how to write data in new row in each iteration of for loop

I am reading images in loop and write data in new row in each iterartion in loop but not able to acheive that.

previous rows are formating to zeros in preceding iteration.

The code is

clc
clear all
close all

%imtool('ref1.jpg');% To know width of fuel
N = 5 %input('Enter the total number of frames N=');
w1 = 78 %input('width of fuel W1=');% entering the width of fuel by plot
w2 = 96 %input('width of fuel W2=');% entering the width of fuel by plot
T = 10 %input('Enter the value of threshold value T=');% entering the threshold value

imgdir = 'I:\Manu Gowda\images';

for i=1:N % reading frames
    basename = sprintf('53 %04d.jpg', i);
    im = imread(fullfile(imgdir, basename));% reading images from folder
    % im=imread(['C:\Users\Nikhil\Desktop\Manu Gowda\images\chromcr' num2str(i) '.jpg']);% reading images from folder
    [x,g]=size(im) % x is no of rows y is no coloums
    y=g/3;% The size of rgb image is 3 times of normal image
    
    for col=w1:w2
        for i=x:-1:1
            if im (i-1, col)-im(i,col)>=T
                pos(N, col-w1+1)=i+1;  % writing posion in excel from firt coloumn  
                
                %s=mean(pos);
                break;
            end
        end
    end
end
xlswrite('Positions.xlsx',pos)

The out put is

0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0   
 
  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0   
 
  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0   
 
  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0   
 
  166  167  167  168  168  168  168  168  168  168  168  167  167  167  167  163  168  155  154   

Answer 1

[Manu Gowda]

Answer 2

[Manu Gowda]