Programming Fundamentals

In Perl, the & symbol before a subroutine name calls that subroutine. Modern Perl style often omits the & when calling subroutines.

The my operator creates lexical variables in Perl, which are private to their scope. This is Perl's mechanism for creating private variables within subroutines.

The next operator in Perl jumps to the end of the current loop iteration and continues with the next iteration. In contrast, last exits the loop entirely, and redo restarts the current iteration.

In Perl, variables are uninitialized (have the special value undef) until they are first assigned a value. This is different from null in some languages or numeric defaults like 0. The undef value evaluates to false in boolean context and as an empty string or zero depending on context.

In Perl, the 'last' operator immediately exits the current loop, similar to 'break' in other languages. The 'next' operator skips to the next iteration, 'redo' restarts the current iteration without re-evaluating the condition, and 'leave' is not a valid Perl loop control operator.

The warn function prints a message to standard error (STDERR) without terminating the execution of the Perl script, unlike die which exits the program.

In Perl, the special variable $? contains the status (return code) of the last external command executed, typically through system(), backticks, or piped opens. It is not for page length, EOL characters, or library error codes. The value is 0 for successful execution and non-zero for errors, following Unix conventions.

In Perl, the special variable $! contains the current value of errno (the error code from a system library routine) when a system call fails. It provides both numeric error code in numeric context and descriptive error string in string context. It is not for page length, EOL characters, or command return codes (which is $?).

The splice function can replace push when used with the current array size as the starting position. By setting the offset to scalar(@array) (the end of the array), length to 0 (nothing removed), and providing @sublist as the elements to insert, splice behaves exactly like push - appending elements to the array's end.

The die function in Perl terminates program execution with a nonzero exit status and outputs an error message to standard error. This makes it the appropriate choice for ensuring your program indicates failure through its exit code, even when errors occur during execution.

The <<= operator in Bash arithmetic evaluation (let) is the left-shift assignment operator. It shifts the bits of var to the left by 2 positions and assigns the result back to var. This is equivalent to multiplying by 4, not 'multiplied by 2 bits' or 'reduced'.

In SAS, the %put statement supports specific keywords to list macro variables in the log. _ALL_ displays all variables, _AUTOMATIC_ displays SAS-defined automatic variables, and _USER_ displays user-defined variables. Other options contain invalid keywords like _AUTO_ or _USERDEFINED_.

In SAS macro quoting, a percent sign (%) is used inside %str() to escape special characters. The sequence %% escapes the percent sign to yield a literal %, and %' escapes the single quote to yield a literal '. Thus, the resolved value stored in myvar is %'.

In SAS, referencing a nonexistent macro variable generates a warning in the log, and the reference is left unresolved. However, referencing an invalid macro variable name (which violates SAS naming rules, such as starting with a number) violates syntax rules and results in a compilation error.