problem with subscript indices and varibles in functions

If I use the following code:

function image_processing_project

M = imread('C:\Users\bostock_h\Documents\Images\130510_162.jpg');
M = int16(M);

x = 975;  %dimensions of the image
y = 1010;
sum=0;
global steps %the number of steps taken for the average
steps = 12;
global lowlimit %minimum value to pass
lowlimit = 40;
global highlimit %maximum value to pass
highlimit = 200;
global movel %amount of pixels moved to the left after check
movel = 10;
prog = 0; 
pixelnum = (x - movel - 1)*y; %Total number of pixels in the image
%Main loop begin
for j = 1:y
    for i1 = movel + 1:x
    c = checkright (i1,j); 

      if c ~= 0
          b = 1;
      else
          b = -1;
      end

      if c == 0
          c = checkleft (i1,j);
      end
      if c == -1 
          if b == 1
          M(j,i1-movel) = 1;
          end 
      end
      if c == 1
          if b == 1
          M(j,i1-movel) = 250;
          end 
      end 
      if c == 1
          if b == -1
          M(j,i1+movel) = 250;
          end 
      end
      if c == -1
          if b == -1
          M(j,i1+movel) = 1;
          end    
      end
      prog = prog + 1;
      percent = (prog / pixelnum) * 100
      end
  end

imagesc(M)
colormap(gray)

function c = checkright(i1,j)
x1 = 1;

c = checkhorizontal(i1,j,x1);
end

function c = checkleft(i1,j)
x1 = -1;

c = checkhorizontal(i1,j,x1);
end

function c = checkhorizontal(i1,j,x1)
for n = 0:steps
        sum = M(j, i1 + n*x1) + sum;
end 
avg = sum / steps;
if avg < lowlimit
    c = -1*x1;
else c = 0;
end

if avg > highlimit
    c = 1*x1;
    else c = 0;
end
end
end

Then I am given the following error message:

Subscript indices must either be real positive integers or logicals.

Error in image_processing_project/checkhorizontal (line 75)
        sum = M(j, i1 + n*x1) + sum;

Error in image_processing_project/checkleft (line 70)
c = checkhorizontal(i1,j,x1);

Error in image_processing_project (line 31)
        c = checkleft (i1,j);

Answer 1

[Walter Roberson]

When i1 is -1 and n is 0, i1 + n*x1 becomes -1, which is not a valid subscript.

Answer 2

[David Sanchez]

It is just that what the error message says, you are trying to access the image with a non integer index. Check your indexes out and you sort out your problems.