How can I find the starting index number when my numbers are changing through a column vector?

How can I find the starting index number when my numbers are changing through a column vector?

[Walter Roberson]

Please give an example.

[Efraim Culfa]

For example, the column vector has the data of repeating numbers from 1 to 100. How can I find the starting index number in this case?

Answer 1

[Star Strider]

If none of the values in your vector are zero, the first index is the first non-zero value. MATLAB will create a vector with defined indices, filling the previous entries with zero:

V(4:7) = randi(9, 1, 4)

V =
     0     0     0     5     8     4     2

Use the find function to return the first non-zero entry:

first_nonzero_index = find(V > 0, 1, 'first')

first_nonzero_index =
     4

EDIT — To find the start and end of a sequence, this works:

V = [1 1 1 1 1 2 2 2 2 2 2 2 2 3 3 3 3 3 3 3 4 4 4 4 4 5 5 5 6 6 6 6 6 6];
first_idx = find(V == 2, 1, 'first');           % Start Of ‘2’ Repeats
last_idx  = find(V == 2, 1, 'last');            % End Of ‘2’ Repeats

first_idx =
     6

last_idx =
    13

Answer 2

[Thorsten]

Your question is somewhat unclear to me. What do you mean by starting index? If you have a vector in Matlab, the starting index into this vector is always 1.

x = rand(1,100);
x(1) % entry at first index of x

Same for a random permutation of the numbers from 1 to 100;

x = perm(100);
x(1) % entry at first index of x

If you want to find the index where x has the value 1, you can use

idx = find(x == 1);

[Efraim Culfa]

So, I want to know how can I make MATLAB find the "new" index number whenever my sequentials stop repeating and starting the new number? For example; 1111122222222333333344444555666666.....100100. How can I make MATLAB say;"Number 2 stops here." ? Can forloop and if be of use?