and while…)

y=0;x=[1 -2 3 -4 5];for k = 1:length(x); if x(k) < 0 break end y = y + sqrt(x(k));end% -- Jump to here on break

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The “continue” Statement

You can skip “bad values” in any looping structure with a “continue”

y=0;x=[1 -2 3 -4 5];for k = 1:length(x); if x(k) < 0 continue end y = y + sqrt(x(k));end % Jump to here on continue

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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.

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Implied Loops (cont.) Many MatLab commands contain

implied loops. Example:

x = [0:5:100];

y = cos(x); Example:

y = find(x>0); See p. 216 for equivalent loops for

these.

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while LOOPS The while loop terminates because

a specified condition is satisfied. The number of passes is NOT

known a prioriSYNTAX:while logical expression statementsendSee p. 223 for flowchart of while loop.

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while LOOP FLOW CHART

Loop structure

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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.

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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.

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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

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SOLUTION

x = 0;

while x <= 100

y = sin(x);

x = x+5;

disp(y)

end

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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.

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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.

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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.