Is there a way to create a zero-cell vector in Mata?

[Salah Mahmud]

Hello,

I could not find a stack implementation in Mata, so I put together a quick one (see code below).
The code works but because the matrix initializer J(r,c,z) does not permit r==0 or c==0, I had to resort to an ugly hack (the "initialized" variable) to simulate an empty stack.
Also, I could not figure out how to shrink or grow a vector without losing content (other than by first copying the contents to another vector and then copying them back).
Is there a better way of implementing this? Other ways to improve the code?

Cheers,

set matastrict on        //all vars must be pre-declared 
mata: 
    version 13.2
    mata clear


//Class vdecStack provides a basic stack data structure
class vdecStack {
    private:
        string colvector _arr
        string colvector _c
        real scalar  initialized
    public:
        void         new()    //constructor
         void        push()
         string scalar pop() 
         void        print()        
}    

void vdecStack::new()
{
    initialized = 0
}

void vdecStack::push(string scalar item) {
    if (initialized  == 1 ) {
        _c    = _arr
        _arr = J(1,cols(_arr)+1,"")
        _arr[1..cols(_c)] = _c
        _arr[cols(_arr)] = item
    } 
    else {
        initialized = 1
        _arr =J(1,1,"")
        _arr[1] = item
    } 
}


string scalar vdecStack::pop() {
    if (initialized  == 0 ) {
        _error("nothing to pop")
        return
    }    
    string scalar item
    if (cols(_arr)==1 ) {
        item = _arr[1]
        _arr[1] = ""
        initialized  = 0
        return(item) 
    }
    item = _arr[cols(_arr)]    
    _arr =_arr[1,cols(_arr)-1]
    return(item)
}

void vdecStack::print()
{
    if (initialized  == 0 ) {
        printf("[]\n")
        return
    }
    real scalar i
    printf("[")
     for (i=1; i< cols(_arr); i ++) {    
        printf("%s,",_arr[i])
     }
    printf("%s]\n",_arr[i])    
}

void numTest() {
    class vdecStack scalar s
    s = vdecStack()
    s.print()    
    printf("pushing 1\n")
    s.push("1")
    s.print()    
    printf("pushing 2\n")
    s.push("2")
    s.print()
    printf("%s popped\n", s.pop())
    s.print()
    printf("%s popped\n", s.pop())    
    s.print()
    printf("try to pop an empty stack\n")
    s.pop()
}

numTest()

end

[Niels Henrik Bruun]

It works fine by me from version 12 onwards

: x=J(0,0,.)

:
: x=J(0,2,.)

:
: x=J(3,0,"")

Kind regards nhb