
- LISP Tutorial
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- Lisp Functions
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- Lisp Predicates
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- Lisp Sequences
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- LISP - every function
- LISP - some function
- LISP - notany function
- LISP - notevery function
- Lisp Lists
- LISP - Lists
- LISP - Accessing Elements of Lists
- LISP - Modifications to Lists
- LISP - Using mapcar on List
- LISP - Using mapc on List
- LISP - Using reduce on List
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- LISP - List vs Vectors
- LISP - Matrix Multiplication
- Lisp Vectors
- LISP - Vectors
- LISP - Creating Vectors
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- LISP - Vector Functions
- Lisp Set
- LISP - Set
- LISP - Adding elements to the Set
- LISP - Getting SubSet from a Set
- LISP - Set Difference
- LISP - Set Exclusive OR
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- LISP - Set Union
- LISP - Representing Set with HashTable
- LISP - List as Set vs HashTable as Set
- Lisp Tree
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- Lisp Hash Table
- LISP - Hash Table
- Adding Values to Hash Table
- Removing Values from Hash Table
- Updating Values of Hash Table
- Iterating Hash Table Entries
- Searching key in HashTable
- Checking Size of HashTable
- Using Custom Equality Check
- Lisp - Input − Output
- LISP - Input − Output
- LISP - Streams
- LISP - Reading Data from Streams
- LISP - Writing Data to Streams
- LISP - File I/O
- LISP - String I/O
- LISP - Formatting with Format
- LISP - Interactive I/O
- LISP - Error Handling
- LISP - Binary I/O
- Lisp - Structures
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- LISP - Structure Options
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- LISP - Applications and Best Practices
- Lisp - CLOS
- LISP - CLOS
- Lisp - Objects
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- LISP - Generic Functions
- LISP - Class Precedence
- LISP - Metaobject Protocol
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- LISP - Method Combinations
- LISP - Method Combinations
- LISP - :before Method Combination
- LISP - :primary Method Combination
- LISP - :after Method Combination
- LISP - :around Method Combination
- LISP - + Method Combination
- LISP - and Method Combination
- LISP - append Method Combination
- LISP Useful Resources
- Lisp - Quick Guide
- Lisp - Useful Resources
- Lisp - Discussion
Lisp - Reading Data From Streams
Understanding how to reading data from streams is very important in order to work with I/O in LISP. In this chapter, we'll explore various methods to read data from streams in various ways.
Core Input Functions
read() function
read() is a primary function to read data or LISP objects from streams.
read() function parses the input stream, interprets the characters read as LISP form and returns the corresponding LISP object.
This function can handle multiple type of data as inputs like symbols, numbers, lists and strings.
By default, read() method reads from *standard-input*. We can specify other streams as well using optional stream parameter.
read-char() function
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read-char() is a specialized function to read a single character from an input stream.
This function is useful in performing char-by-char processing.
read-line() function
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read-char() is a specialized function to read a line of text from an input stream upto new line character. New line character is included in the returned string.
This function returns the new line as a string.
read-byte() function
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read-byte() is a specialized function to read binary data from a stream.
This function reads a single byte from a stream and returns an integer.
peek-char() function
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peek-char() functions allows us to inspect next available character in the stream without actually consuming it.
This function is useful when we need to check what is coming as next in the stream.
Key Considerations
Standard Input
A special variable *standard-input* represents the default input stream, generally connected to the keyboard. read functions use *standard-input* by default but we can provide other stream as input as well.
EOF (End of File) handling
While reading a stream, we should handle EOF to mark end of stream. Most of the input functions accepts optional parameters as eof-error-p and eof-value in order to control how to handle EOF is handled.
eof-error-p − returns true if error occured when EOF is encountered.
eof-value − allows to return a value instead of error when EOF is encountered.
Error Handling
During I/O operations, error can popup in any form like invalid syntax, or error during I/O etc. Error should be handled properly in streams operation.
Example - Reading a File
In following example, we're reading a file test.txt lying in current folder with following content.
Welcome to LISP It is an easy to learn language.
main.lisp
; open test.txt in read mode (let ((input-stream (open "test.txt" :direction :input))) (when input-stream ; if file is readable (loop for line = (read-line input-stream nil) ; read each line using read-line function while line ; if line is present do (format t "Read line: ~a~%" line)) ; print the line (close input-stream))) ; close the stream
Output
When you execute the code, it returns the following result −
Read line: Welcome to LISP Read line: It is an easy to learn language.
Explanation
open function is used to open test.txt in read mode.
read-line function is used to read file line by line until nil is returned.
close function is used to close the file after reading it completely.