How can I use while loops to determine if an element is larger than the element below it and store the results in a vector with 1 representing true and 0 representing false?

Question

Yen Tien Yap 2021-3-26

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编辑： John D'Errico 2021-3-26

I coded for my while loop but I can't get the final answer and the error showed that 'Array indices must be positive integers or logical values.' Can someone help me with this? Thank you so much!!

A=randi([0 9],1,randi([100 300]))'
n=numel(A)-1
B=zeros(n,1)
while i<=n
    if A(i)>A(i+1)
        B(i,1)=true
    else
        B(i,1)=false
    end
    i=i+1   
end

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Stephen23 2021-3-26

You are using i as an index. What is the value of i ? (hint: the square root of -1)

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Answer 1

Matt J 2021-3-26

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A=randi([0 9],1,10)';
n=numel(A)-1;
B=zeros(n,1);
i=1;
while i<=n
    if A(i)>A(i+1)
        B(i,1)=true;
    else
        B(i,1)=false;
    end
    i=i+1;
end
A.',B.'
ans = 1×10
     0     2     1     8     0     9     4     2     6     2
ans = 1×9
     0     1     0     1     0     1     1     0     1

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Answer 2

John D'Errico 2021-3-26

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编辑：John D'Errico 2021-3-26

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"While" this does not answer your question, I would not use a while loop at all. Learn to use MATLAB as it is designed.

A = randn(1,8) % some random numbers
A = 1×8
   -0.2087    0.3007   -0.3599   -0.0566    0.4045   -3.0370   -0.3818    0.8694
B = [false,diff(A)>0] % A vector of the same length as A, true when A(i+1) > A(i)
B = 1×8 logical array
   0   1   0   1   1   0   1   1

B is the same length as A. Note that A(1) will never be greater than the preceding element, since there is no preceding element.

Could you use a while loop? Yes. A for loop would make more sense as a loop here, since you want to test EVERY pair of elements.

B = false(size(A)); % preallocate B
for ind = 2:numel(A)
  B(ind) = A(ind) > A(ind-1);
end
B
B = 1×8 logical array
   0   1   0   1   1   0   1   1

Same result as the single statement, but a loop.

CAN you use a while loop? Well, yes. Learn when to use one type of loop over another when you REALLY need to use a loop. A while loop is good when you don't know how often the loop will run. A for loop is right when you have a known, fixed number of iterations.