If statement in inverse distance weight matrix

[sungwook kim]

Hi,

Now I'm making an inverse distance weight matrix between observations using the Mata function.

In Stata, the code is
-------------------------------------------------
clear mata

mata:
id = st_data(., "DHSCLUST") // this is a location at cluster level.
location = st_data(., ("LONGNUM", "LATNUM")) // this is GIS - longitude and latitude
N = st_nobs()
M = J(N, N, 0)
for (i=1; i<=N; i++) {
for (j=1; j<i; j++) {
delta = location[i,.] - location[j,.]
M[i,j] = M[j,i] = 1/sqrt(delta*delta')
}
}
M
st_matrix("weightmatrix_women", M)
end
-----------------------------------------------------
Main problem is a number of observations share the same location. As a result, LONGNUM and LATNUM of many obs are identical. This results in empty cell (as 1/sqrt(delta*delta') can't be calculated) in the generated matrix.



To solve this problem, I added if statement.

The code is

clear mata
mata:

id = st_data(., "DHSCLUST") // this is a location at cluster level.
location = st_data(., ("LONGNUM", "LATNUM")) // this is GIS - longitude and latitude
N = st_nobs()
M = J(N, N, 0)
for (i=1; i<=N; i++) {
for (j=1; j<i; j++) {
delta = location[i,.] - location[j,.]
if (delta != 0) {
M[i,j] = M[j,i] = 1/sqrt(delta*delta') ;
}
else {
M[i,j]=M[j,i] == 0 ;;
}
}
}
M
st_matrix("weightmatrix_women", M)

end

_______________________________________

However, the result is same. Anyone knows how to sort this out please?

Kind regards,

Kim

[Mike Lacy]

Your delta is a matrix, whereas "0" is a scalar quantity, so (delta != 0) will always be true. If you want to check whether a matrix contains all zeros, you need to compare it to a matrix of the same dimension.

Try this:

mata
delta = (0,0)
delta
"zero scalar"
delta !=  0
//
"zero matrix"
delta !=  (0,0)
end

Also, comparing floating point values for equality is always potentially a source of error. While it might not bite you here, it's safer to compare a floating point absolute value to some small value, e.g. abs(x) <= 1e-10.

In an unrelated matter, I'd also note that your use of ";" is strange (if likely harmless): The ";" in Mata is used to separate multiple statements on a line, but your code as displayed here only has one statement per line.

[sungwook kim]

Dear Mike,

Thank you very much for your reply! It is really helpful. Now it is clear!

Kind regards,

Kim