Loops and lists are important tools for Stata programmers. Although these tools may not be the most computationally efficient technique, they can be of great help during the early stages of code development. By the way, it is always good to keep in mind that a good programming practice is to always keep definitions local to the do-file you are coding whenever possible.
The first example is about using a counter. You can place it on the do-file editor and then run it.
local fruits apple banana pear
local counter 0
foreach d of local fruits{
local counter= `counter’+1
display “ `counter’ – `d’”
}
After running it, the output should be something like this:
1 – apple
2 – banana
3 – pear
Notice that I used the equal sign “=” to build the counter. In short, you have to use it. Just for fun, see what happens when you replace “local counter= `counter’+1” by “local counter `counter’+1”.
Now suppose we have an ordered list of produce 4-digit codes and we would like to display the PLU code next to each fruit name.
local fruits apple banana pear
local plucode 3009 4011 4414
/* The first step is to find out the number of fruits */
local fruitnumber: word count `fruits’
di “Fruit – PLU code”
/* The second step is to build a loop to pair each fruit with its PLU code */
forvalues j = 1/`fruitnumber’ {
/* The third step is to pick the j-th element of each list to have them paired */
local pair1: word `j’ of `fruits’
local pair2: word `j’ of `plucode’
display “`pair1′ – `pair2′”
}
Output:
. local fruits apple banana pear
. local plucode 3009 4011 4414
. /* The first step is to find out the number of fruits */
. local fruitnumber: word count `fruits’
. di “Fruit – PLU code”
Fruit – PLU code
.
. /* The second step is to build a loop to pair each fruit with its PLU code */
. forvalues j = 1/`fruitnumber’ {
/* The third step is to pick the j-th element of each list to have them paired */
local pair1: word `j’ of `fruits’
local pair2: word `j’ of `plucode’
display “`pair1′ – `pair2′”
}
apple – 3009
banana – 4011
pear – 4414
.
end of do-file
So, the “word count” function returns the number of elements of the macro. And the “word `j’ of `fruits’” returns the j-th element of the macro fruits. You can find more about these macro extended functions on Stata by typing “help extended_fcn”.