if(n) Tcl Built-In Commands if(n) 







if - Execute scripts conditionally








if expr1 ?then? body1 elseif
    expr2 ?then? body2 elseif ... ?else?
    ?bodyN?












The if command evaluates expr1 as an expression (in
    the same way that expr evaluates its argument). The value of the
    expression must be a boolean (a numeric value, where 0 is false and anything
    is true, or a string value such as true or yes for true and
    false or no for false); if it is true then body1 is
    executed by passing it to the Tcl interpreter. Otherwise expr2 is
    evaluated as an expression and if it is true then body2 is executed,
    and so on. If none of the expressions evaluates to true then bodyN is
    executed. The then and else arguments are optional
    “noise words” to make the command easier to read. There may be
    any number of elseif clauses, including zero. BodyN may also
    be omitted as long as else is omitted too. 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.








A simple conditional:




if {$vbl == 1} { puts "vbl is one" }




With an else-clause:




if {$vbl == 1} {
   puts "vbl is one"
} else {
   puts "vbl is not one"
}




With an elseif-clause too:




if {$vbl == 1} {
   puts "vbl is one"
} elseif {$vbl == 2} {
   puts "vbl is two"
} else {
   puts "vbl is not one or two"
}




Remember, expressions can be multi-line, but in that case it can
    be a good idea to use the optional then keyword for clarity:




if {
   $vbl == 1 || $vbl == 2 || $vbl == 3
} then {
   puts "vbl is one, two or three"
}












expr(n), for(n), foreach(n)










boolean, conditional, else, false, if, true







  

    
    Tcl