Attempted to access stepcount(0); index must be a positive integer or logical.

Question

Siyao Sui 2016-5-4

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编辑： Weird Rando 2016-5-5

I get the error message:"Attempted to access stepcount(0); index must be a positive integer or logical." But I've already set z11=floor(real(z1)) to make sure it's a positive integer, can someone help?

Here is my code:

a=input('range from = ');
b=input('range to = ');
N=input('pairs of complex number = ');
gcdcount=zeros(1,1);
stepcount=zeros(1,1);
for j=1:M
z1=a + (b-a).*rand(N,1)+ 1 + a + (b-a).*rand(N,1)*1i;
z2=a + (b-a).*rand(N,1)+ 1 + a + (b-a).*rand(N,1)*1i;
z11=floor(real(z1));
end
if(abs(z1)<abs(z2))  % switch z1 and z2, if necessary, so that b<a
    c=z1; % hang onto the value of a
    z1=z2; % even while replacing a with b
    z2=c; % now replace b with a
end
count=0; % initialize counter
while(abs(z2)>0)
    u=z1;
    v=z2;
    z1=z2;
      q=(u/v);
      q1=real(q);
      q2=imag(q);
      if (q1-floor(q1)<=0.5) 
          q1=floor(q1);
      else
          q1=1+floor(q1);
      end
      if (q2-floor(q2)<=0.5) 
          q2=floor(q2);
          else
              q2=1+floor(q2);
      end
      if(length(gcdcount) >=z11)
          gcdcount(z11)=gcdcount(z11)+1; %increment appropriate counter
      else
          gcdcount(z11)=1;
      end
      if(length(stepcount) >=count)
          stepcount(count)=stepcount(count)+1; %increment appropriate counter
      else
          stepcount(count)=1;
      end
      if(length(gcdcount) >=z11)
          gcdcount(z11)=gcdcount(z11)+1; %increment appropriate counter
      else
          gcdcount(z11)=1;
      end
      if(length(stepcount) >=count)
          stepcount(count)=stepcount(count)+1; %increment appropriate counter
      else
          stepcount(count)=1;
      end
end
subplot(2,1,1)
plot(gcdcount/M)
title('Distribution of gcds')
subplot(2,1,2)
plot(stepcount/M)
title('Distribution of algorithm steps')
subplot(111) % means the figure window returns to normal single-graph behavior

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CS Researcher 2016-5-4

floor is not the best option. Use ceil instead.

Siyao Sui 2016-5-4

I tried, it still gave me the same error message

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Answer 1

Azzi Abdelmalek 2016-5-4

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在 MATLAB Online 中打开

stepcount(count) gives an error because count is initialized to 0

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Azzi Abdelmalek 2016-5-4

编辑：Azzi Abdelmalek 2016-5-4

在 MATLAB Online 中打开

I can't tell you what to do, because I don't know the aim of your code. Also your counter count is not incremented in your code. What the following loop is doing?

for j=1:M
z1=a + (b-a).*rand(N,1)+ 1 + a + (b-a).*rand(N,1)*1i;
z2=a + (b-a).*rand(N,1)+ 1 + a + (b-a).*rand(N,1)*1i;
z11=floor(real(z1));
end

each loop, your variables are erased!

I think, you need to revise all your code.

Siyao Sui 2016-5-5

I want to randomly generate N pairs of complex numbers, find their gcds and plot distribution of gcds and distribution of algorithm steps taken.

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Answer 2

Steven Lord 2016-5-4

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MATLAB uses 1-based indexing so the first element in a matrix is element 1. This is different from languages that use 0-based indexing, where the first element is element 0. You will need to adjust your code so you don't try to access or write to element 0 of a matrix. The easiest way to do so (if you wrote your code assuming 0-based indexing) is to add 1 to your indices.