Index exceeds matrix dimensions error for summation

A project Im working on is a random step program with varying steps. We need to know the final step at the end, my code keeps giving me an error for index exceeds the matrix dimensions. My first two sections run perfect and give me what I need, but as I increase the steps it gives me the error. Please help me debug!

%% Part 1 - Random Walk
% Using a similar program to odds vs evens, set initial conditions
 
walkerL = 1 ;
walkerR = -1 ; % Giving the walker  place to walk
x1 = 0.5 ; % probability walker will step right
x2 = -0.5 ; % probability walker will step left
for i = 1:1000
    sp(1) = 0 ; % starting point
    step = rand(1) ; % Generate a random number
    if step<x1 % if step is less than probability, then
        X = walkerR ; % walker takes a step right
    else
        X = walkerL ; % walker takes a step left
    end
    sp(i+1) = sp(i) + X ; % dictates the move
    
end    
 
comet((0:1000),sp) ; 
title('Random Walk')
xlabel('Number of Steps')
ylabel('Left Step(+) vs Right Step(-)')
 
%% Part 2 - Random Walk Ensemble
 
% In the foloowing sections the random walk will be run 5 different times with
% varying number of steps
 
%% 1000 steps
 
walkerL = 1 ;
walkerR = -1 ; 
x1 = 0.5 ; 
x2 = -0.5 ; 
for i = 1:1000
    sp(1) = 0 ; 
    step = rand(1) ; 
    if step<x1 
        X = walkerR ; 
    else
        X = walkerL ; 
    end
    sp(i+1) = sp(i) + X ; 
    
end
 
stepfinal1000 = sp(i+1); % determines final step
 
%% 2000 steps
 
walkerL2 = 1 ;
walkerR2 = -1 ; 
x3 = 0.5 ; 
x4 = -0.5 ; 
for i2 = 1:2000
    sp2(1) = 0 ; 
    step2 = rand(1) ; 
    if step2<x3 
        X2 = walkerR2 ; 
    else
        X2 = walkerL2 ; 
    end
    sp(i2+1) = sp(i2) + X2 ; 
    
end
 
stepfinal2000 = sp2(i2+1);  %% here is where my issue is

[Michael Ballenger]

I'm supposed to continue through 10000, 20000, 100000, and 1000000 steps so a fix to help me with all of these would be great. And I have to create a histogram with my final steps from each program.

Answer 1

[madhan ravi]

You get an error becuase sp2 is a scalar which is 0 and you didn't increment it inside the loop

[Michael Ballenger]

I found my error, I had incremented sp2 I just hadn't fully labeled the summation inside the loop correctly. I had sp(i2+1) = sp(i2) +X2 when it should have been sp2(i2+1)=sp2(i2)+X2

[Michael Ballenger]

Thanks for the advice though

[madhan ravi]

Anytime :)