How do I fix this error?

%$Unable to perform assignment because the size of the left side is 1-by-1 and the size of the right side is 0-by-1.

%$Error in machine_problem (line 39)

%$ p_new(j,1) = w*a + z*b; "

%code


states = [0; 3; 3; 5; 6; 8; 8; 11];

fors = [0.02;0.02;0.02];

capacities =[3;3;5];

p_old = 0;


for i=1:length(capacities)
    
    fprintf('Adding unit: %d \n',i);
    
    p_new = 0;
    for j = 1:(2^i)
        
        
        
        
        if states(j,1) == 0
            [a]= 1;
            
        elseif j >= (2^i)
            [a] =0;
        end
        
        if (states(j,1) - capacities(i,1)) <= 0
            [b] = 1  ;  
        else
            x = find(states == (states(j,1)-capacities(i,1)));
            [b] = p_old(x,1);
        end
        
        [w] = 1 - fors(i,1)
        
        [z] = fors(i,1) 
        
        p_new(j,1) = w*a + z*b;
        
        
    end
    p_old = p_new;
    
    disp(p_new);
    
    
    
end

Answer 1

[Walter Roberson]

        if states(j,1) == 0
            [a]= 1;

        elseif j >= (2^i)
            [a] =0;
        end

what if neither of those is true? What will a be initialized to?

            x = find(states == (states(j,1)-capacities(i,1)));

How do you know that exactly one such location exists?

[Winston Sabellona]

Thank you Mr. Roberson. Honestly, im stuck with this. Do you have any recommendations pls?

[Walter Roberson]

%code


states = [0; 3; 3; 5; 6; 8; 8; 11];

fors = [0.02;0.02;0.02];

capacities =[3;3;5];

p_old = 0;


for i=1:length(capacities)
    
    fprintf('Adding unit: %d \n',i);
    
    p_new = 0;
    for j = 1:(2^i)
        
        
        
        
        if states(j,1) == 0
            [a]= 1;
            
        elseif j >= (2^i)
            [a] =0;
        else
            error('a undefined');
        end
        
        if (states(j,1) - capacities(i,1)) <= 0
            [b] = 1  ;  
        else
            x = find(states == (states(j,1)-capacities(i,1)));
            [b] = p_old(x,1);
        end
        
        [w] = 1 - fors(i,1)
        
        [z] = fors(i,1) 
        temp = w*a + z*b;
        p_new(j,1:numel(temp)) = temp;
    end
    p_old = p_new;
    
    display(p_new);
    
    
end

Adding unit: 1 

w = 0.9800

z = 0.0200

w = 0.9800

z = 0.0200

p_new = 2×1

1.0000 0.0200

Adding unit: 2 

w = 0.9800

z = 0.0200

a undefined