why I get Index exceeds matrix dimensions

clear all; close all; clc;

% Parameter
k1=10000; % Kekakuan Lantai 1 (N/m)
k2=10000; % Kekakuan Lantai 2 (N/m)
k3=10000;% Kekakuan Lantai 3 (N/m)
k4=10000;% Kekakuan Lantai 4 (N/m)
k5=10000;% Kekakuan Lantai 5 (N/m)
k6=10000;% Kekakuan Lantai 6 (N/m)
k7=10000;% Kekakuan Lantai 7 (N/m)
c1=500; %  Koefisien Redaman Lantai 1(kg/s)
c2=500; %  Koefisien Redaman Lantai 2(kg/s)
c3=500;%  Koefisien Redaman Lantai 3(kg/s)
c4=500;%  Koefisien Redaman Lantai 4(kg/s)
c5=500;%  Koefisien Redaman Lantai 5(kg/s)
c6=500;%  Koefisien Redaman Lantai 6(kg/s)
c7=500;%  Koefisien Redaman Lantai 7(kg/s)
m1=1000; % Massa Lantai 1 (kg)
m2=1000; % Massa Lantai 2 (kg)
m3=1000; % Massa Lantai 3 (kg)
m4=1000; % Massa Lantai 4 (kg)
m5=1000; % Massa Lantai 5 (kg)
m6=1000; % Massa Lantai 6 (kg)
m7=1000; % Massa Lantai 7 (kg)

x10=1; % Nilai awal perpindahan lantai 1 (m)
x20=-1; % Nilai awal perpindahan lantai 1 (m)
x30=2;% Nilai awal perpindahan lantai 1 (m)
x40=-2;% Nilai awal perpindahan lantai 1 (m)
x50=3;% Nilai awal perpindahan lantai 1 (m)
x60=-3;% Nilai awal perpindahan lantai 1 (m)
x70=4;% Nilai awal perpindahan lantai 1 (m)
v10=0; % Nilai kecepatan lantai 1 (m/s)
v20=0; % Nilai kecepatan lantai 1 (m/s)
v30=0;% Nilai kecepatan lantai 1 (m/s)
v40=0;% Nilai kecepatan lantai 1 (m/s)
v50=0;% Nilai kecepatan lantai 1 (m/s)
v60=0;% Nilai kecepatan lantai 1 (m/s)
v70=0;% Nilai kecepatan lantai 1 (m/s)

simwaktu=10; % Waktu simulasi (s)
tStep=0.001; 
iterasi=simwaktu/tStep;
t=0:iterasi;


x1=zeros(iterasi,1);
x1(1,:)=x10;
x2=zeros(iterasi,1);
x2(1,:)=x20;
x3=zeros(iterasi,1);
x3(1,:)=x30;
x4=zeros(iterasi,1);
x4(1,:)=x40;
x5=zeros(iterasi,1);
x5(1,:)=x50;
x6=zeros(iterasi,1);
x6(1,:)=x60;
x7=zeros(iterasi,1);
x7(1,:)=x70;
v1=zeros(iterasi,1);
v1(1,:)=v10;
v2=zeros(iterasi,1);
v2(1,:)=v20;
v3=zeros(iterasi,1);
v3(1,:)=v30;
v4=zeros(iterasi,1);
v4(1,:)=v40;
v5=zeros(iterasi,1);
v5(1,:)=v50;
v6=zeros(iterasi,1);
v6(1,:)=v60;
v7=zeros(iterasi,1);
v7(1,:)=v70;
a1=zeros(iterasi,1);
a1(1,:)=-(c1*(v10)-c2*(v20-v10)+k1*(x10)-k2*(x20-x10))/m1;
a2=zeros(iterasi,1);
a2(1,:)=-(c2*(v20-v10)+k2*(x20-x10)-c3*(v30-v20)-k3(x30-x20))/m2;
a3=zeros(iterasi,1);
a3(1,:)=-(c3*(v30-v20)+k3*(x30-x20)-c4*(v40-v30)-k4*(x40-x30))/m3;
a4=zeros(iterasi,1);
a4(1,:)=-(c4*(v40-v30)+k4*(x40-x30)-c5*(v50-v40)-k5*(x50-x40))/m4;
a5=zeros(iterasi,1);
a5(1,:)=-(c5*(v50-v40)+k5*(x50-x40)-c6*(v60-v50)-k6*(x60-x50))/m5;
a6=zeros(iterasi,1);
a6(1,:)=-(c6*(v60-v50)+k6*(x60-x50)-c7*(v70-v60)-k7*(x70-x60))/m6;
a7=zeros(iterasi,1);
a7(1,:)=-(c7*(v70-v60)+k7*(x70-x60))/m7;

% Solve the ODE's with Euler's Method
for n=7:(iterasi+1)
  x1(n,:)=x1(n-1,:)+v1(n-1,:)*tStep; 
  x2(n,:)=x2(n-1,:)+v2(n-1,:)*tStep; 
  x3(n,:)=x3(n-1,:)+v3(n-1,:)*tStep; 
  x4(n,:)=x4(n-1,:)+v4(n-1,:)*tStep; 
  x5(n,:)=x5(n-1,:)+v5(n-1,:)*tStep; 
  x6(n,:)=x6(n-1,:)+v6(n-1,:)*tStep;
  x7(n,:)=x7(n-1,:)+v7(n-1,:)*tStep; 
  v1(n,:)=v1(n-1,:)+a1(n-1,:)*tStep; 
  v2(n,:)=v2(n-1,:)+a2(n-1,:)*tStep; 
  v3(n,:)=v3(n-1,:)+a3(n-1,:)*tStep; 
  v4(n,:)=v4(n-1,:)+a4(n-1,:)*tStep;
  v5(n,:)=v5(n-1,:)+a5(n-1,:)*tStep;
  v6(n,:)=v6(n-1,:)+a6(n-1,:)*tStep;
  v7(n,:)=v7(n-1,:)+a7(n-1,:)*tStep;
  
  a1(n,:)=-(c1*v1(n,:)-c2*(v2(n,:)-v1(n,:))+k1*x1(n,:)-k2*(x2(n,:)-x1(n,:)))/m1;
  a2(n,:)=-(c2*(v2(n,:)-v1(n,:))-c3*(v3(n,:)-v2(n,:))+k2*(x2(n,:)-x1(n,:))-k3*(x3(n,:)-x2(n,:)))/m2;
  a3(n,:)=-(c3*(v3(n,:)-v2(n,:))-c4*(v4(n,:)-v3(n,:))+k3*(x3(n,:)-x2(n,:))-k4*(x4(n,:)-x3(n,:)))/m3;
  a4(n,:)=-(c4*(v4(n,:)-v3(n,:))-c5*(v5(n,:)-v4(n,:))+k4*(x4(n,:)-x3(n,:))-k5*(x5(n,:)-x4(n,:)))/m4;
  a5(n,:)=-(c5*(v5(n,:)-v4(n,:))-c6*(v6(n,:)-v5(n,:))+k5*(x5(n,:)-x4(n,:))-k6*(x6(n,:)-x5(n,:)))/m5;
  a6(n,:)=-(c6*(v6(n,:)-v5(n,:))-c7*(v7(n,:)-v6(n,:))+k6*(x6(n,:)-x5(n,:))-k7*(x7(n,:)-x6(n,:)))/m6;
  a7(n,:)=-(c7*(v7(n,:)-v6(n,:))+k7*(x7(n,:)-x6(n,:)))/m7;
end

% Plot results
subplot(3,1,1)
hold on;
plot(t',x1,'r')
plot(t',x2,'m')
plot(t',x3,'m')
plot(t',x4,'m')
plot(t',x5,'m')
plot(t',x6,'m')
plot(t',x7,'m')
ylabel('Perpindahan (m)')
title('Pepindahan, Kecepatan, Percepatan Terhadap Waktu')
legend('Lantai 1','Lantai 2','Lantai 3','Lantai 4','Lantai 5','Lantai 6','Lantai 7');
subplot(3,1,2)
hold on;
plot(t',v1,'b')
plot(t',v2,'c')
plot(t',v3,'c')
plot(t',v4,'c')
plot(t',v5,'c')
plot(t',v6,'c')
plot(t',v7,'c')
ylabel('Kecepatan (m/s)');
legend('Lantai 1','Lantai 2','Lantai 3','Lantai 4','Lantai 5','Lantai 6','Lantai 7');
subplot(3,1,3)
hold on;
plot(t',a1,'g')
plot(t',a2,'y')
plot(t',a3,'y')
plot(t',a4,'y')
plot(t',a5,'y')
plot(t',a6,'y')
plot(t',a7,'y')
ylabel('Percepatan (m/s^2)');
xlabel('Waktu (s)');
legend('Lantai 1','Lantai 2','Lantai 3','Lantai 4','Lantai 5','Lantai 6','Lantai 7');