for loops question, cell array, cardinal direction

Question:

Finding the shortest distance between two “points” is a class of algorithm called pathfinding. The distancemay be a physical distance between two locations (e.g. driving directions), or something more abstractlike the fastest sequence from start to finish of a construction project. Here you will consider a simplified pathfinding problem where the distance is indices in a matrix. Starting with a matrix of -1s, 0s and 1s, find the distance of every 0-valued entry to the nearest 1-valuedentry, without passing through -1s. For simplicity, being directly next to a 1 is considered a distance of 2. Create a function with the following header:

function [ filled ] = travelDistance( blank )

where: blank is a two-dimensional array comprised of -1s, 0s and 1s, and filled is blank modified to the specifications below. To create filled, replace every 0 in blank with its distance to the nearest 1 (staring at 2), traveling along cardinal directions (up, down, left and right) without passing through a -1 value. That is, all 0s that ared irectly next to a 1 should be changed to a 2 and all 0s next to any new 2s should be changed to 3s, and soon. Spaces with -1 values should be treated as “walls” and should stay -1. If there is no route from a 0 to a 1 without passing through a -1, it should remain a 0. The ultimate result should be that every 0 that is connected to a 1 is replaced with its distance to the nearest 1. For simplicity you may assume that the edges of blank always contain a -1. Your function should be able to reproduce the following test case:

>> m_smallm_small =-1     -1     -1     -1     -1     -1     -1-1      0      0     -1      0      0     -1-1      0      1     -1      0     -1     -1-1      0     -1     -1     -1     -1     -1-1      0     -1      0      1      0     -1-1     -1     -1     -1     -1     -1     -1
>> travelDistance(m_small)
ans =-1     -1     -1     -1     -1     -1     -1-1      3      2     -1      0      0     -1-1      2      1     -1      0     -1     -1-1      3     -1     -1     -1     -1     -1-1      4     -1      2      1      2     -1-1     -1     -1     -1     -1     -1     -1

My answer:

function [filled] = travelDistance( blank )
% blank is a two-dimensional array comprised of -1s, 0s and 1s, 
% and filled is blank modified to the specifications below.
% replace every 0 in blank with its distance to the nearest 1(starting at 2),
% all 0s that are directly next to a 1 should be changed to a 2
% all 0s next to any new 2s should be changed to 3s.
% -1 stay -1

filled = blank;
[a,b] = size(blank);

for i=1:1000
    for X = 1:a
        for Y = 1:b
            for filled(X,Y)==0;
            for filled(X+1,Y)==i||filled(X-1,Y)==i||filled(X,Y+1)==i||filled(X,Y-1)==i
            filled(X,Y)=i+1;
            end
        else
            filled = blank;
        end
    end
end

just wondering which step that I missed

Answer 1

[Walter Roberson]

"for" must be followed by a (unindexed and unqualified) variable name and then a "=" and then an expression .

for filled(X,Y)==0

is not valid syntax. Perhaps you wanted "if" instead of "for"

if filled(X,Y)==0