Chapter 4: Programming in Matlab - College of the Redwoods

266 Chapter 4 Programming in Matlab
Take a moment to check that in each position in B that contains a logical 1 (true),
the corresponding position in matrix A contains a uniform random number that
is less than 0.5.
There are a number of things that we can do with this information. We could
list all the random numbers in A that are less than 0.5 by using matrix B as a
logical index into the matrix A. 2
>> A(B)
ans =
0.4565
0.0185
0.4447
0.1763
0.4057
0.4103
0.0579
0.3529
0.0099
0.1389
0.2028
0.1987
0.2722
0.1988
0.0153
Or we could set each of these positions to zero.
>> A(B)=0
A =
0 0.7919 0.9355 0 0
0 0.9218 0.9169 0.8132 0.6038
0.8214 0.7382 0 0 0
0 0 0.8936 0 0
0.6154 0 0 0 0
2 Matlab has roots that began their growth in Fortran, where the entries in arrays were loaded
by column. The Matlab command A(:) will list all the entries in A, first going down the
first column, then the second, etc. With the command A(B), Matlab first lists all the “true”
positions in the first column, then the second, etc.