TCL Scripting For VLSI Part 2

Part 3: Control Structures and Expressions

If..Else

• The 'if' command is the basic conditional command in Tcl
• If an expression is true then execute one command body else execute another command body
• The second command body (the else clause) is optional
• The syntax of the command is
• if expression? body1 ?else? body2

Example: 

set x 10
if { $x>0} {
puts "x is greater than 0";
} else {
puts "x is lesser than 0";

}

Output: x is greater than 0

Switch Command

• The switch command is used to branch to one of many bodies depending on the value of an expression
• The general form of the command is :
• switch flags value pattern1 body1 pattern2 body2 ...
• any number of pattern body pairs can be specified
• all the pattern body pairs can be grouped into one argument
• switch flags value {pat1 body1 pat2 body2...}

Example:

set vlsi B;
switch $vlsi {
   A {
      puts "Good"
   }
   B {
      puts "Excellent"
   }
   C {
      puts "Not good"
   }
   default {
      puts "Invalid result"   }
}

Output: Excellent

• In the switch command there are four possible flags that determine how value is matched
• -exact: matches the value exactly to one of the patterns
• --glob: uses glob-style pattern matching (using wildcards like *,?)
• -regexp: uses regular expression pattern matching

For Loop

• The for command is a looping construct used to do operations repetitively.
• The syntax is:
• for (initialization} {condition} {increment} { 
• statement(s);
• }

The for command takes 4 parameters:

• The first argument is a command to initialize the loop
• The second argument is a Boolean expression that determines whether the loop body will execute
• The third argument is a command to increment the value of the variable executed after the loop body
• The fourth argument is the body of the loop

Example:
 # for loop execution
for { set i 10} {$i < 20} {incr i} { 
puts "value of a: $i"
}
Output:
value of i: 10
value of i: 11
value of i: 12
value of i: 13
value of i: 14
value of i: 15
value of i: 16
value of i: 17
value of i: 18
value of i: 19

While Loop

• while loop can also be used as a looping construct to execute operations repetitively.
• The syntax is:

while (condition} { 

 statement(s)
   }

Example: 
set i 0 
while {$i < 10} { 
incr i 
puts $i 
}
Output: 1 to 10
Foreach Loop

• foreach takes three arguments
• - the first is the name of a variable
• - the second is a list
• - the third is a Tcl script that forms the body of the loop
• foreach will execute the script once for each element of the list in order
  

set i 1
foreach value { 1 2 3 4 5  } {  
set i [expr $i+ $value] 
}
puts $i

Output: 16

Break and Continue Statements

• The break command causes the innermost enclosing looping command to terminate immediately
• The continue command causes only the current iteration of the innermost loop to be terminated, the loop continues with the next iteration

Example:

Break:

set a 11
while {$a < 20 } {
   puts "value of a: $a"
   incr a
   if { $a > 14} {
      break
   }
}

Output: 

value of a: 11

value of a: 12

value of a: 13

value of a: 14

Continue:

set a 11
while { $a < 15 } {
   if { $a == 15} {
      incr a
      continue
   }
   puts "value of a: $a"
   incr a     
}

Output:

value of a: 11

value of a: 12

value of a: 13

value of a: 14

Expr Command

• Expressions cannot be evaluated by the Tcl interpreter
• The expr command is used to parse and evaluate expressions in Tcl
• It receives the expression as its argument and returns the result after evaluation

Example:

set a [expr 10+20]  # stores expr result in 'a '
puts $a

Output: 30

Operators in TCL

There are basically 5 types of operators in the TCL. They are as follows:

1. Arithmetic Operator

2. Relational Operator

3. Logical Operator

4. Bitwise Operator

5. Ternary Operator

Part 4: String

• Unlike other languages, in Tcl, you need not include double quotes when it's only a single word. An example can be -

set a hello 
puts $a

When the above code is executed, it produces the following result -

hello

When we want to represent multiple strings, we can use either double quotes or curly braces. It is shown below

set a "hello world"

puts $a

set a {hello world}

puts $a

When the above code is executed, it produces the following result -

hello world
hello world

Format Command

• The format command is similar to the C printf function
• It formats a string according to a format specification
• The syntax of the format command is
• format spec value1 value2...
• The spec argument includes literals and keywords

Examples for format:

set x  4 

set y  5
puts [format "In octal: %0o" [expr $x+$y]] 
puts [format "In hexadecimal: %0x" [expr $x+$y]] 
puts [format "In decimal: %d" [expr $x+$y]]

• The literals are placed in the result as it is
• The keyword indicates how to format the corresponding argument
• The keywords are introduced with percent sign % followed by zero or more modifiers and terminate with a conversion specifier
• Example keywords include %f for floating point, %d for integer, and %s for string format
• puts [ format "The square root of 10 is %3f" [expr sqrt (10)]]
• Output: The square root of 10 is 3.162

Scan Command

• The scan command parses a string according to a format specification and assigns values to variables
• The syntax of the command is:
• scan string format var var var...
• It returns the number of successful conversions it made

String Command

• The string command is a collection of operations one can perform on strings
• The first argument to string determines the operation

String Compare: 

% string compare "vlsi" "vlsi"
0
% string compare "vlsi" "4freshers"
-1
% string compare "vlsi4freshers" "vlsi"
1

String equal

• The syntax of the command is
• string equal ?-nocase ? str1 str2
• Compares strings and returns 1 if they are the same or 0 otherwise
• If -nocase is used it makes a case insensitive comparison
• puts [string equal -nocase "VLSI" "vlsi"]
• Output: 1

String First

• The syntax of the command is
• string first str1 str2
• Returns the index in str2 of the first occurrence of str1
• Returns -1 if str1 is not found
• puts [string first "vlsi" "abcdvlsi"]
• index value of occurrence of 'vlsi' in the second string)
• output: 4

String last

• The syntax of the command is
• string last str1 str2
• Returns the index in str2 of the last occurrence of str1
• Returns -1 if str1 is not found

String Index

• The syntax of the command is
• string index string index
• Returns the character at the specified index. If index is less than 0 or greater than or equal to the length of the string, an empty string is returned.
• An index counts from 0. Use keyword 'end' for the last character
• puts [string index "vlsi" 2]
• output: s

String Length

• The syntax of the command is
• string length string
• Return the number of characters in the string
• puts [ string length "vlsi"]
• output: 4

String match

• The syntax of the command is
• string match pattern str
• Returns 1 if str matches the pattern else 0.
• When there is the need for more powerful string matching capabilities, string match can be used in place of string equal, because instead to compare two strings, the command compares a string against a pattern.
• % string match {[0-9]} 4. Output: 1
• % string match vlsi* vlsi. Output: 1

String range

• The syntax of the command is
• string range str i j
• returns the range of characters in str from i to j
• puts [ string range "vlsi4freshers" 1 3]
• Output: lsi 

append: This command is used to append the string value to any string.

Example:

set a "vlsi"

append a 4freshers
puts $a
vlsi4freshers

String replace

• Returns a new string created by replacing characters first through last with newstr or nothing.

String tolower

• Returns string in lowercase.

String toupper

• Returns string in uppercase.

String trim

• Trims the characters in chars from both ends of string. Chars defaults to whitespace. 

String trimleft

• Trims the characters in chars from the beginning of string.

String trimright

• Trims the characters in chars from the end of string. Chars defaults to whitespaces.

Lists

• A Tcl list is a sequence of values
• A list has its elements separated by white space
• Braces or quotes can be used to group words with white space into a single list element
• Tcl lists are just strings with a special interpretation
• Example: A List
• {a b c d e f}
• Tcl list is string consisting of zero or more space separated words.

Some Examples of lists:

• set colorList1 {red black orange}
• set colorList2 [list red white blue]
• set colorList3 [split "red_grey_blue" _]

List Commands

• list command is used to create a list
• list arg1 arg2 arg3 ....
• The list command constructs a list out of its arguments so that there is one list element for each argument
• Example: The list command

set x {1 2 3}

set y vlsi 
set list1 [list $x " a b" $y] 
puts $list1

Output: {1 2 3} {a b} vlsi

lindex

• The lindex command returns the ith element from the list 
• lindex list l
• set list1 [a b c d} 
• puts [lindex $list1 3]
• Output: d

llength

• llength returns the number of elements in the list 
• llength list
• set list1 [a b c d} 
• puts [llength $list1]
• Output: 4

lrange

• The Irange command returns the ith through jth elements from list 
• Irange list i j
• set list { a b c d e} 
• puts [ Irange $list 1 3]
• Output: b c d

lappend

• The lappend appends elements to the value of listvar
•  lappend listvar arg arg...
• set list1 {A B} 
• lappend list1 a b c d
• puts $list1
• Output: A B a b c d

linsert

• linsert inserts elements into list before the element at position index 
• linsert list index arg arg..
• set list1 {A B C} 
• set a [linsert $list1 1 x y z] 
• puts $a
• Output: A x y z B C

lreplace

• Ireplace replaces elements i through j of list with the args 
• Ireplace list i j arg arg...
• set list1 {A B} 
• set a [Ireplace $list1 1 2 x y z] 
• puts $a
• Output: A x y z

lsearch

• Isearch returns the index of the element in list that matches the value according to the mode, which can be -exact, -glob or -regexp
• Isearch ?mode? list value
• -glob is the default search
• Returns -1 if not found
• set list1 {a b c d} 
• set a [Isearch $list1 d] 
• puts $a
• Output: 3

Concat

• concat command joins multiple lists together into one list 
• concat list, list...
• set a {a b}
• set y [ concat 0 $a 2] 
• puts $y
• Output: 0 a b 2

join

• join command merges the elements of a list together by separating them with joinstring
• join list joinstring
• set a [join { 1 {2 3} {4 5 6 7 8}} :] 
• puts $a
• Output: 1:2 3: 4 5 6 7 8

split

• split command splits a string up into list elements using the characters in splitchars as boundaries between list elements
• split string splitchars
• set a [split x:y:z:] 
• puts $a
• Output:  x y z

lreverse

• This command is used to reverse the list of string.
• % lreverse " x y z "
• Output: z y x

lshort

• This command is used to arrange the list in alphabetical order.
• % lsort " c j w b a "
• Output: a b c j w

Arrays

• The array command provides information about the elements currently defined for an array variable
• The first argument to the array command is various information like size, elements etc
• The final argument to the array command is the name of the array being queried
• set ArrayName(Index) value

array size

• array size returns the number of elements in the array

set a(0) 1

set a(1) 2

set a(2) 3

set a(3) 4

puts [array size a]

Output: 4

array names

• array names returns the list of the names of the elements in the array

set a(0) 1

set a(1) 2
set a(2) 3

set a(3) 4

puts [ array names a]

Output: 0 1 2 3

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