How to get a vector using the following index. Please help me.

Hi guys, how to create the vector x = randperm(35)using only logical indexing:

y(x) = 2

if x < 6

y(x) = x - 4

if 6 <= x < 20

y(x) = 36 - x

if 20 <= x <= 35

where the curve should be a triangular shape, always above zero and with a maximum of 16. Im stuck with the my code where:

x=(1:35)

if(x<6)

    y(x)=2
elseif(6<=x<20)
    y(x)=x-4
elseif(20<=x<=35)
    y(x)=36-x
end
plot(x,y)

But i cant seem to get a curve and get y to be right. is there a step where i missed?

[Cedric]

p = randperm(n) returns a row vector containing a random permutation of the integers from 1 to n inclusive. In your case, randperm(35) "shuffles" integers from 1 to 35, so there must be some random number generation somewhere in your solution, .. or what did I misunderstand?

[Amanda Kit Ting]

sorry about that, now that i looked at my question, i realized that i had a typo on randperm(35).

Answer 1

[Image Analyst]

Well here's the code that will make the triangle, but I don't know what you're supposed to do after that to get a list of 35 numbers all scrambled up.

clc;    % Clear the command window.
workspace;  % Make sure the workspace panel is showing.
format long g;
format compact;
fontSize = 35;

% Make x and y;
x = 1:35;
y = 2 * ones(1, length(x)); % Preallocate.
logicalLeft = x < 6
logicalMiddle = x >= 6 & x <= 20
logicalRight = x >= 20
% Make the signal
y(logicalLeft) = 2; % Already 2 actually so does nothing.
y(logicalMiddle) = x(logicalMiddle) - 4;
y(logicalRight) = 36 - x(logicalRight);

plot(x, y, 'bo-', 'LineWidth', 2);
grid on;
xlabel('x', 'FontSize', fontSize);
ylabel('y', 'FontSize', fontSize);
% Enlarge figure to full screen.
set(gcf, 'units','normalized','outerposition',[0 0 1 1]);
% Give a name to the title bar.
set(gcf,'name','Demo by ImageAnalyst','numbertitle','off')

[Amanda Kit Ting]

Hi ImageAnalyst,

thanks for your answer, i was wondering why i cant use the if-else statement to get it work?

[Image Analyst]

You can, but you'd have to put the if inside a for loop over all the x's.

for x = 1 : 35
  if x >= ......etc.

[Amanda Kit Ting]

oic, thanks for your help. i will take note next time.