call different column values in a cycle

Hello!

I'm programming a "digit span task". The task shows strings of numbers sequentially. It starts with 3 numbers and it ends with 10. Each sequence is repeated twice. 

I have a table like this:

span 1 2 3 4 5 6 7 8 9 10
3     3 1 9 x x x x x x x
3     7 2 5 x x x x x x x
4     4 2 9 6 x x x x x x
4     9 3 5 1 x x x x x x

So, in order to tell the script which number is needed, I need to call different columns whithin a trial! I keep thinking as a "for loop" solution, but is not possibble since when you call a col, with @ symbol, then you need to provide an integer, so it's not possible to put a variable after the @.

I have coded my task as follows.

---

task digitspan
     keys 0 1 2 3 4 5 6 7 8 9
    table My_Table
    set &trialcount increase
    set $spannum @1 # this sets how many numbers has the sequence

##### loop stim
    set $spannum2 $spannum #creates an auxiliar variable
    set $colnum 2 #creates a variable indexing where the actual numbers should start (cols 2, 3, 4 ...)
    while $spannum2 != 0 ## loop until span is equal to 0
        show bitmap @$colnum #I want the number after the @ iterate with each while cycle, so it will be, 2, 3, 4 ... BUT @ requires an integer, so it is not possible to call a variable
        delay &stimulus_display_time #define el tiempo estímulo
        set $spannum2 decrease
        set $colnum increase
    while-end

---


I had another script with a random numer generator, BUT, it is appearently too heavy, since it takes few seconds between one trial and another. 

Please, help! I will be soooooooooooo grateful to read you guys!