For Loop or function for repeating action

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JacquelineK
JacquelineK le 13 Sep 2017
Commenté : JacquelineK le 21 Sep 2017
I have A with 225 x 2 vectors. One Column is a variable always ranking from 1-5 (like grades) and the second is also numeric. I now want to calculate the mean, median, first and third quantile of the second vector, for each grade score.
The result I need, need to be interpreted like: mean(age) of A students better than mean(age) of B students
Grades 1 2 3 [etc]
Mean
Median
1st Qntl
3rd Qntl
I did it all by manually, which is kind of a lot, because I have 8 hypothesis for which the calculations are almost the same (the matrix A is in reality 225*11 but I only need 2-3 vectors per hypothesis). Now I wonder if there is a way to "do it faster and more efficient" namely in a for loop?
where I can write something like:
for i = 1:5
if ERM == i
mean_Hyp_1 = nanmean(A(ERM==1;:,2))
meadian_Hyp_1 = nanmedian(A(ERM==i;:,2)
etc
end
end
Thanks in advance

Réponse acceptée

Vishwas
Vishwas le 19 Sep 2017
You had the right idea. "find" function can be used to find all the rows where ERM == 1,2,.. in a loop and the result can be calculated.
Let me show this via an example:
a = [1;3;2;4;5;1;2;4;3;5;3;2;1]
b = [10;15;24;54;36;57;87;98;65;78;05;48;65]
input = [a b]
mean = []
median = []
for i = 1:5
mean(i) = nanmean(input(find(input(:,1)==i), 2))
median(i) = nanmedian(input(find(input(:,1)==i), 2))
end
I the case above, we are using the "find" function on the first column of input, extracting the indices for all values of input(:,1) == i and finding the mean of all the values from the second column.
  9 commentaires
Stephen23
Stephen23 le 20 Sep 2017
@Vishwas Vijaya Kumar: is there a good reason for shadowing the inbuilt input function?
José-Luis
José-Luis le 20 Sep 2017
Modifié(e) : José-Luis le 20 Sep 2017
And mean().
And median().
And that's some pretty tortured indexing.

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Plus de réponses (1)

Tim Berk
Tim Berk le 19 Sep 2017
You can use the condition A(:,1) == i as indexing for which values in A(:,2) to consider, i.e.
A = [1 2 3 1 1 2 3; 4 5 6 7 8 9 0]'
for i = 1:3
mean_A(i) = nanmean(A(A(:,1)==i,2));
% etc..
end
  1 commentaire
JacquelineK
JacquelineK le 21 Sep 2017
Hi Tim,
Thanks to you my codes for all my hypothesis are hapening so much faster. Now I am on my last hypothesis, which is the same method as before with one constraint. Before I open a new question, I just wanted to see, if you can help.
matrix A with 5 columns. First column with grades (1-5) and second column with years ranking from 2008-2013. Rest of columns again numeric.
First: "Cluster" the years 2008-2010, 2011-2013, 2014-2016
Second: Search Grades between the years 2008-2010, 2011-2013, 2014-2016
Third: Calculate the means of every column according to grade and clustered year.
The main problem I have encountered is that Matlab doesn't let me write the expression
for i = 2008:2010 ...etc
I did it again manually (mean of each year for all variables). But I cannot include, like your previous code showed me.
for i= 1:5
...(A(:,i)==i)..etc

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