The ceil command is a part of the [TclX] extension.
TclX created command interfaces to a number of [Tcl]
mathematical functions.  So just read the [expr]
documentation for ceil, and you will know what kind of
arguments to pass to it.

[RS]: Of course you can also write, if you wish
   proc ceil x {expr {ceil($x)}}
so you don't have to require TclX...
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[Category Package]

[Category Mathematics]