using stack command without losing data

Hi everyone.

I'm working with Census data and these are my 5 columns: STATE_str, AGE, TOTAL, MALE, FEMALE (see attached screenshot).

The last three columns represent total population, male population, and female population.

My goal is to combine TOTAL, MALE, and FEMALE into one column (POPULATION) and create a new variable (SEX) that specifies 0, 1, 2 (0 for TOTAL; 1 for MALE, and 2 for FEMALE), but I need to do this by STATE_str and AGE so that for each state and age combination there are now 3 observations - one for each of the three population values. My final data set should have the following four columns:

STATE_str AGE SEX POPULATION

I can successfully combine the three TOTAL, MALE, & FEMALE variables into one column and create a unique code using the stack command and syntax:
But this results in loss of the first two columns (STATE_str & AGE).


I then tried the following syntax:
which resulted in the return "stack may not be combined with by".

Any ideas for how to work around this? I have searched the forums for help using stack and seem to be finding only graphing-related help.

Thanks very much.