Code Execution

Basic code execution

• When we run a chunk of code it runs from top to bottom in order
• It runs the first line, then the second line, and so on
• This means that order is important
• Write code to calculate GDP growth from GDP in 2025 and 2024

    clear           all
    set obs         1
    gen             gdp_d = gdp_25 - gdp_24 / gdp_24
    gen             gdp_24 = 12.9
    gen             gdp_25 = 13.5
    display         gdp_d

• Returns an error because neither gdp_24 or gdp_25 exists yet

• Within a line code executes everything before assigning the output to a variable
• So it starts to the right of the equal sign
• Looks up the value for gdp_25

• Can’t find the variable and so errors

• Rearrange the code so that all variables are created before they are used

    clear           all
    set obs         1
    gen             gdp_24 = 12.9
    gen             gdp_25 = 13.5
    gen             gdp_d = gdp_25 - gdp_24 / gdp_24
    display         gdp_d

• This executes in the following sequence
• The right hand side of the first line creates a scalar
• It is then stored or “assigned” to the variable gdp_24
• The second line then runs creating a scalar and assigning it to the variable gdp_25
• The third line first looks up the variable gdp_25 and replaces it with its value
• It then looks up the the variable gdp_24 and replaces it with its value
• It then again looks up the the variable gdp_24 and replaces it with its value
• It then performs the mathmatical operations
• It assigns the resulting scalar to the variable gdp_d
• The fourth line first looks up the variable gdp_d and then prints its value on the screen

Operator precedence in expressions

• The above code will execute but it will give the wrong answer
• That is because code executes following Stata’s order of operations (similar to the order of operations in math: PEMDAS)
• Within a single command that involves a complex expression (e.g., in a gen or replace command), Stata follows the following order (from first to last):
  • ! (or ~): Logical NOT
  • ^: Exponentiation
  • -: Negation
  • /, *: Division, Multiplication -, +: Subtraction, Addition
  • != (or ~=), >, <, <=, >=, ==: Relational operators (not equal, greater than, etc.)
  • &: Logical AND |: Logical OR
• Like in PEMDAS, parentheses can be used to override this default order and force specific parts of an expression to be evaluated first.
• Stata processes the entire dataset for a single command, then moves to the next command. This is different from some other statistical software, where a command is executed for one observation before moving to the next.

Our example

• The sequence of execution for the third line is
  • Look up the variable gdp_25 and replaces it with its value
  • Look up the the variable gdp_24 and replaces it with its value
  • Again look up the the variable gdp_24 and replaces it with its value
  • Divide gdp_25 by gdp_24
  • Subtract the result of the division from gdp_25
  • It assigns the resulting scalar to the variable gdp_d

• The fourth line first looks up the variable gdp_d and then prints its value on the screen

• The program is dumb and will do exactly what you tell it based on the program’s rules
• If you get an unwanted or unexpected result, the problem is with your directions NOT with how the program executed your directions

    clear           all
    set obs         1
    gen             gdp_24 = 12.9
    gen             gdp_25 = 13.5
    gen             gdp_d = (gdp_25 - gdp_24) / gdp_24
    display         gdp_d

• One needs to take care in how you order actions to the right of the equal sign
• But also need to take care in how you order objects to the right of the command you are executing
• Think carefully on how to use brackets and parenthesis
• Think carefully on how you structure the logic of your commands
• Particularly if they involve
  • Conditionals: if, &, and || (or)
  • Loops: foreach, forevery, assignment of locals
  • Replacing or removing values: replace, drop, drop if

    Summary

• Stata executes code sequentially from top to bottom.
• The order of commands strictly dictates the state of your session.
• Variables must be created before they can be referenced in calculations.