SPSS® 12.0 Command Syntax Reference

254 COMPUTE
Operations
• If the target variable already exists, its values are replaced.
• If the target variable does not exist and the assignment expression is numeric, the program
creates a new variable.
• If the target variable does not exist and the assignment expression is a string, the program
displays an error message and does not execute the command.
• COMPUTE is not executed if it contains invalid syntax. New variables are not created and
existing target variables remain unchanged.
Numeric Variables
• New numeric variables created with COMPUTE are assigned a dictionary format of F8.2
and are initialized to the system-missing value for each case (unless the LEAVE command
is used). Existing numeric variables transformed with COMPUTE retain their original
dictionary formats. The format of a numeric variable can be changed with the FORMATS
command.
• All expressions are evaluated in the following order: first functions, then exponentiation,
and then arithmetic operations. The order of operations can be changed with parentheses.
• COMPUTE returns the system-missing value when it doesn’t have enough information to
evaluate a function properly. Arithmetic functions that take only one argument cannot be
evaluated if that argument is missing. The date and time functions cannot be evaluated if
any argument is missing. Statistical functions are evaluated if a sufficient number of
arguments is valid. For example, in the command
COMPUTE FACTOR = SCORE1 + SCORE2 + SCORE3
FACTOR is assigned the system-missing value for a case if any of the three score values
is missing. It is assigned a valid value only when all score values are valid. In the
command
COMPUTE FACTOR = SUM(SCORE1 TO SCORE3).
FACTOR is assigned a valid value if at least one score value is valid. It is system-missing
only when all three score values are missing.
String Variables
• String variables can be modified but not created on COMPUTE. However, a new string
variable can be created and assigned a width with the STRING command and then
assigned new values with COMPUTE.
• Existing string variables transformed with COMPUTE retain their original dictionary
formats. String variables declared on STRING and transformed with COMPUTE retain the
formats assigned to them on STRING.
• The format of string variables cannot be changed with FORMATS. Instead, use STRING to
create a new variable with the desired width and then use COMPUTE to set the values of
the new string equal to the values of the original.