:TITLE: Numeric Comparisons 101 - if
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:LESSON_TEXT_START_LEVEL 0:
Tcl uses an if construct similar to that used by other languages.
The syntax is:
<P><UL>
<B>if</B> <I>expr1</I> ?then? <I>body1</I><B> elseif</B><I> expr2</I> ?then? 
<I>body2</I><B> elseif</B> ... 
<B>?else?</B> <I>?bodyN?</I>
</UL>
<P>
The words <CODE>then</CODE> and <CODE>else</CODE> are optional, and may be included if you
think they improve the readability of the code, or left out, as you prefer.
<P>
The test expression following the word <CODE>if</CODE> is evaluated in the same manner
as in the <B>expr</B> command.  Hex strings 0xXX will be converted to their numeric
equivalent before evaluation.
<P>
The test expression following <CODE>if</CODE> should return one of:
<PRE>
<B>                      False            True
                      -------          ------ </B>
 a numeric value      0                all others
 yes/no               no               yes 
 true/false           false            true
<P></PRE>
 The case of a textual return is not checked.
	 
<P>
  The test expression following the word <CODE>if</CODE> is evaluated in
the same manner as in the <B>expr</B> command.  Hex strings 0xXX will be
converted to their numeric equivalent before evaluation. 
<P>
The test expression following <CODE>if</CODE> may be enclosed within
quotes, or braces.  If it is enclosed within braces, it will be
evaluated within the if command, and if enclosed within quotes it will
be evaluated during the substitution phase, and then another round of
substitutions will be done within the <B>if</B> command. 
:TEXT_END:
<P>
:LESSON_TEXT_START_LEVEL 1:
Like most languages, Tcl supports an if command.  The syntax is:
<P><UL>
<B>if</B> <I>expr1</I> ?then? <I>body1</I><B> elseif</B><I> expr2</I> ?then? 
<I>body2</I><B> elseif</B> ... 
<B>?else?</B> <I>?bodyN?</I>
</UL>
<P>
The words <CODE>then</CODE> and <CODE>else</CODE> are optional, and may be included if you
think they improve the readability or left out.  If you are used to
programming in a shell language, you may want to leave them in.  If you
are used to C, you will prefer leaving them out.
<P>
The test expression following <CODE>if</CODE> should return one of:
<PRE>
<B>                      False            True
                      -------          ------ </B>
 a numeric value      0                all others
 yes/no               no               yes 
 true/false           false            true
<P></PRE>
  If the test expression returns a string "yes"/"no" or "true"/"false", the
case of the return is not checked.  True/FALSE or YeS/nO are legitimate
returns. 
<P>
  If the test expression evaluates to True, then <I>body1</I> will be 
executed.  
<P>
  If the test expression evaluates to False, then the word after
<I>body1</I> will be examined.  If the next word is 
<CODE>elseif</CODE>, then the next test expression will be tested as a
condition.  If the next word is <CODE>else</CODE> then the final <I>body</I>
will be evaluated as a command.
<P>
  The test expression following the word <CODE>if</CODE> is evaluated in the same manner
as in the <B>expr</B> command.  Hex strings 0xXX will be converted to their numeric
equivalent before evaluation.
<P>
The test expression following <CODE>if</CODE> may be enclosed within
quotes, or braces.  If it is enclosed within braces, it will be
evaluated within the if command, and if enclosed within quotes it will
be evaluated during the substitution phase, and then another round of
substitutions will be done within the <B>if</B> command. 
:TEXT_END:
<P>
:LESSON_TEXT_START_LEVEL 2:
The switch command allows you to compare a string to other strings and
find a match.  Another feature of most computer languages is the ability
to choose between two courses of action based on a numeric test.  
Like most other languages, Tcl supports this functionality with 
the <I>if</I> command.  
<P>
The syntax for the <I>if</I> command is:
<P><UL>
<B>if</B> <I>expr1</I> ?then? <I>body1</I><B> elseif</B><I> expr2</I> ?then? 
<I>body2</I><B> elseif</B> ... 
<B>?else?</B> <I>?bodyN?</I>
</UL>
<P>
The words <CODE>then</CODE> and <CODE>else</CODE> are optional, you
may include them if you
think they improve the readability of your code, or leave them out if you
think that looks better.
<P>
When an <CODE>if</CODE> command is encountered the Tcl interpreter
evaluates <I>test1</I> using the <B>expr</B> command.  If that test is
true, the Tcl interpreter executes <I>body1</I>.  If the test expression 
evaluates to
False, then the word after <I>body1</I> will be examined.  If the
next word is <CODE>elseif</CODE>, then the next expression will be
evaluated as a test condition.  If the next word is <CODE>else</CODE> then the
final <I>body</I> will be evaluated as a command.   
<P>
The test expression following <CODE>if</CODE> should return one of
these values:
<PRE>
<B>                      False            True
                      -------          ------ </B>
 a numeric value      0                all others
 yes/no               no               yes 
 true/false           false            true
<P></PRE>
  If the test expression returns a character string like "yes", "no", "true"
or "false", the case of the return is not checked.  "True", "FALSE",
"YeS" or "nO" are legitimate returns. 
<P>
  The test following <CODE>if</CODE> may be enclosed within quotes, or
braces.  If it is enclosed within braces, it will be parsed by expr
within the <B>if</B> command.  If the test is enclosed within quotes
substitutions will be done during the Tcl parsing substitution phase,
and then another round of substitutions will be done within the <B>if</B>
command.
<P>
  Since the test in an <CODE>if</CODE> is evaluated in the same manner
as in the <B>expr</B> command, hex strings 0xXX will be converted to their
numeric equivalent before evaluation.  You can think of the <I>test</I> in
the <CODE>if</CODE> command as being executed by expr, and all the same rules
will apply.
<P>
<CODE>if {$i == 1} {puts "MATCH}</CODE>
<P>is the same as:<P>
<CODE>if {[expr $i == 1]} {puts "MATCH}</CODE>
<P>
  The <I>body</I> code may be enclosed within quotes or braces. 
Since <I>body</I> will be evaluated just like any other Tcl
command, <I>body</I> can be placed within braces and variable
substitution will still occur.  Note that the opening quote or bracket
must be on the same line as the if command, however. 
<P>
:TEXT_END:

:CODE_START:
set x 1;

if {$x == 2} {puts "$x is 2"} else {puts "$x is not 2"}

if {$x != 1} {
  puts "$x is != 1"
  } else {
  puts "$x is 1"
  }

if $x==1 {puts "GOT 1"}

;# Two passes of substitution will be done on this test string
;#  The initial pass replaces $y with x
;#  Within the if statement, $x is replaced with 1

set y x;
if "$$y != 1" {
  puts "$$y is != 1"
  } else {
  puts "$$y is 1"
  }

:TEXT_END:


