Linear Algebra, 2020a

Topic: Speed of Calculating Determinants 375
Exercises
1 To get an idea of what happens for typical matrices we can use the ability of
computer systems to generate random numbers (of course, these are only pseudorandom
in that they come from an algorithm but they pass a number of reasonable
statistical tests for randomness).
(a) Fill a 5×5 array with random numbers say, in the range [0 ... 1)). See if it is
singular. Repeat that experiment a few times. Are singular matrices frequent or
rare in this sense?
(b) Time your computer algebra system at finding the determinant of ten 10×10
arrays of random numbers. Find the average time per array. Repeat the prior
item for 20×20 arrays, 30×30 arrays, . . . 100×100 arrays, and compare to the
numbers given above. (Notice that, when an array is singular, we can sometimes
decide that quickly, for instance if the first row equals the second. In the light of
your answer to the first part, do you expect that singular systems play a large
role in your average?)
(c) Graph the input size versus the average time.
2 Compute the determinant of each of these by hand using the two methods discussed
above.
2 1 0 0
(a)
∣ 2 1
3 1 1
5 −3∣
(b)
−1 0 5
(c)
1 3 2 0
∣−1 2 −2∣
0 −1 −2 1
∣
0 0 −2 1
∣
Count the number of multiplications and divisions used in each case, for each of
the methods.
3 The use by the timing routine of do_matrix has a bug. That routine does
two things, generate a random matrix and then do gauss_method on it, and the
timing number returned is for the combination. Produce code that times only the
gauss_method routine.
4 What 10×10 array can you invent that takes your computer the longest time to
reduce? The shortest?
5 Some computer language specifications requires that arrays be stored “by column,”
that is, the entire first column is stored contiguously, then the second column, etc.
Does the code fragment given take advantage of this, or can it be rewritten to
make it faster, by taking advantage of the fact that computer fetches are faster
from contiguous locations?