If Else Statement in For Loop

Hello, I have a 1440,721,40 matrix called mag_strength. The first dimension is longitude, the second is latitude, and the third represents 40 time steps of data. I'm trying to create a for loop with an if / else statement embedded that will look at all the data at each time step of the 3rd dimension and keep all values >1 and replace all other values that don't meet the requirement to NaNs. My resulting matrix (of equal size) is named: Fronts. When I run the loop, I get all NaN values, when I know that I should get a mixture of NaNs and positve values greater than 1. Any ideas on what's going on?

Fronts = zeros(1440,721,40);
for F = 1:40
    disp(F)
    if mag_strength(:,:,F)>1
        Fronts(:,:,F) = mag_strength(:,:,F);
    else
        Fronts(:,:,F) = NaN;
    end
end

Answer 1

[Chunru]

n1 = 6;  % 1440
n2 = 4;  % 721
n3 = 2;  % 40
mag_strength = randn(n1, n1, n3)

mag_strength = 
mag_strength(:,:,1) =

   -0.8025    0.9344    1.7522   -0.0025    0.7267    0.4267
   -0.7882    0.1544   -1.3793   -1.1197    1.3624   -0.3141
   -0.7260    1.9888    1.1160   -1.9104    0.4341   -1.6269
    1.6487    1.2641   -0.9046   -1.8453   -0.3283    0.8055
   -0.7476   -0.2142    0.9903    0.0892   -1.5335    0.3169
    0.5660    0.1052   -2.0026   -0.0124    0.9250    0.4308


mag_strength(:,:,2) =

   -0.2306   -0.0103   -1.3022    0.6091   -0.1025    2.0710
   -0.1591   -0.1148    0.0871   -1.6085   -0.6099    0.9604
    0.1962   -1.2370    0.1842    0.9875   -0.0267    0.5523
    1.5947   -0.9226    1.1566   -0.1529    0.5219   -1.0731
    0.4503   -0.5973    1.5207    0.2089   -0.3907   -0.8328
    1.0511    0.7779   -0.3594   -0.0712    0.0253   -0.7505

Fronts  = mag_strength;
Fronts(Fronts<=1) = nan;
Fronts

Fronts = 
Fronts(:,:,1) =

       NaN       NaN    1.7522       NaN       NaN       NaN
       NaN       NaN       NaN       NaN    1.3624       NaN
       NaN    1.9888    1.1160       NaN       NaN       NaN
    1.6487    1.2641       NaN       NaN       NaN       NaN
       NaN       NaN       NaN       NaN       NaN       NaN
       NaN       NaN       NaN       NaN       NaN       NaN


Fronts(:,:,2) =

       NaN       NaN       NaN       NaN       NaN    2.0710
       NaN       NaN       NaN       NaN       NaN       NaN
       NaN       NaN       NaN       NaN       NaN       NaN
    1.5947       NaN    1.1566       NaN       NaN       NaN
       NaN       NaN    1.5207       NaN       NaN       NaN
    1.0511       NaN       NaN       NaN       NaN       NaN

[Brian DeCicco]

Thank you, this makes a lot more sense! I was really over complicating my approach here.