# while loop not ending, juz stopped at i = 3

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Yu Zhi on 15 May 2020
Commented: Yu Zhi on 15 May 2020
Hi, I am new to Matlab.
I wrote a below script to get the vector fwd = [3 8]. If I put breakpoints I could see the fwd I want. but the code never ended.
clc; clear; close all
a = [1 2 3 4 2 3 4 5 6 3 4 5 6 8 9 10 9 8 7 6 7 8 9 8 7 5 4 3 2 4 5 3 2 1];
start = min(a)+2;
ed = max(a) -2;
m = length(a);
f = 1;
i = 2;
while i < m
if a(i) >= start && a(i-1) <= start && a(i+1) > a(i)
fwd(f,1) = a(i);
for j = i+1:m-1
if a(j+1) > ed && a(j+2) >= a(j+1) && a(j) < a(j+1)
fwd(f,2) = a(j);
end
end
else
i = i+1;
end
end

Bjarke Skogstad Larsen on 15 May 2020
Edited: Bjarke Skogstad Larsen on 15 May 2020
In your code, once the following is true, it is always true, since you don't modify any of the variables inside the 'if'
if a(i) >= start && a(i-1) <= start && a(i+1) > a(i)
Thus, your variables i and m stay the same after this point, and you get stuck in an infinite loop.
Maybe you intended i to increase after each iteration regardless of the 'if'?
In this case, the following should work, though it doesn't result in the result you say is correct: [3 8]
while i < m
if a(i) >= start && a(i-1) <= start && a(i+1) > a(i)
fwd(f,1) = a(i);
for j = i+1:m-1
if a(j+1) > ed && a(j+2) >= a(j+1) && a(j) < a(j+1)
fwd(f,2) = a(j);
end
end
end
i = i+1;
end
It's difficult to get any closer to solving this without any explanation of what your code is supposed to do.
Yu Zhi on 15 May 2020
Thank you. I revised my code based on your advice. now it's solved.
clc; clear; close all
a = [1 2 3 4 2 3 4 5 6 3 4 5 6 8 9 10 9 8 7 6 7 8 9 8 7 5 4 3 2 4 5 3 2 1];
start = min(a)+2;
ed = max(a) -2;
m = length(a);
f = 1; r = 1;
i = 2;
while i < m
if a(i) >= start && a(i-1) <= start && a(i+1) > a(i)
fwd(f,1) = a(i);
for j = i+1:m-1
if a(j+1) > ed && a(j+2) >= a(j+1) && a(j) < a(j+1)
fwd(f,2) = a(j);
end
end
i=j+1;
else
i = i+1;
end
end