MATLAB POLYSPACE RELEASE NOTES Instrukcja Użytkownika Pobierz pdf (Strona 50)

2 Matrices and Arra ys

This kind of subs cripting can be do n e in one step by specifying the logical

operation as the subscripting expression. Suppose you have the following

set of data:

x = [2.1 1.7 1.6 1.5 NaN 1.9 1.8 1.5 5.1 1.8 1. 4 2.2 1.6 1.8];

The NaN is a marker for a mis sing observation, such as a failure to respond to

an item on a questionnaire. To remove the missing data with logical indexing,

use

isfinite(x), which is true for all finite numerical values and false for

NaN and Inf:

x = x(isfinite(x))

2.1 1.7 1.6 1.5 1.9 1.8 1.5 5.1 1.8 1.4 2 .2 1.6 1.8

Now there is one observation, 5.1, which seems to b e very different from the

others. It is an outlier. The following statement removes outliers, in this case

those elements more than three standard deviations from the mean:

x = x(abs(x-mean(x)) <= 3*std(x))

2.1 1.7 1.6 1.5 1.9 1.8 1.5 1.8 1.4 2.2 1 .6 1.8

For another exam ple, highlight the location of the prime numbers in Dürer’s

magic square by using logical indexing and scalar expansion to set the

nonprimesto0. (See“ThemagicFunction”onpage2-9.)

A(~isprime(A)) = 0

03213

50110

0070

0000

The find Function

The find function determines the indices of array eleme nts that meet a given

logical condition. In its simplest form,

find returns a column vector of indices.

Transpose that vector to o btain a row vector of indices. For example, start

again with Dürer’s magic square. (See “The magic Function” on page 2-9.)

2-28

1 2 ... 45 46 47 48 49 50 51 52 53 54 55 ... 239 240

Brak uwag

MATLAB POLYSPACE RELEASE NOTES Instrukcja Użytkownika Strona 50