how use the for loop and stem function

Hello, I have a matrix with 3 columns (# of subjects, # of trials, scores). I have 10 subjects (with sometimes the value 999 as score). I need to calculate, for each subject, the mean score and the standard deviation without the value 999. Then, I need to trace the curve of the means for each subjects with the "stem" function. Finally, I have to trace the curve of standard deviation for each subject. I have no idea to do that. Can you help me please.

Answer 1

[Alexandre Williot]

clc; clear all; close all;

%-- extraction des données du fichier xlsx -------------------------
[num,txt,raw] = xlsread('C:\Users\alexandre\Dropbox\UQTR 2013 - 2015\Matlab\MatLab EPK6064\Data_Level4_Exo3.xlsx');

%--création matrice Mat---------------------------------------------
Sujets = num(:,1);
Essais = num(:,2);
Scores = num(:,3);

Mat = [Sujets Essais Scores];

%-- RECHERCHE <20 ---------------------------------------------------------
pos_999 = []; 
pos_999 = find(Mat(:,3)<20);
Mat(pos_999,3)= 999;

%-- Moyenne ---------------------------------------------------------------
for n=1:10
pos_suj = []
pos_suj = find(Mat(:,1) == n)
Mat_suj = []
Mat_suj = Mat(pos_suj,3)
pos_int = []
pos_int = find (Mat_suj~=999)
Mat_val=[]
Mat_val= Mat_suj(pos_int,1)
M_Mean(n,1) = mean (Mat_val)
M_std (n,1) = std (Mat_val)
end

%--graphique---------------------------------------------------------------
figure(1)
stem(M_Mean)
xlabel('sujet')
ylabel('moyenne')
title('graphique des moyennes')

figure(2)
stem(M_std)
xlabel('sujet')
ylabel('écart-type')
title('graphique des écart-types')