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This tutorial discusses find()finding the index of a value in an array using the function in MATLAB.

Use the function in MATLAB find()to find the index of a value in an array

In an array, elements are placed at certain indices starting from 1 and so on. To find the index of a value in a given array, we can use find()the _indices_ function. find()_indices_ function is used to find the index and value of an element in an array or matrix. To find the index of an element, we need to find()define a condition in the _indices_ function. For example, if we want to find the index of a single element, we can find()use the equality operator in the _indices_ function. If the same element appears at different indices, find()the function will return all the indices where that element is present. We can use various conditional statements in this function. For example, suppose we want to find the indices of all elements that are greater than a certain number. In this case, we can use the greater than operator, which will return the indices of all elements that are greater than that particular number. For example, let us find the index of a single element present in a given array. See the code below.

mat = [2 3 1 2];
indices = find(mat==2)

Output:

indices =

     1     4

The variable index contains two values ​​in the above output, which means that the element occurs at index 1 and 4 within the array or vector. Now let us consider that we want to find the index of the element which is greater than 1. We need to change find()the condition inside the function. Instead of using the equal to operator, we will use the greater than operator. See the code below.

mat = [2 3 1 2];
indices = find(mat>1)

Output:

indices =

     1     2     4

In the above output, the variable index contains three values, as you can see, three elements in the given array or vector are greater than one. If we have a matrix and want to find the position of an element in a vector, we can use find()the function. We know that the elements inside the matrix are placed on specific rows and columns, to find the specific row and column, we can use find()the function. If the element is present in multiple positions, find()the function will return multiple values ​​for the rows and columns. See the code below.

mat = [2 3 ; 2 1]
[row, col] = find(mat==2)

Output:

mat =

     2     3
     2     1


row =

     1
     2


col =

     1
     1

In the above output, the matrix has two rows and two columns, the first value of the row and column vector is the first position of the element, i.e. row 1 and column 1, and the second value of the row and column vector is the first position of the element, i.e. row 2 and column 1. If you want to find the index of element 2 in the given matrix, find()the function will return 1 and 2 because in case of a matrix, the values ​​are placed column-wise. This means that value 2 is at index 1 and 2, value 3 is at index 3, and value 1 is at index 4. So, instead of two output variables, if only one variable is passed, find()the function will return the index column. For example, let us find the index of the elements and replace them with another value. See the code below.

mat = [2 3 ; 2 1]
indices = find(mat==2)
mat(indices) = 5

Output:

mat =

     2     3
     2     1


indices =

     1
     2


mat =

     5     3
     5     1

In the above code, first, we find the index of element 2 and then we replace the value with 5 using the index, as you can see, the matrix value has changed. If you don't want to use find()the function for some reason, you can always use a for loop and an if statement to create your function. You need to loop through all the elements of the array or matrix and use the if statement to check if the current value matches the value you want. If it matches the value you want, you can save its index and continue until you have checked all the elements present in the array. For example, let's use a for loop and an if statement to find the index of a vector or array. See the code below.

mat = [2 3 2 1]
indices = [];
for i=1:length(mat)
    if(mat(i) == 2)
        indices = [indices i];
    end
end
indices

Output:

mat =

     2     3     2     1


indices =

     1     3

In the above code, we have initialized the variable index with an empty vector. When the value two matches any element inside the array, we will save that index in indicesthe variable.