For each of the following, provide an answer in Matlab-digestable syntax
1. Assign the value 0.00014 to the variable x, using compact scientific notation.
x = 1.4e-04
2. Set the variable x to a vector whose components are the even numbers between 14 and 36.
x = 14:36
3. Set the variable x to 200 evenly spaced points between 0 and 2.
x = linspace(0, 2, 200)
4. Produce a plot of y = x^2 - 2x + 3 for the x of the previous problem. Label the axes.
plot(x, x.^2 - 2x + 3);
xlabel('x');
ylabel('x^2 - 2x + 3');
Note: We have to use x.^2 rather than x^2 to get the square of each component of the vector x.
5. Plot tan(x) versus x for 200 evenly spaced points between 0 and 1.57 ≈ pi/2, using a logarithmic scale on the y axis.
x = linspace(0, 1.57, 200);
semilogy(x, tan(x));
xlabel('x');
ylabel('tan(x)');
6. Plot sin(x) and cos(x) for 200 evenly spaced points between -π and π, on the same plot, using red for sin(x) and blue for cos(x). Add a legend that indicates which function is which color.
x = linspace(-pi, pi, 200);
plot(x, sin(x), 'r', x, cos(x), 'b');
legend('sin(x)', 'cos(x)');
xlabel('x')
7. Produce a vector of length 13 whose components are random integers between 0 and 10, inclusive.
randi(10, 1, 13)
8. Produce a vector of length 13 whose components are random real numbers between 0 and 10, inclusive.
10*rand(1, 13)
9. Show how you would solve the following system of equations with Matlab
x + 2y - z = 5 3x + y + 6z = 37 -3x + y + 2z = -11
A = [1 2 -1; 3 1 6; -3 1 2]; b = [5; 37; -11]; x = A\b
10. Write a conditional expression that is true if scalar variables x and y are both nonzero and false otherwise.
x ~= 0 && y ~= 0
11. Set variable A to a 3 x 5 matrix of zeros.
A = zeros(3,5)
12. Set variable A to a 4 x 7 matrix of random real numbers, using a guassian (normal) distribution.
A = randn(4,7)
13. Write a conditional expression that is true if a matrix A is square and false otherwise.
dim(A,1) == dim(A,2) % a one-liner, but maybe you don't know dim works this way
or
[M, N] = size(A) M == N
14. Write a conditional expression that is true if either x or y is an integer.
x == round(x) || y == round(y)
Write a Matlab function that
15. returns 1 (true) if its argument is divisible by 3 and 0 (false) if it's not.
function r = divisiblebythree(n)
% divisiblebythree(n) : return 1 if n is divisible by 3 or 0 if it's not
r = 3*round(n/3) == n
end
16. takes a vector x as input and returns 1 if the components of x are sorted in ascending order, 0 if not.
function s = sorttest(x)
% return 1 if x is sorted in ascending order, 0 if not
N = length(x);
% count how many of {x(1) through x(N-1)} are less than {x(2) through x(N)}
% the count will be N-1 if the x is sorted in ascending order
s = sum(x(1:N-1) < x(2:N)) == N-1
end
17. computes the product y = Ax of an M x N matrix A and an N x 1 vector x, according the formula
function y = matvecmult(A,x);
% matvecmult: compute matrix-vector multiplication A x
[M,N] = size(A);
% M x N matrix times N-vector equals an M-vector
y = zeros(M,1);
% evaluate formula for ith component of y
for i=1:M
% evaluate y_i = sum_{j=1}^n A_ij x_j
for j=1:N
y(i) = y(i) + A(i,j)*x(j);
end
end
end
A few more problems, added 1/14/2013
18. Set vector v to the 3rd column of matrix A.
v = A(:,3);
19. Set vector v to the 4th row of matrix A.
v = A(4,:);
20. Set vector v to the first 3 entries in the 7th row of A.
v = A(7,1:3);
21. Set the 4th row of the 8 x 5 matrix A to (1, 2, 9, 5, 0).
A(4,:) = [1 2 9 5 0];
22. Set the 2nd column of the 4 x 3 matrix A to a column vector with entries (3, 0, -1, 5).
A(:,2) = [3; 0; -1; 5];
or
A(:,2) = [3 0 -1 5]';
23. Zero all the entries in the last column of a matrix A.
[M,N] = size(A); A(:,N) = zeros(M,1);
24. Produce a contour plot of 
where x and y range from -10 to 10. Label the axes.
N = length(x);
F = zeros(N,N);
for i=1:N;
for j=1:N;
r = x(i)^2 + y(j)^2;
F(i,j) = sin(r)/sqrt(r);
end
end
% plot matrix with contour
contour(x,y,F);
xlabel('x');
ylabel('y');
% problem didn't ask for these, but they make the plot nicer
title('sin(x^2 + y^2)/sqrt(x^2 + y^2)')
axis equal
axis tight
—-
A few more problems, added late 10/14/2013. You don't have to turn these in, but they are fair game for the exam. I will add more example problems over the next few days.
25. Write an expression that returns the positive elements of a vector v.
v(v>0)
26. Write an expression that returns the sum of the positive elements of a vector v.
sum(v(v>0))
27. Write an expression that returns the number of positive elements in a vector v.
sum(v>0)
Even more practice problems! Hooray! added 10/22/3013
28. Write a function that computes the factorial of an integer n using a while loop.
function f = factorial2(n)
% factorial2: compute n!
f = 1;
while (n>1);
f = f*n;
n = n-1;
end
end
29. Write an isPrime(n) function that returns 1 (true) is n is prime
and 0 (false) if n is not prime. Don't worry about making the test efficient
or doing it with integer arithmetic.
function r = isprime(n)
% isprime(n): return 1 (true) of n is prime, 0 (false) if it isn't. (crude!)
r = 1; % set answer to true to start
% check if n is divisble by 2, return false if it is
if 2*round(n/2) == n
r = 0; % set answer to false
return; % exit function right away
end
% now check all odd numbers between 3 and sqrt(n)
for m=3:2:round(sqrt(n))
if m*round(n/m) == n
r = 0;
return;
end
end
end
30. Write a function that returns a vector of all the integer divisors of an integer n. Again, don't worry about efficiency or integer arithmetic.
function d = divisors(n)
% divisors(n): return vector of all divisors of n
m = 0; % set number of divisors to zero
for k = 1:n
if k*round(n/k) == n
m = m+1; % increment number of divisors
d(m) = k; % add m to vector of divisors
end
end
end
