Error in Linear Convolution code MATLAB

I am trying to run this code :

Question : Linear convolution and circular convolution of two sequences.

%%Linear convolution of two sequences.

clc; %clears the console window
clear all; %deletes the user defined variable in variable browser 
close all; %close the figure window

x=input('Enter the 1st sequence : '); 
nx=input('Enter the time index sequence : '); 
h=input('Enter the 2nd sequence : '); 
nh=input('Enter the time index sequence : '); 
[y,ny]=findconv(x,nx,h,nh);
figure; subplot(3,1,1);
stem(nx,x);

xlabel('Time'); 
ylabel('Amplitude'); 
title('1st sequence'); 
subplot(3,1,2);
stem(nh,h);
xlabel('Time'); 
ylabel('Amplitude'); 
title('2nd sequence'); 
subplot(3,1,3);
stem(ny,y);
xlabel('Time'); 
ylabel('Amplitude'); 
title('Linear convolution'); 
disp(y);
disp(ny);


function [y,ny]=findconv(x,nx,h,nh) 
nybegin=nx(1)+nh(1); 
nyend=nx(length(nx))+nh(length(nh)); 
ny=nybegin:nyend;
%y=conv(x,h); %calling inbuilt function 
y=calcconv(x,h)
end

function [y] = calcconv(x,h) 
l1=length(x); 
l2=length(h);
N = l1+l2-1; 
%length of linear convolution
%y=linear convolution of x[n] and h[n]
%note: in matlab index starts with 1 and not 0 
for n=1:1:N
    y(n)=0;
    for k=1:1:l1
        if(n-k+1>=1 & n-k+1<=l2) % to avoid negative index
            y(n)=y(n)+x(k)*h(n-k+1);
        end
    end 
end 
end

Everythings seemed runnning flawlessly until I got this error -

P.S : I am running this file on MATLAB live script

[Dyuman Joshi]

From the error, the lengths of y and ny must not be not equal.

What is a sample input to x, nx, h and nh? And please copy paste the full error message.

Also, what is the logic behind calculating the values y and ny?

Additionally, instead of

y(n)=0; 

do this

N = l1+l2-1;
y=zeros(1,N);
for 
    ...
end

Answer 1

[Sulaymon Eshkabilov]

Hi,

Here is the corrected code. Note also two variable names are changed to avoid confusion l1 to L1 and l2 to L2.

%%Linear convolution of two sequences.

clc;            %clears the console window
clearvars;      %deletes the user defined variable in variable browser 
close all;      %close the figure window

  x=input('Enter the 1st sequence : '); 
  nx=input('Enter the time index sequence : '); 
  h=input('Enter the 2nd sequence : '); 
  nh=input('Enter the time index sequence : '); 

%% This Example simulated here
%  x = randi([-2, 4], 1, 7);
%  nx = 1:length(x);
%  h = randi([-2, 0], 1,7);
%  nh=1:length(h);
%% 
[y,ny]=findconv(x,nx,h,nh);

y = 1×13

0 -2 -3 0 -1 2 -5 -1 2 -1 4 -6 4

figure; subplot(3,1,1);
stem(nx,x);

xlabel('Time'); 
ylabel('Amplitude'); 
title('1st sequence'); 
subplot(3,1,2);
stem(nh,h);
xlabel('Time'); 
ylabel('Amplitude'); 
title('2nd sequence'); 
subplot(3,1,3);
stem(ny,y);
xlabel('Time'); 
ylabel('Amplitude'); 
title('Linear convolution'); 

disp(y);

     0    -2    -3     0    -1     2    -5    -1     2    -1     4    -6     4

% compare the computed y  with conv(x, h)
disp(conv(x,h))

     0    -2    -3     0    -1     2    -5    -1     2    -1     4    -6     4

disp(ny);

     2     3     4     5     6     7     8     9    10    11    12    13    14

function [y,ny]=findconv(x,nx,h,nh) 
nybegin=nx(1)+nh(1); 
nyend=nx(length(nx))+nh(length(nh)); 
ny=nybegin:nyend;
%y=conv(x,h); %calling inbuilt function 
y=calcconv(x,h)
end

function y = calcconv(x,h) 
L1=length(x);       
L2=length(h);
N = (L1+L2)-1; 
%length of linear convolution
%y=linear convolution of x[n] and h[n]
%note: in matlab index starts with 1 and not 0 
%y = zeros(1, N);
for n=1:N
    y(n)=0;
    for k=1:L1
     if(n-k+1>=1 && n-k+1<=L2) % to avoid negative index
            y(n)=y(n)+x(k)*h(n-k+1);
       end
    end
end
end

[Achyuth S.S]

Thank you so much, Will look into it

[Sulaymon Eshkabilov]

Most welcome!