This command is for reading the file line by line. Now we decide that we need to see the entire file. For that we need the read command. But remember that if we have read one line before, the read command will read only from the second line.
For resetting the channel to the first position, we need the seek command. The offset and origin arguments specify the position at which the next read or write will occur for channelId. Offset must be an integer which may be negative and origin must be one of the following - start, current or end.
Now we have to read the file contents. So we move on to If numChars is not specified, the whole file is read and its content will be returned. You can read a file and then get all the lines into a list with every line as an item in the list. This can be done by On to the example - for the last time.
That's about it with file handling. How do I read and write files in Tcl. One way to get file data in Tcl is to 'slurp' up the file into a text variable. This works really well if the files are known to be small.
Slurp up the data file. Checks if the values of two operands are equal or not, if values are not equal then condition becomes true. Checks if the value of left operand is greater than the value of right operand, if yes then condition becomes true. Checks if the value of left operand is less than the value of right operand, if yes then condition becomes true.
Checks if the value of left operand is greater than or equal to the value of right operand, if yes then condition becomes true. Checks if the value of left operand is less than or equal to the value of right operand, if yes then condition becomes true. Called Logical OR Operator.
If any of the two operands is non-zero, then condition becomes true. 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.
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.
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