Styjun

I am doing some simple hex comparison in an if statement.

0x7843E0 is greater than 0x780000 but the code below doesn't output anything.

if "780000" <= "7843E0" 
 puts "True!"

>>

Omitting the trailing 0 works fine however.

if "780000" <= "7843E" 
 puts "True!"

>>> True!

There must be something wrong with the trailing 0 but I don't understand what it is. Any ideas?

You're having problems with the way the expr command parses numbers. (The rest of Tcl is more relaxed about this.) The issue is that:

• 
  "780000" gets interpreted as a decimal integer.


• 
  "7843E0" gets interpreted as a double precision floating point number. (Compare with 1.2e10; the number parser thinks it is fitting the same sort of pattern.)


• 
  "780000" gets interpreted as a decimal integer.


• 
  "7843E" gets interpreted as a non-numeric string (a fallback because no numeric interpretation is legal).

The <= operator will happily compare two numbers if they're both numbers, or two strings if at least one of the parameters to it is non-numeric. (Yes, this does make for weird semantics occasionally.) Moreover, he expr command is eager to interpret values as numbers if it possibly can, but it still has Tcl's syntactic rules for what actually is numeric, and what type of numeric those things are. When you don't stick to the rules, it gets a bit odd.

To get a value interpreted as hexadecimal, you have to either prefix its string representation with 0x (e.g., 0x7843E0) or force things with a command such as scan with %x:

scan "780000" %x a
scan "7843E0" %x b
if $a <= $b 
 puts "True"

Forcing interpretations with scan is considered to be one of the best ways of dealing with this, as that only writes canonical values into variables. (If you'd been wanting to handle octal numbers, or were wanting to really always be decimal, you'd use %o and %d respectively; %f is for floating-point numbers.)

Finally, if you're really comparing values as strings with normal ASCII-like rules, look at string compare instead of using <= directly.

if [string compare $input1 $input2] <= 0 
 ...