Creating new variables with for loop

[Dmitry Ilin]

Dear all,

A simple code:

for (x = 1; x <= 5; x++) {
m_x = x
}

doesn't create five variables (m_1...m_5), as I expected, but rather creates one variable m_x.
Anyone has any idea about how to make Mata generate these 5 variables in a loop? Would be much appreciated.

Dmitry

[Nick Cox]

You are expecting Mata scalars to behave like Stata local macros. Short answer is that won't work. Another answer is that if this were true, then there would be a horrible class of bugs. If I really want m_x or a variable excellent inside a loop with index x, I am scuppered. Mata would instead create e1cellent, etc. But it doesn't work like that.

A longer answer starts by saying that there are different and usually better ways to do what you want. One is that a set of related scalars is usually best created as a vector. Another is that you need to know about pointers.

A fuzzier truth is that features you like and are familiar with aren't always available in other languages.

Some references:

SJ-11-2 pr0052 . . . . Stata tip 100: Mata and the case of the missing macros
. . . . . . . . . . . . . . . . . . . . . . . . W. Gould and N. J. Cox
Q2/11 SJ 11(2):323--324 (no commands)
tip showing how to do the equivalent of Stata's macro
substitution in Mata

SJ-8-3 pr0040 . . . . . . . . . . . . . . . . . . . . . Mata Matters: Macros
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . W. Gould
Q3/08 SJ 8(3):401--412 (no commands)
describes why macros are seldom needed in Mata and
outlines what you might do with macros in Mata

[Dmitry Ilin]

Thank you, Nick!

The only thing which I should have mentioned in the first place is that m_x in my application are, actually, matrices, not scalars. Hence, I doubt that the solution with a vector of matrices would work. But thank you for the references and pointers - I will read it carefully.

Dmitry

[Dmitry Ilin]

To be more precise, what I need is the following:

for (x = 1; x <= 1000; x++) {
P_x = J(10,20,.)
}

In words, I need to generate 1000 for now missing matrices, each of them should be labeled P_1, P_2 ... P_1000.
Now, the above code doesnt work and it is clear why. So far my understanding of pointers also cannot help to solve this problem.
I am a bit frustrated as it is a very easy task for Stata, but in Mata it becomes for some reason a puzzle for me...

[Nick Cox]

You need a thousand of these simultaneously? Could you work with J(10000,20,.)? If so and if not, then it sounds like pointers.

[Joseph Coveney]

As Nick mentioned, a set of related scalars--or in this instance, an array of related matrices--is best handled as a Mata vector, and you'll benefit from a knowledge of Mata pointers. In this case, you can put the two together with a pointer vector that can point to your 1000 missing-valued matrices.

version 13.1

mata:
mata set matastrict on

void function assignMatrix(real scalar x, pointer(real matrix) vector P) {

    real matrix M
    M = J(10, 20, .)

    P[x] = &M
}

void function demo() {

    pointer(real matrix) vector P
    P = J(1, 1000, NULL)

    real scalar x
    for (x=1; x<=1000; x++) {
        assignMatrix(x, P)
    }

    // Dereference P[index] and do stuff with the missing-valued matrix
}

end

exit

[Christophe Kolodziejczyk]

In interactive mode you can write

forval x = 1/1000 {
          mata: p_`x' = J(10, 20,.)

}

mata:p_1

An alternative to Joseph solution could be

mata:

M = J(1,1000,NULL)

for (i=1;i<=1000;i++)
{
          M[1,i] = &J(10,20,.)
}

end

You access the first matrix by typing

*M[1,1]

In non-interactive mode you have to use a vector of pointers to collect the different matrices in one place. See the help on pointers from the Mata manual.

[Dmitry Ilin]

Great, thanks a lot!!

[Dmitry Ilin]

For those interested in the question: the quoted nice piece of code

In interactive mode you can write

forval x = 1/1000 {
mata: p_`x' = J(10, 20,.)

}

mata:p_1

unfortunately doesn't work with if statements. For example, the following code won't be executed:

forvalues x = 1/5 {
    mata: if (`x' >= 2) `x'
}

[Matthew White]

One-line blocks (if, for, etc.) need to be followed by two semicolons in interactive mode. Your code works when they're added:

forvalues x = 1/5 {
    mata: if (`x' >= 2) `x';;
}

Apparently the `x' statement needs one semicolon, and the if another. Why Mata has trouble parsing this line without them, I'm not sure.

[Christophe Kolodziejczyk]

See this post about the issue raised by Matthew. http://www.stata.com/statalist/archi.../msg00434.html