Calculating Mean inside a loop

Hi,

 I am new to matlab and am struggling with a very simple problem that I cannot solve. I have a set of data that needs to be binned by temperature. Once I have them in bins, I need to calculate the mean and std dev. of variables. I wrote the following code but it does not work i.e. does not give me the correct mean and std dev. Any help will be greatly appreciated.

for i = 1:59

if(T(i)>-30. && T(i)<=-25.)

    MnT30 = Mean(T(i)); SnT30 = std(T(i));
    MnIWC30 = Mean(IWC(i)); SnIWC30 = std(IWC(i));
    MnDe30 = Mean(De(i)); SnDe30 = std(De(i));
    MnVm30 = Mean(Vm(i)); SnVm30 = std(Vm(i));
elseif(T(i)>-35 && T(i)<=-30.)
    MnT35 = Mean(T(i)); SnT35 = std(T(i));
    MnIWC35 = Mean(IWC(i)); SnIWC35 = std(IWC(i));
    MnDe35 = Mean(De(i)); SnDe35 = std(De(i));
    MnVm35 = Mean(Vm(i)); SnVm35 = std(Vm(i));
end 
end

Answer 1

[Matt Tearle]

If you have Statistics Toolbox, you can use grpstats. In general, if you're doing a lot with variables in groups, you might want to make a categorical grouping array:

T = randi(100,300,1); % some fake data
levels = [-Inf,20,35,60,80,Inf];
Tgrp = ordinal(T,num2str((1:(length(levels)-1))'),[],levels);

or, simply, as Matt Fig suggests, use histc to bin:

[~,Tgrp] = histc(T,levels);

Then

[m,s]=grpstats(T,Tgrp,{@mean,@std})

gives you the mean and std dev of each group in the vectors m and s, respectively.

[Subhashree]

Thank you. This gives me a good start point.

Answer 2

[Matt Fig]

Use HISTC for binning problems. See both the one and two output functionality.