How to find first instance of a value in array?

Hi, I am writing a code that as follows:

arr(p)=abs(c); %absolute value of c
if(arr(p)<1)
if(arr(p)>0.9)
    
    array(t)=white1;
    cor(t)=arr(p);   %correlation for c>.95 && c<1
    t=t+1;
end
end
maximum=max(cor); %maximum value of cor(t)

Now, I am trying to find the first instance the value of maximum comes in the "arr" Array.

I have tried the following:

1.

for v=1:x
    if(maximum==arr(v)) 
  % x=find(arr==maximum);
        u=u+1;
        if(u==1)      %making sure only the first max value is taken
        q=value(v);      %q is becoming always 0.5,have to fix it
        end
    end
end

and,

2.

x=find(arr==maximum);

and,

3.

 x = find(maximum == arr,'first') ;

In the 1st and 2nd try, when I use fprintf to detect the x, it prints blank, such as,

"x is "

In the 3rd try the following error occurs,

Second argument must be a positive scalar integer.

How can I fix it?

Answer 1

[Steven Lord]

You could use logical indexing.

rng('default')
c = randn(1, 10)

c = 1×10

0.5377 1.8339 -2.2588 0.8622 0.3188 -1.3077 -0.4336 0.3426 3.5784 2.7694

arr = abs(c)

arr = 1×10

0.5377 1.8339 2.2588 0.8622 0.3188 1.3077 0.4336 0.3426 3.5784 2.7694

% Overwrite those values in arr that are out of bounds with NaN. I'm using
% wider bounds to show the technique with a small data set.
arr(arr > 1.5) = NaN

arr = 1×10

0.5377 NaN NaN 0.8622 0.3188 1.3077 0.4336 0.3426 NaN NaN

arr(arr < 0.5) = NaN

arr = 1×10

0.5377 NaN NaN 0.8622 NaN 1.3077 NaN NaN NaN NaN

[value, location] = max(arr, [], 'omitnan')

value = 1.3077

location = 6

If all you're interested in is the value and not its location in c you could delete the elements of arr that fall out of bounds.

arr2 = abs(c);
arr2(arr2 > 1.5) = [];
arr2(arr2 < 0.5) = []

arr2 = 1×3

0.5377 0.8622 1.3077

[value2, location2] = max(arr2)

value2 = 1.3077

location2 = 3

location2 is not the same as location because arr2 does not have the same number of elements as either arr or c.

[Tawsif Mostafiz]

Thanks @Steven Lord!

Answer 2

[Jiro Doke]

Is this what you're looking for?

maximum = 5;
arr = [2 4 3 5 7 3 4];

find(arr == maximum, 1)

ans = 4

[Tawsif Mostafiz]

@jiro in this code:

arr(p)=abs(c); %absolute value of c
if(arr(p)<1)
if(arr(p)>0.9)
    
    array(t)=white1;
    cor(t)=arr(p);   %correlation for c>.95 && c<1
    t=t+1;
end
end
maximum=max(cor);

when I do this:

x=find(arr == maximum, 1);

It works for some input. It finds the first index of arr==maximum. However, when I print maximum with fprintf, for some input the x shows blank, such as:

x is 

To investivage the problem, I printed arr(p) too. And found that the maximum I got from maximum=max(cor); is not present in the arr(p)

Let me give you the output:

Here's arr(p) in output:

4.595946e-02
2.556745e-02
6.488689e-02
6.076814e-02
8.913487e-03
6.457960e-02
1.031540e-01
1.213301e-02
5.870144e-02
1.425681e-01
1.397469e-03
1.525214e-01
1
5.967187e-02
7.100746e-02
7.474598e-02
1.066773e-01
1.259531e-01
7.464354e-01
7.420288e-01
7.217714e-01
4.300225e-01
3.690350e-01
3.326089e-01
3.141340e-01
1.852171e-01
1.726543e-01
1.494046e-01
1.428864e-01
1.347272e-01
1.168218e-01
1.118371e-01
9.120090e-02
8.713045e-02
5.995163e-02
6.033924e-02
5.797939e-02
5.797939e-02
5.797939e-02
5.797939e-02
5.797939e-02
5.797939e-02
5.797939e-02

And here's maximum:

maximum is 9.682138e-01

As you can see, maximum does not match any arr(p) values.

But when I print cor(t), the value of maximum can be found in it. Among other values.

What I don't understand is that maximum is the maximum value of arr(p) when arr(p)>0.95 and <1, which is stored in cor(t). But when I do max(cor), why maximum value is not present in arr(p)?

Answer 3

[madhan ravi]

help find % read the third explanation

 FIND   Find indices of nonzero elements.
    I = FIND(X) returns the linear indices corresponding to 
    the nonzero entries of the array X.  X may be a logical expression. 
    Use IND2SUB(SIZE(X),I) to calculate multiple subscripts from 
    the linear indices I.
  
    I = FIND(X,K) returns at most the first K indices corresponding to 
    the nonzero entries of the array X.  K must be a positive integer, 
    but can be of any numeric type.
 
    I = FIND(X,K,'first') is the same as I = FIND(X,K).
 
    I = FIND(X,K,'last') returns at most the last K indices corresponding 
    to the nonzero entries of the array X.
 
    [I,J] = FIND(X,...) returns the row and column indices instead of
    linear indices into X. This syntax is especially useful when working
    with sparse matrices.  If X is an N-dimensional array where N > 2, then
    J is a linear index over the N-1 trailing dimensions of X.
 
    [I,J,V] = FIND(X,...) also returns a vector V containing the values
    that correspond to the row and column indices I and J.
 
    Example:
       A = magic(3)
       find(A > 5)
 
    finds the linear indices of the 4 entries of the matrix A that are
    greater than 5.
 
       [rows,cols,vals] = find(speye(5))
 
    finds the row and column indices and nonzero values of the 5-by-5
    sparse identity matrix.
 
    See also SPARSE, IND2SUB, RELOP, NONZEROS.

    Documentation for find
       doc find

    Other functions named find

       codistributed/find    gpuArray/find    mtree/find    tall/find