:TITLE: Results of a command - Math 101 
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:LESSON_TEXT_START_LEVEL 0:
The command for doing math type operations is <CODE>expr</CODE>.

<P>

Here's some excerpts from the man page.

<P>

Expr concatenates arg's (adding separator spaces between them),
evaluates the result as a Tcl expression, and returns  the
value.   The  operators permitted in Tcl expressions are a
subset of the operators permitted in  C  expressions,  and
they  have  the  same meaning and precedence as the corresponding C operators.   Expressions  almost  always  yield
numeric  results  (integer or floating-point values).  For
example, the expression

<P>

<CODE>expr 8.2 + 6</CODE>

<P>

evaluates to 14.2.  Tcl expressions differ from C  expressions  in  the way that operands are specified.  Also, Tcl
expressions support non-numeric operands and  string  comparisons.

<P>

<B>OPERANDS</B><P> A Tcl expression consists of a combination of
operands, operators, and parentheses.  White space may be used between
the operands and operators and parentheses; it is ignored by the
expression processor.  Where possible, operands are interpreted as
integer values.  Integer values may be specified in decimal (the normal
case), in octal (if the first character of the operand is 0), or in
hexadecimal (if the first two characters of the operand are 0x).
<P> Note that the octal and hexadecimal conversion takes place differently
in the expr command than in the Tcl substitution phase.
<P>If an
operand does not have one of the integer formats given above, then it is
treated as a floating-point number if that is possible.  Floating-point
numbers may be specified in any of the ways accepted by an ANSI-
compliant C compiler.  For example, all
of the following are valid floating-point numbers: 2.1, 3., 6e4,
7.91e+16.  If no numeric interpretation is possible, then an operand is
left as a string (and only a limited set of operators may be applied to
it). 

<P>

Operands may be specified in any of the following ways:
<UL>
<LI>          As  an  numeric  value, either integer or floating-
              point.
<LI>          As a Tcl variable, using standard $ notation.   The
              variable's value will be used as the operand.
</UL><P>

<B>OPERATORS</B><P>
       The valid operators are listed below, grouped in  decreasing order of precedence:
<DL><DT>
       -  +  ~  !          <DD>Unary minus, unary plus, bit-wise NOT,
                           logical NOT.  None of  these  operands
                           may be applied to string operands, and
                           bit-wise NOT may be  applied  only  to
                           integers.
<DT>
       *  /  %             <DD>Multiply,  divide, remainder.  None of
                           these  operands  may  be  applied   to
                           string  operands, and remainder may be
                           applied only to integers.  The remainder  will always have the same sign as
                           the  divisor  and  an  absolute  value
                           smaller than the divisor.
<DT>
       +  -                <DD>Add   and  subtract.   Valid  for  any
                           numeric operands.
<DT>
       &lt;&lt;  &gt;&gt;              <DD>Left and right shift.  Valid for integer operands only.
<DT>
       &amp;                   <DD>Bit-wise   AND.    Valid  for  integer
                           operands only.
<DT>
       ^                   <DD>Bit-wise  exclusive  OR.   Valid   for
                           integer operands only.
<DT>
       |                   <DD>Bit-wise   OR.    Valid   for  integer
                           operands only.
<DT>
       &amp;&amp;                  <DD>Logical AND.  Produces a 1  result  if
                           both  operands  are non-zero, 0 otherwise.  Valid for numeric operands only
                           (integers or floating-point).
<DT>
       ||                  <DD>Logical  OR.   Produces  a 0 result if
                           both operands are zero,  1  otherwise.
                           Valid for numeric operands only (integers or floating-point).
<DT>
       x?y:z               <DD>If-then-else, as in C.  If x evaluates
                           to  non-zero,  then  the result is the
                           value of y.  Otherwise the  result  is
                           the  value  of  z.  The x operand must
                           have a numeric value.
<P></UL>
<B>MATH FUNCTIONS</B><P>
      Tcl  supports  the  following  mathematical  functions  in
      expressions:
<PRE>
             acos        cos         hypot      sinh
             asin        cosh        log        sqrt
             atan        exp         log10      tan
             atan2       floor       pow        tanh
             ceil        fmod        sin
</PRE>
<B>TYPE CONVERSIONS</B><P>
	Tcl supports the following functions to convert from one
	representation of a number to another:
<PRE>
 	abs double int round </PRE>

<P> If you haven't hit the <B>Run Example</B> button yet, do so now, and
examine the results of the example.  Play around a bit with the example
code, and then go on to the next lesson. 

:TEXT_END: 

:LESSON_TEXT_START_LEVEL 1:
The command for doing math type operations
is <CODE>expr</CODE>.  The following discussion of the <CODE>expr</CODE>
command is extracted and adapted from the <CODE>expr</CODE> man page. 


<P>

<CODE>Expr</CODE> concatenates arg's (adding separator spaces
between them), evaluates the result as a Tcl expression, and returns the
value.  The operators permitted in Tcl expressions include all the
standard math functions.  Expressions almost always yield numeric
results (integer or floating-point values).  

<P>

 <B>OPERANDS</B> 

<P>

 A
Tcl expression consists of a combination of operands, operators, and
parentheses.  White space may be used between the operands and operators
and parentheses; it is ignored by the expression processor.  Where
possible, operands are interpreted as integer values.  Integer values
may be specified in decimal (the normal case), in octal (if the first
character of the operand is 0), or in hexadecimal (if the first two
characters of the operand are 0x).  
<P> Note that the octal and hexadecimal conversion takes place differently
in the expr command than in the Tcl substitution phase.  In the substitution
phase, a <B>\x32</B> would be converted to an ascii "2",   while the <CODE>expr</CODE>
command would covert a <B>0x32</B> to a decimal 50.
<P>

If an operand does not have one of the integer formats given above, then
it is treated as a floating-point number if that is possible. 
Floating-point numbers may be specified in any of the ways accepted by
an ANSI-compliant C compiler.  For example, all of the following are
valid floating-point numbers: 2.1, 3., 6e4, 7.91e+16.  If no numeric
interpretation is possible, then an operand is left as a string (and
only a limited set of operators may be applied to it). 

<P>


 Operands may be specified in any of the following ways:
<UL>
<LI>          As  an  numeric  value, either integer or floating-
              point.
<LI>          As a Tcl variable, using standard $ notation.   The
              variable's value will be used as the operand.
</UL>

<P>


<B>OPERATORS</B>

<P>


       The valid operators are listed below, grouped in  decreasing order of precedence:
<DL><DT>
       -  +  ~  !          <DD>Unary minus, unary plus, bit-wise NOT,
                           logical NOT.  None of  these  operands
                           may be applied to string operands, and
                           bit-wise NOT may be  applied  only  to
                           integers.
<DT>
       *  /  %             <DD>Multiply,  divide, remainder.  None of
                           these  operands  may  be  applied   to
                           string  operands, and remainder may be
                           applied only to integers.  The remainder  will always have the same sign as
                           the  divisor  and  an  absolute  value
                           smaller than the divisor.
<DT>
       +  -                <DD>Add   and  subtract.   Valid  for  any
                           numeric operands.
<DT>
       &lt;&lt;  &gt;&gt;              <DD>Left and right shift.  Valid for integer operands only.
<DT>
       &amp;                   <DD>Bit-wise   AND.    Valid  for  integer
                           operands only.
<DT>
       ^                   <DD>Bit-wise  exclusive  OR.   Valid   for
                           integer operands only.
<DT>
       |                   <DD>Bit-wise   OR.    Valid   for  integer
                           operands only.
<DT>
       &amp;&amp;                  <DD>Logical AND.  Produces a 1  result  if
                           both  operands  are non-zero, 0 otherwise.  Valid for numeric operands only
                           (integers or floating-point).
<DT>
       ||                  <DD>Logical  OR.   Produces  a 0 result if
                           both operands are zero,  1  otherwise.
                           Valid for numeric operands only (integers or floating-point).
<DT>
       x?y:z               <DD>If-then-else, as in C.  If x evaluates
                           to  non-zero,  then  the result is the
                           value of y.  Otherwise the  result  is
                           the  value  of  z.  The x operand must
                           have a numeric value.


<P>

</UL>
<B>MATH FUNCTIONS</B><P>
      Tcl  supports  the  following  mathematical  functions  in
      expressions:
<PRE>
             acos        cos         hypot      sinh
             asin        cosh        log        sqrt
             atan        exp         log10      tan
             atan2       floor       pow        tanh
             ceil        fmod        sin
</PRE>
<B>TYPE CONVERSIONS</B><P>
	Tcl supports the following functions to convert from one
	representation of a number to another:
<PRE>
 	abs double int round </PRE>
<P>
If you haven't hit the <B>Run Example</B> button yet, do so now, and examine
the results of the example.  Play around a bit with the example code, and
then go on to the next lesson.
:TEXT_END:

:LESSON_TEXT_START_LEVEL 2:
The driving force for using a computer has traditionally been to perform
mathematic manipulations of data.  Some languages like BASIC or C do
math operations whenever they appear in an expression.  Tcl, however,
has a special command for doing math operations.  The command for doing
mathematic operations is <CODE>expr</CODE>.  The following discussion of the <CODE>expr</CODE>
command is extracted and adapted from the <CODE>expr</CODE> man page. 

<P>

Expr accepts the strings that follow it, evaluates them as a Tcl
expression, and returns the value.  The operators permitted in Tcl
expressions include all the standard math functions.  Mathematic
expressions always yield numeric results (integer or floating-point
values) while logical operations which return boolean (true or false)
results. 

<P>

Like regular arithmetic, the Tcl math expression consists of operators,
operands, and parentheses.  White space may be used between the operands,
operators and parentheses. 

<P>

An operand is:
<UL>
<LI>  An integer value.
<LI>  A floating point value.
<LI>  A $variable_name
<LI>  A text string.
</UL>
<P>

Where possible, operands are interpreted as integer values.  Integer
values may be specified in decimal (the normal case), in octal (if the
first character of the operand is 0), or in hexadecimal (if the first
two characters of the operand are 0x).
<P> 

Note that the octal and hexadecimal conversion takes place differently
in the expr command than in the Tcl substitution phase.
<P>

For example, in the Tcl substitution phase, a <B>\x32</B> (the ascii
code for the number 2) would be converted to an ascii "2" while
<B>0x32</B> would be converted within the <CODE>expr</CODE> command to a
decimal 50. 

<P>
This code is a true
statement: <P><CODE> expr \x32 == 0x2</CODE><P> It is true because
during the substitution phase the \x32 was replaced with an ascii "2",
while the 0x2 was passed to <CODE>expr</CODE> unchanged.  <CODE>Expr</CODE>
then interprets 0x2 as a hexadecimal 2,
compares decimal 2 to hexadecimal 2, and finds that
they are equal, which is obviously true. 

<P>
  If an operand does not have one of the integer formats given above,
then it is treated as a floating-point number if that is possible. 
Floating-point numbers may be specified in any of the ways accepted by
an ANSI-compliant C compiler.  For example, all of the following are
valid floating-point numbers: 2.1, 3., 6e4, 7.91e+16.  If no numeric
interpretation is possible, then an operand is left as a string (and
only a limited set of operators may be applied to it). 

<P>

If there are no parentheses to explicitly declare the order of
operations, then Tcl does arithmetic operations in a particular order,
just as you would do them on paper.  This is called the order of
precedence of the operators.  

<P>

As you see in the list below, the command <CODE> expr "5 + -3 * 4" </CODE> is evaluated in
this order:
<OL>
<LI> The unary minus sign in front of the 3 (3 is negative)
<LI> The multiplication sign (-3 is multiplied 4)
<LI> The plus sign ( add 5 to the product of -3 and 4)
</OL>
<P>
Operands may be specified in any of the following ways:
<UL>
<LI>          As  an  numeric  value, either integer or floating-
              point.
<LI>          As a Tcl variable, using standard $ notation.   The
              variable's value will be used as the operand.
</UL><P>

<B>OPERATORS</B><P>
       The valid operators are listed below, grouped in  decreasing order of precedence:
<DL><DT>
       -  +  ~  !          <DD>Unary minus, unary plus, bit-wise NOT,
                           logical NOT.  None of  these  operands
                           may be applied to string operands, and
                           bit-wise NOT may be  applied  only  to
                           integers.
<DT>
       *  /  %             <DD>Multiply,  divide, remainder.  None of
                           these  operands  may  be  applied   to
                           string  operands, and remainder may be
                           applied only to integers.  The remainder  will always have the same sign as
                           the  divisor  and  an  absolute  value
                           smaller than the divisor.
<DT>
       +  -                <DD>Add   and  subtract.   Valid  for  any
                           numeric operands.
<DT>
       &lt;&lt;  &gt;&gt;              <DD>Left and right shift.  Valid for integer operands only.
<DT>
       &amp;                   <DD>Bit-wise   AND.    Valid  for  integer
                           operands only.
<DT>
       ^                   <DD>Bit-wise  exclusive  OR.   Valid   for
                           integer operands only.
<DT>
       |                   <DD>Bit-wise   OR.    Valid   for  integer
                           operands only.
<DT>
       &amp;&amp;                  <DD>Logical AND.  Produces a 1  result  if
                           both  operands  are non-zero, 0 otherwise.  Valid for numeric operands only
                           (integers or floating-point).
<DT>
       ||                  <DD>Logical  OR.   Produces  a 0 result if
                           both operands are zero,  1  otherwise.
                           Valid for numeric operands only (integers or floating-point).
<DT>
       x?y:z               <DD>If-then-else, as in C.  If x evaluates
                           to  non-zero,  then  the result is the
                           value of y.  Otherwise the  result  is
                           the  value  of  z.  The x operand must
                           have a numeric value.
<P></UL>
<B>MATH FUNCTIONS</B><P>
      Tcl  supports  the  following  mathematical  functions  in
      expressions:
<PRE>
             acos        cos         hypot      sinh
             asin        cosh        log        sqrt
             atan        exp         log10      tan
             atan2       floor       pow        tanh
             ceil        fmod        sin
</PRE>
<B>TYPE CONVERSIONS</B><P>
	Tcl supports the following functions to convert from one
	representation of a number to another:
<PRE>
 	abs double int round </PRE>
The <CODE>expr</CODE> command supports most of the standard C library math calls.  

<P>

If you haven't hit the <B>Run Example</B> button yet, do so now, and examine
the results of the example.  Play around a bit with the example code, and
then go on to the next lesson.
:TEXT_END:

:CODE_START:
set X 100;
set Y 256;
set Z [expr "$Y + $X"]
set Z_LABEL "$Y plus $X is "

puts "$Z_LABEL $Z"
puts "The square root of $Y is [expr sqrt($Y)]\n"

puts "Because of the precedence rules \"5 + -3 * 4\"   is: [expr -3 * 4 + 5]"
puts "Because of the parentheses      \"(5 + -3) * 4\" is: [expr (5 + -3) * 4]"

puts "\n................. more examples of differences between  \" and \{"
puts {$Z_LABEL [expr $Y + $X]}
puts "$Z_LABEL {[expr $Y + $X]}"
puts "The command to add two numbers is: \[expr \$a + \$b]"
:TEXT_END:


