How to find the index of the first and last non-zero value of a vector ?

Hello, I have a vector suppose a function f(x), such that

f(x)= ...0,0,0,0,0,0,0,0,5,6,7,4,3,5,6,7,8,0,0,0,0,0,0,0... and

x = 1,2,3,4,5,....

I don't know which command can I use to find those values of x for which the values of f(x) are 5 and 8 (i.e the first and last non-zero values)?

Answer 1

[Daniel Pollard]

The built in command find sounds like what you need. The code could look something like

f = ...0,0,0,0,0,0,0,0,5,6,7,4,3,5,6,7,8,0,0,0,0,0,0,0...;
x_idx = find(f);
x = [x_idx(1), x_idx(end)];

where x is the positions of the first non-zero component and the last non-zero component.

Edit To correct a mis-type.

[Adnan Hayat]

Dear Daniel, thanks. Actually it is working for x_idx(1), but for x_idx(end) it is giving an error. My MATLAB seriel number is R2017a.

[Daniel Pollard]

The documentation says that the syntax for 'end' was released pre-2006, so I don't think that is the error. Can you paste the exact error message?

[Adnan Hayat]

Thanks Daniel, it worked now. It was my typing mistake.

[Adnan Hayat]

Hello Daniel, what if f(x) be like

f = ...0,0,0,0,0,0,0,0,5,6,7,4,3,5,6,7,8,-1,-3,-5,...0,0,0,0,0,0,0...;

and I have to find the value of x for which the last value of f is positive ? I mean the value of x when f=8, after which all other values are negative and then after that 0.

[Daniel Pollard]

Have a look at the documentation for find - either click the link in my answer, google "matlab find" or type doc find into your matlab command window. You can give it a condition, so for example

x_ind = find(f > 0).

[Rik]

Note that f_x=f(x);ind=[find(f_x,1,'first') find(f_x,1,'last')]; might be faster, as the JIT might be able to optimize this call.

[Adnan Hayat]

Thank you both, I got my issue resolved :)