CODEWITHSUNDEEP
tclternary-operator
LionKimbro
LionKimbro

Reputation: 797

Why doesn't tcl's "if" return values?

Why doesn't this work like I would think?

set x [if {1} {return 2} {return 3}]

The command output shows 2, but when you check the value of x, it is found to be undefined..!

I have read a workaround, ...

proc give {x} {return $x}
set x [if {1} {give 2} {give 3}]

...but I don't understand why the first one doesn't work.

From the tcl 8.5 documentation for if:

"The return value from the command is the result of the body script that was executed, or an empty string if none of the expressions was non-zero and there was no bodyN."

Questions:

  1. Why doesn't set x [if {1} {return 2} {return 3}] set a value for x?
  2. Given that set x [if {1} {return 2} {return 3}] doesn't work, how does tclsh display "2" on the interactive shell?
  3. Given that tclsh displays "2" on the interactive shell, how is it that set x remains undefined?

Upvotes: 4

Views: 1548

Answers (3)

This isn't my real name
This isn't my real name

Reputation: 5029

The reason that set x [if {1} {return 2} {return 3}] doesn't work the way you expect, is, as already pointed out, the fact that return is a command that causes script evaluation to terminate. The "workaround", using a give procedure, demonstrates the fact that by default, the return command terminates 1 level of the call stack.

However, the "workaround" really is a pointless exercise in masochism: The return command allows you to specify where to return to, using the -level option introduced in Tcl 8.5. Try this, see how it works for you.

set x [if {1} {return -level 0 2} {return -level 0 3}]

Upvotes: 2

flup
flup

Reputation: 27104

See http://www.tcl.tk/man/tcl8.6/TclCmd/return.htm

Return codes are used in Tcl to control program flow. A Tcl script is a sequence of Tcl commands. So long as each command evaluation returns a return code of TCL_OK, evaluation will continue to the next command in the script. Any exceptional return code (non-TCL_OK) returned by a command evaluation causes the flow on to the next command to be interrupted. Script evaluation ceases, and the exceptional return code from the command becomes the return code of the full script evaluation. This is the mechanism by which errors during script evaluation cause an interruption and unwinding of the call stack. It is also the mechanism by which commands like break, continue, and return cause script evaluation to terminate without evaluating all commands in sequence.

So the evaluation of the script is aborted, x does not get set, and 2 is returned and printed as a result.

Upvotes: 7

glenn jackman
glenn jackman

Reputation: 246807

The expr command returns a value, so

set x [if 1 {expr 2} {expr 3}]

But if you're doing that, you might as well use the ternary operator

set x [expr {1 ? 2 : 3}]

Upvotes: 7

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