matlab programming-4.ppt

1、,4 Loops,王素珍 青岛理工大学 ： Mobile Tel：132-1002-1986,4.1 The While Loop,A while loop is a block of statements that is repeated indefinitely as long as some condition is satisfied. The general form of a while loop is while expression end If the expression is non-zero (true), the code block will be executed

2、, and then control will return to the while statement. If the expression is still non-zero (true), the statements will be executed again.,4.1 The While Loop,The pseudocode corresponding to a while loop is While expre end Example 4.1,(4.1),(4.2),4.2 The for Loop,The for loop is a loop that executes a

3、 block of statements a specified number of times. The for loop has the form for index=expr statement 1 statement n end Where index is the loop variable ( also known as the loop index) and expr is the loop control expression. The expression usually takes the forms of a vector on short notation first:

4、incr:last.,Body,4.2 The for Loop,An example: In this case, the control expression generated a 110 array, so statements 1 through n will be executed 10 times.,for ii=1:10 statement 1 statement n end,4.2 The for Loop,An example: In this case, the control expression generated a 15 array, so statements

5、1 through n will be executed 5 times.,for ii=1:2:10 statement 1 statement n end,4.2 The for Loop,An example: Here, the control expression is an explicity written 13 array, so statements 1 through n will be executed 3 times with the loop index set to 5 the first time, 9 the second time, and 7 the fin

6、al time. The loop index ii will set to 7 after the loop finished executing.,for ii=5 9 7 statement 1 statement n end,4.2 The for Loop,An example: In this case, the control expression generated a 23 array, so statements 1 through n will be executed 3 times. The loop index ii will be the column vector

7、 1;4 the first time, 2;5 the second time, and 3;6 the third time. This example illustrates the fact that a loop index can be a vector.,for ii=1 2 3; 4 5 6 statement 1 statement n end,4.2 The for Loop,Example 4.2 Example 4.3 Example 4.4,4.2.1 Details of Operation,Always indent the body of a for loop

8、by 2 or more spaces to improve the readability of the code Never modify the value of a loop index within the body of the loop. Always pre-allocate all arrays used in a loop before executing the loop. This practice greatly increases the execution speed of the loop. If it is possible to implement a ca

9、lculation either with a for loop or using vectors, always implement the calculation with vectors. Your program will be much faster. Example 4.5,4.2.2 the break and continue statements,There are two additional statements that can be used to control the operation of while loops and for loops: the brea

10、k and continue statements. The break statement terminates the execution of a loop and passes control to the next statement after the end of the loop, while the continue statement terminates the current pass through the loop and returns control to the top of the loop.,4.2.2 the break and continue sta

11、tements,An example for ii=1:5 if ii= =3 ; break; end fprintf ( ii= %dn , ii) end disp (end of loop!) When this program is executed, the output is test_break ii=1 ii=2 end of loop!,4.2.2 the break and continue statements,If a continue statement is executed in the body of a loop. The execution of the

12、current pass through the loop will stop and control will return to the top of the loop. The controlling variable in the for loop will take on its next value, and the loop will be executed again. an example:,4.2.2 the break and continue statements,an example: While this program is executed, the outpu

13、t is,for ii=1:5 if ii= =3 ; continue; end fprintf ( ii= %dn , ii) end disp (end of loop!), test_continue ii=1 ii=2 ii=4 ii=5 end of loop!,4.2.3 Nesting Loops,It is possible for one loop to be completely inside another loop. If one loop is completely inside another one, the two loops are called neste

14、d loops. An example:,for ii=1:3 for jj=1:3 product=ii*jj; fprintf ( %d* %d =%dn , ii,jj, product) end end,4.2.3 Nesting Loops,The resulting output is Note that the inner for loop executes completely before the index variable of the outer for loop is incremented.,1*1=1 1*2=2 1*3=3 2*1=2 2*2=4 2*3=6 3

15、*1=1 3*2=6 3*3=9,4.2.3 Nesting Loops,If a break or continue statement appears inside a set of nested loops, then the break statement refers to the innermost of the loops containing it. An example:,for ii=1:3 for jj=1:3 if jj=3; break; end product=ii*jj; fprintf ( %d* %d= %dn , ii, jj, product); end

16、fprinf ( end of inner loopn ) end fprintf ( end of outer loopn );,4.2.3 Nesting Loops,The resulting output values are,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,4.3 Logical Arrays and Vectorization,Logical arrays are standard numeric a

17、rrays with a special “logical” property added and are created by all relational and logical operators. They can be distinguished from standard arrays because the (logical) modifier appears after them in the whos command or the Workspace Browser. For example, consider the following statement: a=1 2 3

18、; 4 5 6; 7 8 9; b=a5; then b=0 0 0; 0 0 1; 1 1 1,4.3 Logical Arrays and Vectorization,When the whos command is executed, the results are as shown below. Note that array b has the (logical) modifier applied to it. whos Name Size Bytes Class a 33 72 double array b 33 72 double array (logical) Grand to

19、tal is 18 elements using 144 bytes.,4.3 Logical Arrays and Vectorization,It is also possible to add the logical property to an array by using the logical function. For example, the statement c=logical (a) will assign the values in array a to array c, while setting the logical property for the array.

20、 c=logical (a) C=1 2 3; 4 5 6; 7 8 9 whos Name Size Bytes Class a 33 72 double array b 33 72 double array (logical) b 33 72 double array (logical) Grand total is 27 elements using 216 bytes.,4.3 Logical Arrays and Vectorization,The logical property can be removed from an array by performing almost a

21、ny arithmetic operation on it. For example, if we add zero to array c, the values in the array will be unchanged, but the logical property will be lost. c=c+0 c=1 2 3; 4 5 6; 7 8 9 whos Name Size Bytes Class a 33 72 double array b 33 72 double array (logical) b 33 72 double array Grand total is 27 e

22、lements using 216 bytes.,4.3.1 The Significance of Logical Arrays,A mask is an array that selects the elements of another arrays for use in an operation. The specified operation will be applied to the selected elements, but not to the remaining elements. a(b)=sqrt(a(b) a= 1.0000 2.0000 3.0000 4.0000

23、 5.0000 2.4495 2.6458 2.8284 3.0000 This is a very fast and very clever way of performing an operation on a subset of an array without needing loops and branches.,Take the square root of all elements for which the logical array b is nonzero and leave all the other elements in the array unchanged.,4.3.1 The Significance of Logical Arrays,The following two code fragments bot