So, put the following source code in a test. Let us now see the basic structure of Tcl program, so that it will be easy for you to understand basic building blocks of the Tcl language. In Tcl, we use new line or semicolon to terminate the previous line of code. But semicolon is not necessary, if you are using newline for each command.
Comments are like helping text in your Tcl program and the interpreter ignores them. A Tcl identifier is a name used to identify a variable, function, or any other user-defined item. Thus Manpower and manpower are two different identifiers in Tcl. The following list shows a few of the reserved words in Tcl.
These reserved words may not be used as constant or variable or any other identifier names. A line containing only whitespace, possibly with a comment, is known as a blank line , and a Tcl interpreter totally ignores it.
Whitespace is the term used in Tcl to describe blanks, tabs, newline characters, and comments. Whitespace separates one part of a statement from another and enables the interpreter to identify where one element in a statement, such as puts, ends and the next element begins. As you know, Tcl is a Tool command language, commands are the most vital part of the language. Tcl commands are built in-to the language with each having its own predefined function.
These commands form the reserved words of the language and cannot be used for other variable naming. The advantage with these Tcl commands is that, you can define your own implementation for any of these commands to replace the original built-in functionality. Tcl command is actually a list of words, with the first word representing the command to be executed. The next words represent the arguments. As said before, we have used "" to group two words.
Here, stdout makes the program to print in the standard output device. In command substitutions, square brackets are used to evaluate the scripts inside the square brackets. A simple example to set a value to a variable and print it is shown below. These are commonly called escape sequences ; with each backslash, followed by a letter having its own meaning.
The primitive data-type of Tcl is string and often we can find quotes on Tcl as string only language. These primitive data-types in turn create composite data-types for list and associative array. Let's look into the details about each of the above. In Tcl, whether it is an integer number, boolean, floating point number, or a string.
When you want to use a variable, you can directly assign a value to it, there is no step of declaration in Tcl. There can be internal representations for these different types of objects. It can transform one data-type to another when required. The above statement will create a variable name myVariable and stores it as a string even though, we have not used double quotations. Now, if we try to make an arithmetic on the variable, it is automatically turned to an integer.
One important thing to note is that, these variables don't have any default values and must be assigned value before they are used. If we try to print using puts, the number is transformed into proper string.
Having two representations, internal and external, help Tcl to create complex data structures easily compared to other languages. Also, Tcl is more efficient due to its dynamic object nature. Unlike other languages, in Tcl, you need not include double quotes when it's only a single word. When we want to represent multiple strings, we can use either double quotes or curly braces.
List is nothing but a group of elements. A group of words either using double quotes or curly braces can be used to represent a simple list. Associative arrays have an index key that is not necessarily an integer. It is generally a string that acts like key value pairs. Tcl handles are commonly used to represent files and graphics objects. The following is an example where a file handle is created.
In Tcl, there is no concept of variable declaration. Once, a new variable name is encountered, Tcl will define a new variable. The name of variables can contain any characters and length. You can even have white spaces by enclosing the variable in curly braces, but it is not preferred. Tcl is a dynamically typed language. The value of the variable can be dynamically converted to the required type when required. For example, a number 5 that is stored as string will be converted to number when doing an arithmetic operation.
As you can see in the above example, expr is used for representing mathematical expression. The default precision of Tcl is 12 digits. In order to get floating point results, we should add at least a single decimal digit.
A simple example explains the above. In the above example, you can see three cases. In the first case, the dividend and the divisor are whole numbers and we get a whole number as result.
In the second case, the divisor alone is a decimal number and in the third case, the dividend is a decimal number. In both second and third cases, we get a decimal number as result. An operator is a symbol that tells the compiler to perform specific mathematical or logical manipulations.
This chapter will explain the arithmetic, relational, logical, bitwise, and ternary operators one by one. Following table shows all the arithmetic operators supported by Tcl language.
Following table shows all the relational operators supported by Tcl language. Following table shows all the logical operators supported by Tcl language. Bitwise operator works on bits and perform bit-by-bit operation. The Bitwise operators supported by Tcl language are listed in the following table.
Operator precedence determines the grouping of terms in an expression. This affects how an expression is evaluated. Certain operators have higher precedence than others; for example, the multiplication operator has higher precedence than the addition operator.
Here, operators with the highest precedence appear at the top of the table, those with the lowest appear at the bottom. Within an expression, higher precedence operators will be evaluated first. Decision making structures require that the programmer specifies one or more conditions to be evaluated or tested by the program, along with a statement or statements to be executed if the condition is determined to be true, and optionally, other statements to be executed if the condition is determined to be false.
An 'if' statement can be followed by an optional 'else' statement, which executes when the Boolean expression is false. We have covered conditional operator? The value of a '? If it is true, then Exp2 is evaluated and becomes the value of the entire '? An example is shown below. There may be a situation, where you need to execute a block of code several number of times. In general, statements are executed sequentially: The first statement in a function is executed first, followed by the second, and so on.
Programming languages provide various control structures that allow for more complicated execution paths. Repeats a statement or group of statements while a given condition is true.
It tests the condition before executing the loop body. Executes a sequence of statements multiple times and abbreviates the code that manages the loop variable. Loop control statements change execution from its normal sequence. When execution leaves a scope, all automatic objects that were created in that scope are destroyed. Terminates the loop or switch statement and transfers execution to the statement immediately following the loop or switch.
Causes the loop to skip the remainder of its body and immediately retest its condition prior to reiterating. A loop becomes infinite loop if a condition never becomes false. The while loop is traditionally used for this purpose.
You can make an endless loop by leaving the conditional expression as 1. When the conditional expression is absent, it is assumed to be true.
An array is a systematic arrangement of a group of elements using indices. The syntax for the conventional array is shown below. Though, array indices can be non-continuous like values specified for index 1 then index 10 and so on.
But, in case they are continuous, we can use array iteration to access elements of the array. A simple array iteration for printing elements of the array is shown below. In Tcl, all arrays by nature are associative. Arrays are stored and retrieved without any specific order. Associative arrays have an index that is not necessarily a number, and can be sparsely populated.
A simple example for associative array with non-number indices is shown below. You can use the indices of array to iterate through the associative array. These strings can contain alphanumeric character, just numbers, Boolean, or even binary data.
Tcl uses 16 bit unicode characters and alphanumeric characters can contain letters including non-Latin characters, number or punctuation. A character literal can be a plain character e. Compares string1 and string2 lexographically. Returns 0 if equal, -1 if string1 comes before string2, else 1. Return the index in findstring of the character after the word containing the character at index.
Return the index in findstring of the first character in the word containing the character at index. Scan command is used for parsing a string based to the format specifier.
Some examples are shown below. List is one of the basic data-type available in Tcl. It is used for representing an ordered collection of items. It can include different types of items in the same list. Further, a list can contain another list. An important thing that needs to be noted is that these lists are represented as strings completely and processed to form individual items when required.
So, avoid large lists and in such cases; use array. A dictionary is an arrangement for mapping values to keys. Procedures are nothing but code blocks with series of commands that provide a specific reusable functionality. It is used to avoid same code being repeated in multiple locations. Procedures are equivalent to the functions used in many programming languages and are made available in Tcl with the help of proc command.
Default arguments are used to provide default values that can be used if no value is provided. Packages are used for creating reusable units of code. A package consists of a collection of files that provide specific functionality. This collection of files is identified by a package name and can have multiple versions of same files.
The package can be a collection of Tcl scripts, binary library, or a combination of both. Package uses the concept of namespace to avoid collision of variable names and procedure names. Check out more in our next ' namespace ' tutorial. A package can be created with the help of minimum two files. One file contains the package code. Other file contains the index package file for declaring your package.
Create code for package inside a folder say HelloWorld. Let the file be named HelloWorld. Open tclsh. First two steps create the package. Namespace is a container for set of identifiers that is used to group variables and procedures. Namespaces are available from Tcl version 8. Before the introduction of the namespaces, there was single global scope. Now with namespaces, we have additional partitions of global scope. Namespaces are created using the namespace command.
In the above program, you can see there is a namespace with a variable myResult and a procedure Add. This makes it possible to create variables and procedures with the same names under different namespaces. You can see in the previous namespace examples, we use a lot of scope resolution operator and it's more complex to use.
We can avoid this by importing and exporting namespaces. You can remove an imported namespace by using forget subcommand. Tcl supports file handling with the help of the built in commands open, read, puts, gets, and close. Tcl uses the open command to open files in Tcl. Opens an existing text file for reading purpose and the file must exist. This is the default mode used when no accessMode is specified.
Opens a text file for writing, if it does not exist, then a new file is created else existing file is truncated. Opens a text file for writing in appending mode and file must exist.
Here, your program will start appending content in the existing file content. Opens a text file for reading and writing both. It first truncate the file to zero length if it exists otherwise create the file if it does not exist. It creates the file if it does not exist. The reading will start from the beginning, but writing can only be appended. Any file that has been opened by a program must be closed when the program finishes using that file.
In most cases, the files need not be closed explicitly; they are closed automatically when File objects are terminated automatically. When the above code is compiled and executed, it creates a new file input.
Error handling in Tcl is provided with the help of error and catch commands. The syntax for each of these commands is shown below. In the above error command syntax, message is the error message, info is set in the global variable errorInfo and code is set in the global variable errorCode.
In the above catch command syntax, script is the code to be executed, resultVarName is variable that holds the error or the result. The catch command returns 0 if there is no error, and 1 if there is an error. As you can see in the above example, we can create our own custom error messages. Similarly, it is possible to catch the error generated by Tcl.
Tcl provides a number of built-in functions procedures for various operations. Functions for creating namespaces and packages. Each of the above except for math and system functions are covered in earlier chapters.
Math and system built-in functions are explained below. App; About; Custom text. Fonts available at Fonts2u. Add to Dash. Add Docset to Dash. Examples: serif sans-serif fantasy cursive Andale Mono, monospace.
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