TCL factorial calculation code: extra characters after close-brace

Question

This is the code in TCL that is meant to produce factorial of a number given as parameter by the user.

if {$argc !=1}{
    puts stderr \"Error! ns called wi         


        

Answer 1

Tcl is a little bit more sensitive about whitespace than most languages (though not as much as, say, Python). For instance, you can't add unescaped newlines except between commands as command separators. Another set of rules are that 1) every command must be written in the same manner as a proper list (where the elements are separated by whitespace) and 2) a command invocation must have exactly the number of arguments that the command definition has specified.

Since the invocation must look like a proper list, code like

... {$x>1}{incr x -1} ...

won't work: a list element that starts with an open brace must end with a matching close brace, and there can't be any text immediately following the close brace that matches the initial open brace. (This sounds more complicated than it is, really.)

The number-of-arguments requirement means that

for {set result 1} {$x>1}{incr x -1}{
    set result [expr $result * $x]
}

won't work because the for command expects four arguments (start test next body) and it's only getting two, start and a mashup of the rest of other three (and actually not even that, since the mashup is illegal).

To make this work, the arguments need to be separated:

for {set result 1} {$x>1} {incr x -1} {
    set result [expr {$result * $x}]
}

Putting in spaces (or tabs, if you want) makes the arguments legal and correct in number.