The Control Flow

If Condition

The Syntax:

if { test1 } {
body1 
} elseif { test2 } {
body2 
} else {
bodyn
}

If you don't like the curly brackets, you can take them out. But then, you will have to give the full thing in one line. I know that after you had a look at that syntax, you are still wondering what in the world that was. Don't worry - an example will set things straight.


#The marriage status script...
#Change the status please
set marital_status "After"

label .thesis -text "Marriage is a three ring circus..."

if { $marital_status=="Before" } {
set ring "Engagement"
} elseif { $marital_status=="During" } {
set ring "Wedding"
} else {
set ring "suffe -"
}

label .proof -text "$marital_status Marriage: $ring ring"

pack .thesis
pack .proof

Run this script and give the 'marital_status' to "Before", "During" and "After" and see the different results.

Operators

You will need operators to do the checking. Fortunately, the operators in Tcl are same as in any other language. So a small experience with any language will guarantee that you know all them.

Tcl has two kinds of operators.
Relational operators: <, <=, >, >=, ==, != return 0 (false) or 1 (true).

Operator in Tcl	Meaning	Example
==	is equal to	5 == 6
!=	is not equal to	5 != 6
<	is less than	5 < 6
<=	is less than or equal to	5 <= 6
>	is greater than	5 > 6
>=	is greater than or equal to	5 >= 6

Logical Operators: && , ! and || operator - AND, NOT and OR Operators.

Operator	Logical Equivalent	Example
! expression	NOT	!$a
expression1 && expression2	AND	$a > 6 && $a < 10
expression1 \|\| expression2	OR	$a != 6 \|\| $a!=5

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Comments

rauphel79 at 21 Sep, 2007 12:45

Hi,
I have question to ask about if statement in term of condition expression

if [ -n $a ]

what is the equivalent in tcl ?

Regards,

Reply to this.

Anonymous at 05 Sep, 2008 06:43

if {exists $n}

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Anonymous at 24 Apr, 2008 07:39

I believe this [ -n $a ] would be referring to shell-scripting ? (test whether variable a is NOT set)

Reply to this.

sandeep at 18 Jun, 2008 01:52

Error: syntax error in expression " $i != "" && $i == Exported "
//when used in if loop
if { $i != "" && $i == Exported } {
puts -nonewline $fpw "$i "
}

Reply to this.

Gaurav Thakur at 22 Sep, 2009 01:27

shouldn't it be "Exported" and not Exported ???

Reply to this.

khajaahmed at 28 Sep, 2009 07:11

Hi,
how to take input from the keyboard like scanf in C, is there any function like scanf in TCl also.

Reply to this.

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