Storing Outputs from a Nested Loop

A M
2017 1 30
2 个回答
回答已采纳
更新时间：2017 2 1
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0 个投票

I have the following problem, wherein, i want to store outputs from the nested loops. I am just presenting a simple scenario to depict the situation:
for a=[1:1:10]
for b=[1:1:10]
c=a*b
end
end
c in this case would store the last value i.e. 100. How to develop an array which stores all the values of c from 1 to 100 as c(1,1)=1, c(1,2)=2 and so on.

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Iddo Weiner
Iddo Weiner 2017-1-31

1 个投票

By initiating an empty array and then filling it with the looping indices:
c = nan(10,10);
for a = 1:10
for b= 1:10
c(a,b)=a*b;
end
end
% visualize c
imagesc(c)

8 个评论
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A M
A M 2017-1-31
Thank you for this support. I am still stuck. Can you look into this code:
c=nan(5,5);
for a=[20:20:100];
for b =[5:-1:1];
c(a,b)=(a+b);
end
end
Please correct me. To me belief, this should have returned a 5x5 matrix with all values stored in it such that the result of c(20,5)=25 is appeared. Instead it shows that 'c' is a 5x100 with 0 stored in 1-19 and then values appearing at 20. How to solve this issue so i can have a clean matrix of 5x5.
Stephen23
Stephen23 2017-1-31
编辑：Stephen23 2017-1-31
A = 20:20:100;
B = 5:-1:1;
C = NaN(numel(A),numel(B));
for a = 1:numel(A)
for b = 1:numel(B)
C(a,b) = A(a)+ B(b);
end
end
You were using the values as indices. Instead you need to create separate indices and use those to index into the vectors and output matrix.
Jan
Jan 2017-1-31
@A M: Do you see, that the suggested codes contain less square brackets? See http://www.mathworks.com/matlabcentral/answers/35676-why-not-use-square-brackets .
A M
A M 2017-1-31
Thank you. This appears to be a promising solution. It worked out and i got a 5*5 matrix. So, if i am not mistaken, the NaN creates a n*n matrix. What does the 'Numel' exactly does? Is it not the same as NaN(5,5) creating a 5*5 matrix?
Niels
Niels 2017-1-31
Matlab uses (as you should also do) smart shortcuts for its functions and variables, such as numel for Number of Elements
Iddo Weiner
Iddo Weiner 2017-2-1
I think you are supposed to get a 10*10 matrix in the example you posted. Yes, nan() initiates an empty array. You can also use zeros() to initiate a matrix of zeros. BTW - this code works also without the imitation line, i.e.:
for a = 1:10
for b= 1:10
c(a,b)=a*b;
end
end
BUT this is highly un-recommended, because the shape of c changes constantly - which is a very inefficient use of memory. When you pre-allocate then c's shape is constant and the whole code runs much quicker (of course in the example case the array is very small and there won't be a significant difference).
Hope this helps, good luck

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更多回答（1 个）

Niels
Niels 2017-1-31

1 个投票

some changes:
aMax=10; % equal to #of rows
bMax=10; % equal to #of colums
c=zeros(aMax,bMax);
for a=1:aMax % let a run from 1 to aMax, stepwidth:=1
for b=1:bMax
c(a,b)=a*b; % hope you know that c is not running fom 1 to 100
end
end
if you want c to cointain to values 1:(aMax*bMax) (100 in current state) change
c(a,b)=(a-1)*aMax+b;

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