how to make a loop(for...end)

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Tian Lin
Tian Lin 2011-3-22
评论： Walter Roberson 2021-4-5
I'm a new guy for matlab loop. Righi now I want to now some rools about loop(for...end). Such as how to make a loop to get all odd numbers from a matrix x=[1:100] or numbers like 1,5,9,13,17,21...? p.s.I know this x1=x(1:2:100) and x4=(1:4:100),but I want to know how to get it from a loop(for...end).

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采纳的回答

Matt Fig
Matt Fig 2011-3-22
It would be good if you learned to pre-allocate your vectors so your code runs efficiently...
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EDIT In response to question about generalization.
The general case can be written:
N = 100; % The largest number. Change to whatever...
a = 1; % The starting point. Change to 3,5... whatever
n = zeros(1,ceil((N-a)/2)); % Pre-allocate the array...
for ii = 1:length(n)
n(ii) = 2*(ii)+(a-2);
end
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Matt Fig
Matt Fig 2011-3-22
Boy, you keep changing the problem! That is o.k., you will just have to realize that changing the problem changes the approach:
N = length(x); % The largest number. Change to whatever...
a = 1; % The starting point. Change to 3,5... whatever
S = 3;
n = zeros(1,floor((N-a)/(S))+1); % Pre-allocate the array...
for ii = 1:length(n)
n(ii) = x(S*(ii)+(a-S));
end
Tian Lin
Tian Lin 2011-3-22
man,that's cool,I only need to change S and a to get the best result.Thank you very much.I believe some day I can write the code by myself,like you.

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更多回答（3 个）

Paulo Silva
Paulo Silva 2011-3-22
n=[];
for a=1:2:100
n=[n a];
end
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Walter Roberson
Walter Roberson 2011-3-22
for K = 1:100
a = K:2:100;
%here, do something with the vector "a"
end
Tian Lin
Tian Lin 2011-3-22
if I have numbers larger than 100,just like 10000,whether I will write 10000 "for"?

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Walter Roberson
Walter Roberson 2011-3-22
Please don't do that: please read this FAQ instead.
Paulo Silva
Paulo Silva 2011-3-22
clc
n={};
c=0;
for b=2:100
n1=[];
for a=b:2:100
n1=[n1 a];
end
c=c+1;
n{c,1}=n1;
end
The result is inside the n variable, n is a cell, each element of it contains the results n{1,1} gives you the odd numbers for 2:2:100, n{2,1} gives the odd numbers for 3:2:100 and so on...
No pre-allocation, Matt is right, it's faster with pre-allocation of memory but a little more effort from your part in understand it.
  1 个评论
Tian Lin
Tian Lin 2011-3-22
thank you Paulo.It's hard to me to understand something like reduce using memory.Also,I don't know the cell very much,so I find Matt's code much esaier.whatever,thanks a lot.

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