# How to Apply Convolution in MATLAB Without Using the Function

MATLAB contains a pre-built convolution function.
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Convolution is a mathematical operation that blends two functions relative to the overlap of one function as it is shifted over another. Although MATLAB contains a pre-built convolution function, it is possible to calculate the discrete convolution integral yourself. The discrete convolution of two functions f and g is defined as the sum over the range 0 to j of f(j) * g(k-j).

## Step 1

Define two vectors, f and g, containing the two functions you want to convolve. The lengths of f and g do not have to be equal. The length of result of the convolution, k, will be one less than the sum of the length of f and g:

m = length(f); n = length(g); k = m + n - 1;

## Step 2

Define the range j over which the convolution will occur. The value of j is the range where subscripts of the two functions to be convolved, f(j) and g(k+1-n), are legal. The value of 1 added to k is to account for the fact that MATLAB begins indexing vectors at 1 rather than 0:

j = max(1,k+1-n):min(k,m)

## Step 3

Preallocate space for the result of the convolution:

my_result = zeros(k);

## Step 4

Write a for loop to iterate through values of k:

for result_index = 1:k

## Step 5

Calculate the convolution for all values of j:

my_result(k) = sum(f(j) .* g(k-j+1));

## Step 6

Close the for loop with the "end" command.

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