Index an array using specific indices

I have a vector

x = [0,1,1,0,1,...,N] % x is vector containing either 1s or 0s of size N = 107199.

then I have another vector s, which contains index locations in x.

s= [23,100,500, 9888,....N] % s is of size 1x518

so I want to filter s to contain only indices where the values in x are 1s.

I tried something like this,

  idx = x(s)~=1;
  new_s = s(idx);

but it gives me this error Index exceeds the number of array elements (107520).. any idea what im doing wrongly?

Answer 1

[Daniel Neubauer]

x=[1 0 1 1 0 0 0 0 1 0];
s=[9 4 6];
snew=s(x(s)==1)

snew = 1×2

9 4

[KALYAN ACHARJYA]

Yes, I noticed the array length later.

Answer 2

[DGM]

Try

% some test vectors
x = randi([0 1],1,20)

x = 1×20

0 0 0 1 1 0 1 0 1 0 1 1 1 0 1 0 1 1 1 0

s = randi([1 numel(x)],1,10)

s = 1×10

9 6 20 9 12 9 17 11 7 3

% this is the content selected by s
x(s)

ans = 1×10

1 0 0 1 1 1 1 1 1 0

% and then this would be cases where s selects 1s within x
new_s = s(x(s)==1)

new_s = 1×7

9 9 12 9 17 11 7

[Book]

try this:

%% Load audio and extract 3s–6s

[x, Fs] = audioread('test.wav');

t = (0:length(x)-1)/Fs;

segment = x(Fs*3:Fs*6);

%% Flip audio

x_flip = flipud(segment);

%% Downsample (aliasing)

alias1 = downsample(segment, 2);

alias2 = downsample(segment, 3);

%% Spectrograms

figure;

subplot(3,1,1);

spectrogram(segment,256,200,256,Fs,'yaxis');

title('Original Segment');

subplot(3,1,2);

spectrogram(alias1,256,200,256,Fs/2,'yaxis');

title('Alias 1 (↓2)');

subplot(3,1,3);

spectrogram(alias2,256,200,256,Fs/3,'yaxis');

title('Alias 2 (↓3)');

%% Frequency analysis

Y = abs(fft(segment));

f = (0:length(Y)-1)*(Fs/length(Y));

[~, idx] = sort(Y, 'descend');

topFreqs = f(idx);

%% Save outputs

audiowrite('part3_segment.wav', segment, Fs);

audiowrite('part3_flip.wav', x_flip, Fs);

audiowrite('part3_alias1.wav', alias1, Fs/2);

audiowrite('part3_alias2.wav', alias2, Fs/3);

saveas(gcf, 'part3_spectrogram.jpg');% Sampling & Aliasing (Audio)

[Book]

a = input('Rows for x1: '); b = input('Columns for x1: '); x1 = zeros(a,b); for i = 1:a for j = 1:b x1(i,j) = input('Enter elements for x1: '); end end

c = input('Rows for x2: '); d = input('Columns for x2: '); x2 = zeros(c,d); for i = 1:c for j = 1:d x2(i,j) = input('Enter element for x2: '); end end

disp('Matrix x1:'); disp(x1) disp('Matrix x2:'); disp(x2)

[Book]

% Step 1: Create two 3x3 matrices x1 = [1 2 3; 4 5 6; 7 8 9]; % you can replace with your own values x2 = [9 8 7; 6 5 4; 3 2 1]; % you can replace with your own values

% Step 2: Perform the required operations

% a) y1 = x1 - 2x2 y1 = x1 - 2*x2;

% b) y2 = 3x2 + (1/2)x1 y2 = 3*x2 + 0.5*x1;

% c) y3 = (2/3)x1x2 (matrix multiplication) y3 = (2/3) * (x1 * x2);

% d) y4 = x2x1 (matrix multiplication) y4 = x2 * x1;

% e) y5 = x1^(-4) + x2^3 % x1^(-4) means inverse(x1)^4, must be invertible try y5 = (inv(x1)^4) + (x2^3); catch disp('Matrix x1 is not invertible, choose another set of values.'); y5 = NaN; end

% Display results disp('y1 ='); disp(y1); disp('y2 ='); disp(y2); disp('y3 ='); disp(y3); disp('y4 ='); disp(y4); disp('y5 ='); disp(y5);

Answer 3

[dpb]

Well, if size N = 107199 while s is of size 1x518, then 107199 > 518 so any location in x past numel(s) is going to be an out of bounds reference.

It's not clear in isolation what relationship there is (if any) between x and s so why one would expect those two to be commensurate is puzzling.

Perhaps you're looking for locations that are in s that were found in some other fashion and wanting to know if there's a corresponding matching "1" value in x at the specific location? That's about the only way I can think of at the moment to relate the two -- if that were the case then try

new_s=s(ismember(s,find(~x)));

Answer 4

[fadams18]

Ah i got it. In my original code I was doing something to s before the indexing, which was chanching its values. My code also works.

Thanks for your insights and time guys, @KALYAN ACHARJYA @dpb @Daniel Neubauer @DGM.