# How to find first instance of a value in array?

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Tawsif Mostafiz le 4 Juil 2021
Commenté : Tawsif Mostafiz le 5 Juil 2021
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?
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### Réponse acceptée

Steven Lord le 4 Juil 2021
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.
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Tawsif Mostafiz le 5 Juil 2021
Thanks @Steven Lord!

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### Plus de réponses (2)

Jiro Doke le 4 Juil 2021
Is this what you're looking for?
maximum = 5;
arr = [2 4 3 5 7 3 4];
find(arr == maximum, 1)
ans = 4
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Tawsif Mostafiz le 4 Juil 2021
Modifié(e) : Tawsif Mostafiz le 4 Juil 2021
@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)?

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madhan ravi le 4 Juil 2021
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
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