CODEWITHSUNDEEP
tcl
my_question
my_question

Reputation: 3255

Why Tcl expr does not equal 2 true boolean values?

I have this script:

$ cat tmp/bool.tcl
puts "[expr [expr True] == [expr true] ]"

I expect it prints 1 because both "True" and "true" are boolean true so they should equal, but I get 0:

$ tclsh tmp/bool.tcl
0

Of course this is simplified code. The real code generates boolean values in string, "True", "true", "Yes", "yEs", etc.

Do I have to make them all lower case before comparing them?

I do not understand why the above code does not equal.

Upvotes: 1

Views: 885

Answers (2)

Donal Fellows
Donal Fellows

Reputation: 137807

The == operator is defined to do (the documentation in the expr page itself is more obscure):

numeric comparison if possible, exact string comparison otherwise

Neither True nor true is numeric, so exact string comparison is used. The two values differ in the first character of their string representation; they are therefore not equal in the sense of ==.

To compare uncontrolled boolean values for equality, invert them first as the result of the ! operator is both boolean and numeric (i.e., it's 0 or 1):

set a Yes
set b tRuE
expr {!$a == !$b}

Upvotes: 1

glenn jackman
glenn jackman

Reputation: 247240

% puts "[expr [expr True] == [expr true] ]"
0

Let's look at what each side does:

% expr True
True
% expr true
true

So, both recognized as boolean values, but not equal. Let's convert those strings to a "canonical" boolean form:

% set a True
True
% set b true
true
% expr {$a == $b}
0
% expr {!!$a == !!$b}
1

What's going on here: step-by-step

% expr {$a}
True
% expr {!$a}
0
% expr {!!$a}
1

The native "true/false" values are "1/0" respectively. And clearly 1 == 1

Opinion: this feels like it goes back to Tcl's "everything is a string" roots, and the strings "True" and "true" are unequal.

Upvotes: 3

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