ECE 102 Engineering Computation

Transcription

1 ECE 102 Engineering Computation Phillip Wong MATLAB Loops for while break / continue

2 Loops A loop changes the execution flow in a program. What happens in a loop? For each iteration of the loop, statements in the body of the loop are executed. A conditional test is evaluated once per iteration to determine whether or not to exit the loop. The loop s body could be executed multiple times or even not at all. 1

3 Two categories of loops: Counted loops Statements are repeated a specified number of times. MATLAB for loop Conditional loops Statements are repeated as long as a conditional test remains true. MATLAB while loop 2

4 Most loops require these operations: Statements A Initialization Loop initialization Conditional test Update Conditional test TRUE Stay in loop Statements L FALSE Loop update Exit loop Statements B 3

5 for The for loop is often used if the desired number of iterations is known in advance. for var = first:step:last Statements Loop body Iteration range var loop or iterator variable first value of var in the first iteration step increment between values last value of var in the last iteration 4

6 Example: t=1:2:11 6 iterations, t values are 1, 3, 5, 7, 9, 11 x=1:0.5:2 3 iterations, x values are 1, 1.5, 2 If step is omitted, then default is +1: k=0:3 4 iterations, k values are 0, 1, 2, 3 step can be negative for a descing count: k=4:-2:-6 6 iterations, k values are 4, 2, 0, -2, -4, -6 5

7 for var = first:step:last Special cases If first == last, the loop is executed only once. If first > last and step > 0, the loop is not executed at all. if first < last and step < 0, the loop is not executed at all. 6

8 How does the for loop know when to stop? for var = first:step:last Vector: [ v first v 2 v 3 v last ] At each iteration, var takes on a consecutive value from the vector. The loop stops when the last element in the vector is reached. 7

9 Example: for m = 1:3 fprintf('hello!\n'); Output: Hello! Hello! Hello! for m = 1:3 s = m^2; fprintf('%d %d\n',m,s); Output:

10 Example: % Factorial n = 4; % Find 4! fact = 1; for a = 1:n fact = fact * a; #1 : a 1 fact = 1 * 1 1 #2 : a 2 fact = 1 * 2 2 #3 : a 3 fact = 2 * 3 6 #4 : a 4 fact = 6 *

11 Example: vec = [ ]; for k = 1:length(vec) fprintf('%d\n', vec(k)); vec = [ ]; for k = length(vec):-1:1 fprintf('%d\n', vec(k)); vec = [ ]; for k = 2:2:3 fprintf('%d\n', vec(k)); Output: Output: Output: 4 10

12 Example: u = []; for k = 1:6 x = input('x? '); if x > 0 u(k) = 'p'; elseif x < 0 u(k) = 'n'; else u(k) = 'z'; Sample run: x? 9 x? -3 x? 0 x? -9 x? -2 x? pnznnp disp(u) disp(char(u)) 11

13 Nested for loops: A for loop can be placed within the body of another for loop. for var1 = first1:step1:last1 Statements for var2 = first2:step2:last2 Statements Statements outer loop inner loop 12

14 Assumption: Outer miterations, Inner niterations For each iteration of the outer loop, the inner loop is executed n times. Total number of iterations = m n. Nested for loops are often used to manipulate 2-D arrays (e.g., matrices). for loops can be nested multiple levels deep. 13

15 Example: % This script prints out a matrix A = [1 3 2; 4-2 4]; [num_rows num_cols] = size(a); for m = 1:num_rows for n = 1:num_cols fprintf('%3d', A(m,n)); fprintf('\n'); Output:

16 Example: % This script multiplies two matrices A = [1 3 2; 4-2 4]; B = [4-1; 3 2; 5 3;]; T = []; [nr1 nc1] = size(a); [nr2 nc2] = size(b); if nc1 == nr2 % Check for compatible dimensions for r = 1:nr1 % Row index of A for c = 1:nc2 % Column index of B sum = 0; for k = 1:nc1 sum = sum + A(r,k)* B(k,c); T(r,c) = sum; T :

17 while The while loop is often used if the number of iterations is not known in advance. while conditional test Statements Loop body As long as the conditional test is true, the loop repeats and the loop body is executed. Once the conditional test is false, the loop is exited. 16

18 Conditional test Combination of logical and/or relational expressions (evaluates to true or false) Variables used by the conditional test must have initial values before entering the loop. Usually, the test variable(s) is updated within the loop so the conditional test will eventually be false. Example: count = 0; while count < 5 count=count+1; Initial value for conditional variable Conditional test Update 17

19 Example: % Factorial fact = 1; a = 1; while a <= 4 fact = fact * a; a = a + 1; fact: 24 w = [ ]; k = 1; while k < 5 w(k) = w(k+1)+3; k = k + 1; w:

20 Example: % for loop version for m = 3:2:9 s = m^2; fprintf('%d %d\n',m,s); Output: % while loop version m = 3; while m <= 9 s = m^2; fprintf('%d %d\n',m,s); m = m + 2; Output:

21 Example: % Error check menu disp('options:'); disp('1. Red'); disp('2. Green'); disp('3. Blue'); done = false; while ~done c = input('choice?'); if c >= 1 && c <= 3 done = true; else disp('error: Please try again'); 20

22 In general, any type of loop can be nested: A for loop can contain a for or while loop. A while loop can contain a while or for loop. Warning: An infinite loop can occur if the conditional test never becomes false. TIP: Type CTRL-C at the keyboard to abort out of an infinite loop in MATLAB. 21

23 break Inside a loop, the break command terminates execution of the loop (exits). Inside a nested loop, break only exits the loop that contains it. If break appears outside a loop in a script or function file, it terminates execution of the file. break is often used with a selection statement. 22

24 Example: % Use a state variable done = false; % State sum = 0; while ~done x = input('x? '); sum = sum + x; if sum > 1000 done = true; % Use a break statement sum = 0; while true x = input('x? '); sum = sum + x; if sum > 1000 break; 23

25 continue Inside a loop, the continue command: Immediately stops the current iteration Restarts at the next iteration continue is often used with a selection statement. 24

26 Example: % Add only positive input values to the sum sum = 0; % Process five input values for k = 1:5 x = input('x? '); if x < 0 disp('negative value rejected'); continue; sum = sum + x; 25