# why is this function running infinitely?

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Tyler Franks
on 29 Nov 2021

Commented: Image Analyst
on 29 Nov 2021

I am trying to create an eye matrix that spreads until all the inside numbers inside the border of zeros are ones, I did this using loops and it works it just never stops even after the sum of the entire matrix equals 400

Master1 = eye(22,22);

Master1(1, :) = zeros(1, 22);

Master1(22, :) = zeros(1, 22);

scan=[3:3]

temp=Master1

i=0;

while sum(sum(Master1))<400

for r=2:21

for c=2:21

scan=((r-1):(r+1):(c-1):(c+1))

if sum(scan)<1

temp(r,c)=1

else sum(scan)>1

temp(r,c)=1

end

end

end

end

Master1=temp

i=i+1

##### 1 Comment

Jan
on 29 Nov 2021

A simplification of:

Master1(1, :) = zeros(1, 22);

Master1(22, :) = zeros(1, 22);

scan=[3:3]

is

Master1(1, :) = 0;

Master1(22, :) = 0;

scan = 3;

### Accepted Answer

Image Analyst
on 29 Nov 2021

Edited: Image Analyst
on 29 Nov 2021

You never change Master1 inside the loop so the sum of it never changes and the loop continues forever.

Not sure what you're thinking with this weird syntax:

scan=((r-1):(r+1):(c-1):(c+1));

Maybe you mean

% Extract 3x3 sub-array:

scan = temp((r-1):(r+1), (c-1):(c+1));

but here is an improvement:

Master1 = eye(22,22);

Master1(1, :) = zeros(1, 22);

Master1(22, :) = zeros(1, 22);

scan=[3:3]

temp=Master1;

loopCounter=0;

maxIterations = 100000; % Failsafe

while sum(temp(:)) < 400 && loopCounter < maxIterations

for r=2:21

for c=2:21

% Define scan with some bizarre 4 element syntax:

scan=((r-1):(r+1):(c-1):(c+1));

if sum(scan) < 1

temp(r,c) = 1;

else

temp(r,c) = 1;

end

end

end

loopCounter=loopCounter+1;

fprintf('After %d iterations, then sum of temp = %d.\n', loopCounter, sum(temp(:)));

end

Master1=temp;

fprintf('Done after %d iterations with sum of temp = %d.\n', loopCounter, sum(temp(:)))

##### 2 Comments

Image Analyst
on 29 Nov 2021

### More Answers (1)

Jan
on 29 Nov 2021

Edited: Jan
on 29 Nov 2021

% Simplified version:

Master1 = eye(22,22);

Master1(1, 1) = 0;

Master1(22, 22) = 0;

scan = 3;

temp = Master1;

i = 0;

while sum(Master1(:)) < 400

for r = 2:21

for c = 2:21

scan = ((r-1):(r+1):(c-1):(c+1)); % ???

if sum(scan) ~= 1

temp(r,c)=1

end

end

end

end

Master1 = temp;

i = i+1;

This will most likely not do, what you expect:

(r-1):(r+1):(c-1):(c+1)

The colon operator is defined with two : only: a:b:c is the vector from a to c in steps of b. This replies a vector. In the next step this vector is used as 1st input for the next colon operator:

((r-1):(r+1):(c-1)) : (c+1)

% ^ evaluated at first ^ then this follows

If a vector is used as input of the colon operator, only the first element is considered and the rest is ignored. Therefore your code is equivalent to:

% (r-1):(r+1):(c-1):(c+1)

(r-1):(c+1)

What is the purpose of the variable scan? Do you want to collect indices or the values of the matrix?

The problem causing the infinite loop, is that the body of the loop does not change the condition. If sum(Master1(:)) < 400 is true initially, this will never change, because Master1 is not modified inside the loop.

Why do you use the copy temp of Master1 and not the data directly? Maybe you want:

M = eye(22,22);

M(1, 1) = 0;

M(22, 22) = 0;

i = 0;

while sum(M(:)) < 400

for r = 2:21

for c = 2:21

scan = M((r-1):(r+1), (c-1):(c+1));

if sum(scan(:)) ~= 1

M(r,c) = 1;

end

end

end

M

i = i+1;

end

i

##### 0 Comments

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