calculate mean using while and iteration?

Question

Madan Kumar 2019-4-30

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https://ww2.mathworks.cn/matlabcentral/answers/459334-calculate-mean-using-while-and-iteration

评论： Madan Kumar 2019-5-3

Hi,

I have data of ~3500x2. I want to calulate mean of second column for a particular condition in first column using 'while'.

let data (a, b) be like

0.5 1.8

0.6 1.5

0.9 1.8

1.0 1.5

1.1 1.4

1.2 1.4

1.5 1.6

1.8 1.2

2.1 1.2

2.3 1.3

2.4 1.5

2.6 1.8

2.9 2.0

3.0 3.0

3.12 3.2

3.15 1.9

3.16 1.7

3.18 2.2

I need to calculate mean of b, if a> 0.5 and a<1.5. Then increase 'a' by 1 and calculate mean of b (i.e for a > 1.5 and a<2.5) and so on. It may be a silly question but I am stuck with it. My code is

del=0.5;
k=1;
a(k)=1;
while(a(k) >(a(k)-del) && a(k)< (a(k)+del))
xn(k)=mean(b(k));
 k= k+1;
a(k)=a(k)+1;
end
but it shows error Index exceeds array bounds.
Error in untitled (line 12)
a(k)=a(k)+1;
Thank you for your help.

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David Wilson 2019-4-30

编辑：David Wilson 2019-4-30

在 MATLAB Online 中打开

My code below is a bit ugly, but I think it does what you want:

cutoff = [0.5 1.5]; % band of interest
maxA = ceil(max(a))+0.5; 
bmean = []; 
for i=1:maxA
   idx = find(a>cutoff(1) & a<cutoff(2)); 
   bmean(i) = mean(b(idx));
   cutoff = cutoff+1; 
end 

The means of column "b" are in variable bmean.

I note that you specified strict < as opposed to <= which may, or may not be what you really want.

Note that column a need not be sorted in increasing order.

Madan Kumar 2019-4-30

Thank you...

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Answer 1

Rik 2019-4-30

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https://ww2.mathworks.cn/matlabcentral/answers/459334-calculate-mean-using-while-and-iteration#answer_372943

在 MATLAB Online 中打开

So you want to calculate these values?

xn(1)=mean(b(a>0.5 & a<1.5));
xn(2)=mean(b(a>1.5 & a<2.5));
xn(3)=mean(b(a>2.5 & a<3.5));
etc?

You don't need a while loop for that:

a=[0.5 0.6 0.9 1.0 1.1 1.2 1.5 1.8 2.1 2.3 2.4 2.6 2.9 3.0 3.12 3.15 3.16 3.18];
b=[1.8 1.5 1.8 1.5 1.4 1.4 1.6 1.2 1.2 1.3 1.5 1.8 2.0 3.0 3.2 1.9 1.7 2.2];
del=0.5;
xn=zeros(1,ceil(max(a-del)));
for k=1:size(xn,2)
    xn(k)=mean(b(a>(k-del) & a<(k+del)));
end

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Stephen23 2019-4-30

编辑：Stephen23 2019-4-30

"Suppose, I need xn(0.5)=... "

You can't. Indices must be whole integers greater than zero.

Either change del to 0.25 and leave it at that, or write a function which lets you have any input values that you desire. But you certainly cannot have indexing with non-integer values.

Madan Kumar 2019-5-3