You might also want to wrap the operands in the expr command in curly braces for better performance. See the expr man page's 'performance considerations' section for details:
http://www.tcl.tk/man/tcl8.4/TclCmd/expr.htm
Enclose expressions in braces for the best speed and the smallest storage requirements. This allows the Tcl bytecode compiler to generate the best code.
As mentioned above, expressions are substituted twice: once by the Tcl parser and once by the expr command. For example, the commands
set a 3
set b {$a + 2}
expr $b*4
return 11, not a multiple of 4. This is because the Tcl parser will first substitute $a + 2 for the variable b, then the expr command will evaluate the expression $a + 2*4.
Most expressions do not require a second round of substitutions. Either they are enclosed in braces or, if not, their variable and command substitutions yield numbers or strings that don't themselves require substitutions. However, because a few unbraced expressions need two rounds of substitutions, the bytecode compiler must emit additional instructions to handle this situation. The most expensive code is required for unbraced expressions that contain command substitutions. These expressions must be implemented by generating new code each time the expression is executed.
log 10.x.y.z local1.info "$request_log_line,\
[expr {[clock clicks -milliseconds] - $http_request_time}]"
Aaron
Nimbostratus
