$tcl_precision, a built-in Tcl variable, specifies the number of digits to generate when converting floating-point values to strings.

See Also

IEEE binary float to string conversion


official reference
TIP 132
Describes several changes to the conversion between floating-point numbers and strings, primarily to ensure that string equality implies value equality. New in Tcl 8.5


As of Tcl 8.5, changing $tcl_precision to a value other than its default value of 0 is deprecated. If you need to control the display precision of floating point values, use format (eg format %.12g $x).

The default value for $tcl_precision is 0, meaning that when Tcl converts a number to a string, the fewest digits necessary to distinguish a floating point number from its nearest neighbours. The advantage of this behaviour is that the string representation of an inexact number like 1.4 is 1.4 rather than 1.3999999999999999, allowing Tcl to maintain its EIAS semantics, and providing a more intuitive representation for script authors. If $tcl_precision is set to a value other than 0, scenarios that violates EIAS semantics can occur:

% set tcl_precision 12
% set a [expr 1.00000000000123]
% set b [expr 1.0]
% expr { $a == $b }
% expr { $a eq $b }

Additionally, numbers that shimmer to strings and back again can lose precision:

expr (1/3.0) == .3333333333333333
expr {(1/3.0) == .3333333333333333}
set x [expr 1/3.0]
expr $x == .3333333333333333
expr {$x == .3333333333333333}

when $tcl_precision is 0, the results are:


but when $tcl_precision is 15, the results change to:


$x was handed unbraced to expr, which then concatenated its arguments to form an expression, and in the process used the string value of $x. Subsequently, expr converted that string value, which was truncated due to the $tcl_precision of 15, back into a floating-point value, and in the process, some precision was lost. One more reason to Brace your expr-essions, and not to mess with the value of $tcl_precision.

Historically, the default value of $tcl_precision was 12.

17 digits, the maximum allowed value, is "perfect" for IEEE floating-point in that it allows double-precision values to be converted to strings and back to binary with no loss of information. However, using 17 digits prevents any rounding, which produces longer, less intuitive results. For example, expr 1.4 returns 1.3999999999999999 with $tcl_precision set to 17, vs. 1.4 if $tcl_precision is 12.

All interpreters in a process share a single $tcl_precision: changing it in one interpreter will affect all other interpreters as well. However, safe interpreters are not allowed to modify it.

Since the incorporation TIP #132 into Tcl 8.5, Tcl behaves like this :

% set tcl_precision 17
% expr 1.4

This change doesn't affect the inherent limitations to the precision of binary floating point numbers:


is still less than the Platonic real number, 7/5. This can be proven by multiplying it by three:

% expr {3 * 1.4}

Interestingly enough, multiplying by 5 yields an exact result:

% expr {5 * 1.4}

sbron: KBK mentioned in the Tcl Chatroom that after TIP #132 a value can be displayed the same way as before by using format with a formatstring of %#.12g

KBK 2014-05-14 in the Tcl Chatroom: The work put into Tcl's floating-point conversion - making float->string->float bit-for-bit lossless - wasn't so much for precision or performance as it was to continue to satisfy EIAS.

Donald Porter's wise advice, in comp.lang.tcl, slightly edited:

It is best to use $tcl_precision in a global way. Set it early in your program's execution, then leave it alone. For situations like above where you want to control precision of single assignment, use format.

Historically, if you were doing any math at all where you care about getting the right answer, you would just set ::tcl_precision 17, and then forget anyone suggested anything else. The current recommendation is not to change ::tcl_precision at al.

RS: The funny thing about $tcl_precision is that it's not there in the beginning - it is just created on demand (trace?):

% info vars
tcl_rcFileName tcl_version argv argv0 tcl_interactive auto_oldpath auto_path err
orCode errorInfo auto_index env tcl_patchLevel argc tcl_libPath tcl_platform tcl
% set tcl_precision
% info vars
tcl_rcFileName tcl_version argv argv0 tcl_interactive auto_oldpath auto_path err
orCode tcl_precision errorInfo auto_index env tcl_patchLevel argc tcl_libPath tc
l_platform tcl_library
% unset tcl_precision
% set tcl_precision

Now you see it - now you don't. $tcl_precision is the only variable I know that can be read after being unset.

DGP: You don't know about $tcl_interactive then? Common trait - they are Tcl_LinkVar variables, script level representations of C values.