About checking each elements in a vector using if loop (some what...)

Hello I'm new to MATLAB and I'd like to check an inputted array, which contain non-numeric elements and perform plotting, the code is as follow:

     filename = 'HetCtxDIV1720164171motion.xls';
     x = xlsread(filename,'D:D');
     t = xlsread(filename,'C:C');
     y = xlsread(filename,'E:E');
     d = xlsread(filename,'B:B');
     if  isnumeric(d(i))
         if  isnumeric(t(i+2))
             t1(i:i+600) = t(i+3:i+603);
             x1(i:i+600) = x(i+3:i+603);
             y1(i:i+600) = y(i+3:i+603);
             tc = (t1)/(max(t1))
             c = tc;
             sz = 30;
             scatter(x1,y1,sz,c,'filled');
             colorbar('Ticks',[(0.1),(0.2),(0.3),(0.4),(0.5),(0.6),(0.7),(0.8),(0.9)],'TickLabels',
                      {'0.1','0.2','t=0.3','0.4','0.5','0.6','0.7','0.8','0.9*fulltime'})
         end
     end

however, output t1 is an empty vector...why?

[Stephen23]

@LEE Min Hsun: What is i, and where is it defined?

[LEE Min Hsun]

i is refer to elements in vectors(d,t,x,y), and it's not defined.

[Stephen23]

If i is not defined what value do you expect it to have? (hint: think about complex numbers).

[LEE Min Hsun]

seems like d(i) shall not exist cause there shall not have "i"th element in d, but isnumeric(d(i)) turn out to be true...?