**J**

This

**programming language**may be used to instruct a computer to perform a task.

Official website |
---|

Execution method: | Interpreted |
---|---|

Type safety: | Safe |

Type strength: | Strong (but regular) |

Type checking: | Dynamic |

Lang tag(s): | j |

See Also: |

## Contents

## The J language[edit]

A frequent reaction when one first encounters a J program is often something along the lines of "that's cheating". The thought here is that the problem could not possibly be that simple, so -- instead -- the issue must be that J was specifically designed to tackle that problem.

The flip side of this issue is that J is a dialect of APL -- a language whose development started in the 1950s and which was implemented in the early 1960s. And, originally, APL was designed as a language to describe computer architecture. The implementation as a programming language was motivated by its original successes in documenting computer hardware and instructions, and the relative simplicity of its concepts.

## Introduction[edit]

J is a notational programming language designed for interactive use.

It is an array language; data is universally structured as rectangular arrays.

It is a functional language; creation and composition of functions is emphasized.

Object-module and imperative techniques are supported, but not required.

The J programming language was designed and developed by Ken Iverson and Roger Hui. It is a closely related successor to APL, also by Iverson which itself was a successor to the notation Ken Iverson used to teach his classes about computers in the 1950s.

The notation draws heavily from concepts of Abstract algebra and Tensor calculus, simplified for describing computer architecture and design to a pragmatic business audience. (The ideas themselves are simple, but for some reason the topics scare most teachers.)

## Reading J[edit]

J is meant to be read with the aid of a computer. J sentences are single lines and trying variations and simplifications of an expression is common practice. The first step in understanding any J sentence is to understand the data you started with and the data which resulted. When learning how a J sentence works, you can also try simpler sentences with the same data or perhaps related data. When trying to understand contexts that use large data structures, it can often be wise to investigate small, representative samples until you understand how the code works.

Unless you attend an institution which has made a J interpreter available to you through your web browser (or preinstalled on your machine), if you want to see how J works you should probably install a copy of J -- or you can try one of the "try me" links, below. If you want to understand how to experiment with alternative expressions you should probably also be studying some of its documentation.

For example, the phrase `(+/ % #)`

finds the average of a list of numbers.

(+/ % #) 1 2 3

2

To understand how this works, you might try working with simpler sentences and their variations.

+/ 1 2 3

6

+/4 5 6

15

# 1 2 3

3

# 2 3 4

3

6 % 3

2

15 % 3

5

(+/ % #) 4 5 6

5

By themselves, these experiments mean nothing, but if you know that +/ was finding the sum of a list and # was finding the length of a list and that % was dividing the two quantities (and looks almost like one of the old school division symbols) then these experiments might help confirm that you have understood things properly.

## Some Perspective[edit]

If you wish to use J you will also have to learn a few grammatical rules (J's parser has 9 reduction rules and "shift" and "accept" - the above examples use four of those rules). J verbs have two definitions - a single argument "monadic" definition and a two argument "dyadic" definition. These terms are borrowed from music and are distinct from Haskell's use of the word "monad". The dyadic definitions are in some sense related to LISP's "cons cell" but are implemented as grammar rather than data structure, and are a pervasive part of the language.

Another pervasive feature of the language is rank.

The language represents capabilities of hardware. For example, if language did not have an internal stack, a word's definition could not be used during the execution of that word. All current J implementations support recursion, but in some sense this is a convenience, and it's reasonable to imagine J implementations which do not (perhaps in a "compile to silicon" implementation).

Perhaps also worth noting is that when thinking about J programs, it can be convenient to think of an instance of an array as a type. This is somewhat different from the usual treatment of type (where all potential values in a context are treated as a type).

## J on RosettaCode[edit]

Discussion of the goals of the J community on RC and general guidelines for presenting J solutions takes place at House Style.

## Jedi on RosettaCode[edit]

- Roger Hui: contributions, J wiki
- Tracy Harms: contributions, J wiki
- Dan Bron: contributions, J wiki
- Arie Groeneveld: contributions
- Raul Miller: contributions, J wiki
- Jose Quintana: contributions, J wiki
- Ric Sherlock: contributions, J wiki
- Avmich: contributions
- VZC: contributions
- Alex 'bathala' Rufon: contributions, J wiki
- David Lambert:contributions
- JimTheriot: contributions
- Devon McCormick: contributions

## Try me[edit]

Want to try one of those cryptic J lines you see peppered through RC? Try pasting it into this browser-based implementation of J. Here's a short video intro, for people who would prefer some guidance.

If you want to be a bit more interactive, and get some guidance from J gurus, you can join the actual J IRC channel on Freenode, #jsoftware. Buubot and several other J eval bots run there. If you don't have an IRC client you can try freenode's web interface (or just give it a quick spin). More details about the J IRC community is available.

If any of that piques your interest, and you want to explore a little more, you can download J and join the J forums.

If you have problems executing any of the J code here on Rosetta, please make a note of it either on the task page itself, on the talk page, or on the appropriate J forum, whichever is best. It might be that there's a version dependency that needs to be documented, or you might have found an actual bug.

## Todo[edit]

Reports:Tasks_not_implemented_in_J

## Subcategories

This category has the following 3 subcategories, out of 3 total.

## Pages in category "J"

The following 1,147 pages are in this category, out of 1,147 total.

### A

- A* search algorithm
- A+B
- Abbreviations, automatic
- Abbreviations, easy
- Abbreviations, simple
- ABC problem
- Abelian sandpile model
- Abelian sandpile model/Identity
- Abstract type
- Abundant odd numbers
- Abundant, deficient and perfect number classifications
- Accumulator factory
- Achilles numbers
- Ackermann function
- Active object
- Add a variable to a class instance at runtime
- Additive primes
- Address of a variable
- AKS test for primes
- Algebraic data types
- Align columns
- Aliquot sequence classifications
- Almkvist-Giullera formula for pi
- Almost prime
- Amb
- Amicable pairs
- Anagrams
- Anagrams/Deranged anagrams
- Angle difference between two bearings
- Angles (geometric), normalization and conversion
- Animate a pendulum
- Animation
- Anonymous recursion
- Anti-primes
- Append a record to the end of a text file
- Apply a callback to an array
- Apply a digital filter (direct form II transposed)
- Approximate equality
- Arbitrary-precision integers (included)
- Archimedean spiral
- Arena storage pool
- Arithmetic coding/As a generalized change of radix
- Arithmetic evaluation
- Arithmetic-geometric mean
- Arithmetic-geometric mean/Calculate Pi
- Arithmetic/Complex
- Arithmetic/Integer
- Arithmetic/Rational
- Array concatenation
- Array length
- Arrays
- Ascending primes
- ASCII art diagram converter
- Aspect oriented programming
- Assertions
- Assertions in design by contract
- Associative array/Creation
- Associative array/Iteration
- Associative array/Merging
- Attractive numbers
- Average loop length
- Averages/Arithmetic mean
- Averages/Mean angle
- Averages/Mean time of day
- Averages/Median
- Averages/Mode
- Averages/Pythagorean means
- Averages/Root mean square
- Averages/Simple moving average
- AVL tree

### B

- Babbage problem
- Babylonian spiral
- Bacon cipher
- Balanced brackets
- Balanced ternary
- Banker's algorithm
- Barnsley fern
- Base64 decode data
- Base64 encode data
- Bell numbers
- Benford's law
- Bernoulli numbers
- Best shuffle
- Bilinear interpolation
- Bin given limits
- Binary digits
- Binary search
- Binary strings
- Bioinformatics/base count
- Bioinformatics/Sequence mutation
- Biorhythms
- Birthday problem
- Bitmap
- Bitmap/Bresenham's line algorithm
- Bitmap/Bézier curves/Cubic
- Bitmap/Bézier curves/Quadratic
- Bitmap/Flood fill
- Bitmap/Histogram
- Bitmap/Midpoint circle algorithm
- Bitmap/Read a PPM file
- Bitmap/Write a PPM file
- Bitwise IO
- Bitwise operations
- Boolean values
- Box the compass
- Brace expansion
- Brazilian numbers
- Break OO privacy
- Brilliant numbers
- Brownian tree
- Bulls and cows
- Bulls and cows/Player
- Burrows–Wheeler transform

### C

- Caesar cipher
- Calculating the value of e
- Calendar
- Calendar - for "REAL" programmers
- Calkin-Wilf sequence
- Call a foreign-language function
- Call a function
- Call a function in a shared library
- Call an object method
- Canny edge detector
- Canonicalize CIDR
- Cantor set
- Card shuffles
- Carmichael 3 strong pseudoprimes
- Cartesian product of two or more lists
- Case-sensitivity of identifiers
- Casting out nines
- Catalan numbers
- Catalan numbers/Pascal's triangle
- Catamorphism
- Catmull–Clark subdivision surface
- Chaocipher
- Chaos game
- Character codes
- Chebyshev coefficients
- Check Machin-like formulas
- Check output device is a terminal
- Check that file exists
- Checkpoint synchronization
- Chernick's Carmichael numbers
- Cheryl's birthday
- Chinese remainder theorem
- Chinese zodiac
- Cholesky decomposition
- Chowla numbers
- Church numerals
- Cipolla's algorithm
- Circles of given radius through two points
- Circular primes
- Cistercian numerals
- Classes
- Closest-pair problem
- Closures/Value capture
- Collections
- Color of a screen pixel
- Color quantization
- Color wheel
- Colorful numbers
- Colour bars/Display
- Colour pinstripe/Display
- Combinations
- Combinations and permutations
- Combinations with repetitions
- Comma quibbling
- Command-line arguments
- Commatizing numbers
- Comments
- Common sorted list
- Compare a list of strings
- Compare length of two strings
- Compare sorting algorithms' performance
- Compile-time calculation
- Compiler/AST interpreter
- Compiler/code generator
- Compiler/lexical analyzer
- Compiler/syntax analyzer
- Compiler/virtual machine interpreter
- Composite numbers k with no single digit factors whose factors are all substrings of k
- Compound data type
- Concurrent computing
- Conditional structures
- Conjugate transpose
- Constrained genericity
- Constrained random points on a circle
- Continued fraction
- Continued fraction/Arithmetic/Construct from rational number
- Continued fraction/Arithmetic/G(matrix ng, continued fraction n)
- Convert decimal number to rational
- Convert seconds to compound duration
- Convex hull
- Conway's Game of Life
- Coprimes
- Copy a string
- Count in factors
- Count in octal
- Count occurrences of a substring
- Count the coins
- Cousin primes
- Cramer's rule
- CRC-32
- Create a file
- Create a two-dimensional array at runtime
- Create an HTML table
- Create an object/Native demonstration
- CSV data manipulation
- CSV to HTML translation
- Cuban primes
- Cumulative standard deviation
- Currency
- Currying
- Curve that touches three points
- CUSIP
- Cut a rectangle
- Cycle detection
- Cyclotomic polynomial

### D

- Damm algorithm
- Date format
- Date manipulation
- Day of the week
- Days between dates
- De Bruijn sequences
- Deal cards for FreeCell
- Death Star
- Deceptive numbers
- Decimal floating point number to binary
- Decision tables
- Deconvolution/1D
- Deconvolution/2D+
- Deepcopy
- Define a primitive data type
- Delegates
- Delete a file
- Deming's Funnel
- Department numbers
- Descending primes
- Detect division by zero
- Determinant and permanent
- Determine if a string has all the same characters
- Determine if a string has all unique characters
- Determine if a string is collapsible
- Determine if a string is numeric
- Determine if a string is squeezable
- Dice game probabilities
- Digit fifth powers
- Digital root
- Digital root/Multiplicative digital root
- Dijkstra's algorithm
- Dinesman's multiple-dwelling problem
- Dining philosophers
- Discordian date
- Discrete Fourier transform
- Display a linear combination
- Display an outline as a nested table
- Distinct power numbers
- Diversity prediction theorem
- DNS query
- Documentation
- Dot product
- Doubly-linked list/Definition
- Doubly-linked list/Element definition
- Doubly-linked list/Element insertion
- Doubly-linked list/Traversal
- Dragon curve
- Draw a clock
- Draw a cuboid
- Draw a pixel
- Draw a rotating cube
- Draw a sphere
- Duffinian numbers
- Dutch national flag problem
- Dynamic variable names

### E

- Eban numbers
- Egyptian division
- Egyptian fractions
- EKG sequence convergence
- Element-wise operations
- Elementary cellular automaton
- Elementary cellular automaton/Infinite length
- Elementary cellular automaton/Random Number Generator
- Elliptic curve arithmetic
- Emirp primes
- Empty directory
- Empty program
- Empty string
- Enforced immutability
- Entropy
- Entropy/Narcissist
- Enumerations
- Environment variables
- Equal prime and composite sums
- Equilibrium index
- Esthetic numbers
- Ethiopian multiplication
- Euler method
- Euler's identity
- Euler's sum of powers conjecture
- Evaluate binomial coefficients
- Even or odd
- Evolutionary algorithm
- Exceptions
- Exceptions/Catch an exception thrown in a nested call
- Executable library
- Execute a Markov algorithm
- Execute a system command
- Execute Brain****
- Execute HQ9+
- Execute SNUSP
- Exponentiation operator
- Exponentiation order
- Exponentiation with infix operators in (or operating on) the base
- Extend your language
- Extensible prime generator
- External sort
- Extra primes
- Extract file extension
- Extreme floating point values

### F

- Faces from a mesh
- Factorial
- Factorial base numbers indexing permutations of a collection
- Factorions
- Factors of a Mersenne number
- Factors of an integer
- Fairshare between two and more
- Farey sequence
- Fast Fourier transform
- FASTA format
- Faulhaber's formula
- Faulhaber's triangle
- Feigenbaum constant calculation
- Fermat numbers
- Fibonacci n-step number sequences
- Fibonacci sequence
- Fibonacci word
- Fibonacci word/fractal
- File extension is in extensions list
- File input/output
- File modification time
- File size
- File size distribution
- Filter
- Find common directory path
- Find first and last set bit of a long integer
- Find first missing positive
- Find largest left truncatable prime in a given base
- Find limit of recursion
- Find palindromic numbers in both binary and ternary bases
- Find prime numbers of the form n*n*n+2
- Find squares n where n+1 is prime
- Find the intersection of a line with a plane
- Find the intersection of two lines
- Find the last Sunday of each month
- Find the missing permutation
- First 9 prime Fibonacci number
- First class environments
- First perfect square in base n with n unique digits
- First power of 2 that has leading decimal digits of 12
- First-class functions
- First-class functions/Use numbers analogously
- Five weekends
- Fivenum
- Fixed length records
- FizzBuzz
- Flatten a list
- Flipping bits game
- Flow-control structures
- Floyd's triangle
- Floyd-Warshall algorithm
- Forest fire
- Fork
- Formal power series
- Formatted numeric output
- Forward difference
- Four bit adder
- Four is magic
- Four sides of square
- Fractal tree
- Fraction reduction
- Fractran
- Free polyominoes enumeration
- Frobenius numbers
- FTP
- Function composition
- Function definition
- Function frequency
- Function prototype
- Functional coverage tree
- Fusc sequence

### G

- Galton box animation
- Gamma function
- Gapful numbers
- Gauss-Jordan matrix inversion
- Gaussian elimination
- General FizzBuzz
- Generalised floating point addition
- Generate Chess960 starting position
- Generate lower case ASCII alphabet
- Generate random chess position
- Generator/Exponential
- Generic swap
- Geometric algebra
- Get system command output
- Globally replace text in several files
- Go Fish
- Goldbach's comet
- Graph colouring
- Gray code
- Grayscale image
- Greatest common divisor
- Greatest element of a list
- Greatest subsequential sum
- Greyscale bars/Display
- Guess the number
- Guess the number/With feedback
- Guess the number/With feedback (player)
- GUI component interaction
- GUI enabling/disabling of controls

### H

- Hailstone sequence
- Hamming numbers
- Happy numbers
- Harshad or Niven series
- Hash from two arrays
- Hash join
- Haversine formula
- Hello world/Graphical
- Hello world/Line printer
- Hello world/Newbie
- Hello world/Newline omission
- Hello world/Standard error
- Hello world/Text
- Hello world/Web server
- Here document
- Heronian triangles
- Hickerson series of almost integers
- Higher-order functions
- Hilbert curve
- History variables
- Hofstadter Figure-Figure sequences
- Hofstadter Q sequence
- Hofstadter-Conway $10,000 sequence
- Holidays related to Easter
- Home primes
- Honeycombs
- Horizontal sundial calculations
- Horner's rule for polynomial evaluation
- Host introspection
- Hostname
- Hough transform
- HTTP
- HTTPS
- Huffman coding
- Humble numbers

### I

- I before E except after C
- IBAN
- Identity matrix
- Idiomatically determine all the characters that can be used for symbols
- Idiomatically determine all the lowercase and uppercase letters
- Image convolution
- Image noise
- Imaginary base numbers
- Implicit type conversion
- Include a file
- Increasing gaps between consecutive Niven numbers
- Increment a numerical string
- Index finite lists of positive integers
- Infinity
- Inheritance/Multiple
- Inheritance/Single
- Input loop
- Input/Output for lines of text
- Input/Output for pairs of numbers
- Integer comparison
- Integer overflow
- Integer roots
- Integer sequence
- Interactive programming
- Intersecting number wheels
- Introspection
- Inverted index
- Inverted syntax
- ISBN13 check digit
- Isqrt (integer square root) of X
- Iterated digits squaring
- Iterators

### J

### K

- K-d tree
- K-means++ clustering
- Kahan summation
- Kaprekar numbers
- Kernighans large earthquake problem
- Knapsack problem/0-1
- Knapsack problem/Bounded
- Knapsack problem/Continuous
- Knapsack problem/Unbounded
- Knight's tour
- Knuth shuffle
- Knuth's algorithm S
- Knuth's power tree
- Koch curve
- Kolakoski sequence
- Kosaraju
- Kronecker product
- Kronecker product based fractals

### L

- Lah numbers
- Langton's ant
- Largest five adjacent number
- Largest int from concatenated ints
- Largest number divisible by its digits
- Largest proper divisor of n
- Last Friday of each month
- Last letter-first letter
- Latin Squares in reduced form
- Law of cosines - triples
- Leap year
- Least common multiple
- Left factorials
- Legendre prime counting function
- Leonardo numbers
- Letter frequency
- Levenshtein distance
- Levenshtein distance/Alignment
- Linear congruential generator
- Linux CPU utilization
- List comprehensions
- List rooted trees
- Literals/Floating point
- Literals/Integer
- Literals/String
- Logical operations
- Long literals, with continuations
- Long multiplication
- Long primes
- Long year
- Longest common prefix
- Longest common subsequence
- Longest common substring
- Longest common suffix
- Longest increasing subsequence
- Longest string challenge
- Look-and-say sequence
- Loop over multiple arrays simultaneously
- Loops/Break
- Loops/Continue
- Loops/Do-while
- Loops/Downward for
- Loops/For
- Loops/For with a specified step
- Loops/Foreach
- Loops/Increment loop index within loop body
- Loops/Infinite
- Loops/N plus one half
- Loops/Nested
- Loops/While
- Loops/With multiple ranges
- Loops/Wrong ranges
- LU decomposition
- Lucas-Lehmer test
- Lucky and even lucky numbers
- Ludic numbers
- Luhn test of credit card numbers
- Lychrel numbers
- LZW compression

### M

- MAC Vendor Lookup
- Mad Libs
- Magic 8-ball
- Magic squares of doubly even order
- Magic squares of odd order
- Magic squares of singly even order
- Magnanimous numbers
- Make directory path
- Man or boy test
- Mandelbrot set
- Map range
- Markov chain text generator
- Matrix chain multiplication
- Matrix digital rain
- Matrix multiplication
- Matrix transposition
- Matrix with two diagonals
- Matrix-exponentiation operator
- Maximum difference between adjacent elements of list
- Maximum triangle path sum
- Mayan calendar
- Mayan numerals
- Maze generation
- Maze solving
- McNuggets problem
- MD4
- MD5
- MD5/Implementation
- Median filter
- Memory allocation
- Memory layout of a data structure
- Menu
- Merge and aggregate datasets
- Mertens function
- Metallic ratios
- Metaprogramming
- Metered concurrency
- Metronome
- Mian-Chowla sequence
- Middle three digits
- Miller–Rabin primality test
- Mind boggling card trick
- Minesweeper game
- Minimal steps down to 1
- Minimum multiple of m where digital sum equals m
- Minimum number of cells after, before, above and below NxN squares
- Minimum positive multiple in base 10 using only 0 and 1
- Minkowski question-mark function
- Modified random distribution
- Modular arithmetic
- Modular exponentiation
- Modular inverse
- Modulinos
- Monads/List monad
- Monads/Maybe monad
- Monads/Writer monad
- Monte Carlo methods
- Monty Hall problem
- Morpion solitaire
- Morse code
- Mosaic matrix
- Most frequent k chars distance
- Motzkin numbers
- Move-to-front algorithm
- Multi-dimensional array
- Multifactorial
- Multiline shebang
- Multiple distinct objects
- Multiple regression
- Multiplication tables
- Multiplicative order
- Multisplit
- Munchausen numbers
- Munching squares
- Musical scale
- Mutual recursion
- Möbius function

### N

- N'th
- N-body problem
- N-queens minimum and knights and bishops
- N-queens problem
- N-smooth numbers
- Named parameters
- Names to numbers
- Naming conventions
- Narcissist
- Narcissistic decimal number
- Native shebang
- Natural sorting
- Nautical bell
- Negative base numbers
- Nested function
- Nested templated data
- Next highest int from digits
- Nice primes
- Nim game
- Nimber arithmetic
- Non-continuous subsequences
- Non-decimal radices/Convert
- Non-decimal radices/Input
- Non-decimal radices/Output
- Non-transitive dice
- Nonoblock
- Nth root
- Null object
- Number names
- Number reversal game
- Numbers divisible by their individual digits, but not by the product of their digits.
- Numbers which binary and ternary digit sum are prime
- Numeric error propagation
- Numerical integration
- Numerical integration/Adaptive Simpson's method
- Numerical integration/Gauss-Legendre Quadrature

### O

- Object serialization
- Odd word problem
- Old lady swallowed a fly
- Old Russian measure of length
- One of n lines in a file
- One-dimensional cellular automata
- OpenGL
- OpenGL/Utah Teapot
- Operator precedence
- Optional parameters
- Order disjoint list items
- Order two numerical lists
- Ordered partitions
- Ordered words

### P

- Padovan sequence
- Palindrome detection
- Palindromic gapful numbers
- Pangram checker
- Paraffins
- Parallel calculations
- Parametric polymorphism
- Parse an IP Address
- Parse command-line arguments
- Parsing/RPN calculator algorithm
- Parsing/RPN to infix conversion
- Parsing/Shunting-yard algorithm
- Partial function application
- Particle swarm optimization
- Partition an integer x into n primes
- Partition function P
- Pascal matrix generation
- Pascal's triangle
- Pascal's triangle/Puzzle
- Password generator
- Pathological floating point problems
- Peano curve
- Pell numbers
- Pell's equation
- Penney's game
- Pentagram
- Percentage difference between images
- Percolation/Mean cluster density
- Percolation/Mean run density
- Percolation/Site percolation
- Perfect numbers
- Perfect shuffle
- Perfect totient numbers
- Perlin noise
- Permutation test
- Permutations
- Permutations by swapping
- Permutations with repetitions
- Permutations/Derangements
- Permutations/Rank of a permutation
- Pernicious numbers
- Phrase reversals
- Pi
- Pick random element
- Pierpont primes
- Pig the dice game
- Pig the dice game/Player
- Pinstripe/Display
- Piprimes
- Plasma effect
- Playfair cipher
- Playing cards
- Plot coordinate pairs
- Pointers and references
- Poker hand analyser
- Polymorphic copy
- Polymorphism
- Polynomial long division
- Polynomial regression
- Polynomial synthetic division
- Polyspiral
- Population count
- Power set
- Practical numbers
- Pragmatic directives
- Price fraction
- Primality by trial division
- Primality by Wilson's theorem
- Prime conspiracy
- Prime decomposition
- Prime numbers p for which the sum of primes less than or equal to p is prime
- Prime triangle
- Primes - allocate descendants to their ancestors
- Primes with digits in nondecreasing order
- Primorial numbers
- Priority queue
- Probabilistic choice
- Problem of Apollonius
- Program name
- Program termination
- Proof
- Proper divisors
- Pseudo-random numbers/Middle-square method
- Pythagoras tree
- Pythagorean quadruples
- Pythagorean triples

### Q

### R

- Ramanujan's constant
- Ramer-Douglas-Peucker line simplification
- Ramsey's theorem
- Random Latin squares
- Random number generator (device)
- Random number generator (included)
- Random numbers
- Range consolidation
- Range expansion
- Range extraction
- Ranking methods
- Rare numbers
- Rate counter
- Ray-casting algorithm
- Read a configuration file
- Read a file character by character/UTF8
- Read a file line by line
- Read a specific line from a file
- Read entire file
- Real constants and functions
- Recaman's sequence
- Recursive descent parser generator
- Reduced row echelon form
- Reflection/Get source
- Reflection/List methods
- Reflection/List properties
- Regular expressions
- Remove duplicate elements
- Remove lines from a file
- Rename a file
- Rep-string
- Repeat
- Repeat a string
- Resistor mesh
- Respond to an unknown method call
- Return multiple values
- Reverse a string
- Reverse the gender of a string
- Reverse words in a string
- Rhonda numbers
- Robots
- Rock-paper-scissors
- Roman numerals/Decode
- Roman numerals/Encode
- Roots of a function
- Roots of a quadratic function
- Roots of unity
- Rosetta Code/Count examples
- Rosetta Code/Find unimplemented tasks
- Rosetta Code/Fix code tags
- Rosetta Code/Rank languages by popularity
- Rot-13
- RPG attributes generator
- RSA code
- Run-length encoding
- Runge-Kutta method
- Runtime evaluation
- Runtime evaluation/In an environment
- Ruth-Aaron numbers

### S

- S-expressions
- Safe addition
- Safe primes and unsafe primes
- Sailors, coconuts and a monkey problem
- Same fringe
- Sattolo cycle
- Scope modifiers
- Scope/Function names and labels
- Search a list
- Search a list of records
- Search in paragraph's text
- SEDOLs
- Self-describing numbers
- Self-hosting compiler
- Semiprime
- Semordnilap
- Send an unknown method call
- Separate the house number from the street name
- Sequence of non-squares
- Sequence of primes by trial division
- Sequence of primorial primes
- Sequence: nth number with exactly n divisors
- Sequence: smallest number greater than previous term with exactly n divisors
- Sequence: smallest number with exactly n divisors
- Set
- Set consolidation
- Set of real numbers
- Set puzzle
- Seven-sided dice from five-sided dice
- Sexy primes
- SHA-1
- SHA-256
- Shell one-liner
- Shoelace formula for polygonal area
- Short-circuit evaluation
- Show ASCII table
- Show the epoch
- Sierpinski carpet
- Sierpinski triangle
- Sierpinski triangle/Graphical
- Sieve of Eratosthenes
- Simple database
- Simple turtle graphics
- Simple windowed application
- Simulated annealing
- Sine wave
- Singleton
- Singly-linked list/Element definition
- Singly-linked list/Element insertion
- Singly-linked list/Traversal
- Sleep
- Smarandache prime-digital sequence
- Smith numbers
- Snake and Ladder
- Sockets
- Solve a Holy Knight's tour
- Solve the no connection puzzle
- Solve triangle solitare puzzle
- Sort a list of object identifiers
- Sort an array of composite structures
- Sort an integer array
- Sort disjoint sublist
- Sort numbers lexicographically
- Sort primes from list to a list
- Sort stability
- Sort three variables
- Sort using a custom comparator
- Sorting algorithms/Bead sort
- Sorting algorithms/Bogosort
- Sorting algorithms/Bubble sort
- Sorting Algorithms/Circle Sort
- Sorting algorithms/Cocktail sort
- Sorting algorithms/Comb sort
- Sorting algorithms/Counting sort
- Sorting algorithms/Cycle sort
- Sorting algorithms/Gnome sort
- Sorting algorithms/Heapsort
- Sorting algorithms/Insertion sort
- Sorting algorithms/Merge sort
- Sorting algorithms/Pancake sort
- Sorting algorithms/Patience sort
- Sorting algorithms/Permutation sort
- Sorting algorithms/Quicksort
- Sorting algorithms/Radix sort
- Sorting algorithms/Selection sort
- Sorting algorithms/Shell sort
- Sorting algorithms/Sleep sort
- Sorting algorithms/Stooge sort
- Sorting algorithms/Strand sort
- Sorting algorithms/Tree sort on a linked list
- Soundex
- Sparkline in unicode
- Special characters
- Special divisors
- Special variables
- Spiral matrix
- Split a character string based on change of character
- Square but not cube
- Square-free integers
- Stable marriage problem
- Stack
- Stack traces
- Stair-climbing puzzle
- Start from a main routine
- State name puzzle
- Statistics/Basic
- Statistics/Normal distribution
- Stem-and-leaf plot
- Stern-Brocot sequence
- Stirling numbers of the first kind
- Stirling numbers of the second kind
- Straddling checkerboard
- String append
- String case
- String comparison
- String concatenation
- String interpolation (included)
- String length
- String matching
- String prepend
- Strip a set of characters from a string
- Strip block comments
- Strip comments from a string
- Strip control codes and extended characters from a string
- Strip whitespace from a string/Top and tail
- Strong and weak primes
- Subleq
- Subset sum problem
- Substitution cipher
- Substring
- Substring/Top and tail
- Subtractive generator
- Successive prime differences
- Sudoku
- Suffix tree
- Sum and product of an array
- Sum digits of an integer
- Sum multiples of 3 and 5
- Sum of a series
- Sum of elements below main diagonal of matrix
- Sum of first n cubes
- Sum of square and cube digits of an integer are primes
- Sum of squares
- Sum of the digits of n is substring of n
- Sum of two adjacent numbers are primes
- Sum to 100
- Summarize and say sequence
- Summarize primes
- Sunflower fractal
- Super-d numbers
- Superellipse
- Superpermutation minimisation
- Sutherland-Hodgman polygon clipping
- Symmetric difference
- Synchronous concurrency
- System time

### T

- Table creation
- Table creation/Postal addresses
- Take notes on the command line
- Tau function
- Tau number
- Taxicab numbers
- Teacup rim text
- Temperature conversion
- Terminal control/Clear the screen
- Terminal control/Coloured text
- Terminal control/Cursor positioning
- Terminal control/Dimensions
- Terminal control/Display an extended character
- Terminal control/Hiding the cursor
- Terminal control/Inverse video
- Terminal control/Ringing the terminal bell
- Ternary logic
- Test a function
- Test integerness
- Text between
- Text processing/1
- Text processing/2
- Text processing/Max licenses in use
- Textonyms
- The Name Game
- The Twelve Days of Christmas
- Thiele's interpolation formula
- Thue-Morse
- Tic-tac-toe
- Time a function
- Tokenize a string
- Tokenize a string with escaping
- Tonelli-Shanks algorithm
- Top rank per group
- Topic variable
- Topological sort
- Topological sort/Extracted top item
- Topswops
- Total circles area
- Totient function
- Towers of Hanoi
- Trabb Pardo–Knuth algorithm
- Transportation problem
- Tree from nesting levels
- Tree traversal
- Trigonometric functions
- Triplet of three numbers
- Truncatable primes
- Truncate a file
- Truth table
- Twelve statements
- Twin primes
- Two identical strings
- Two sum
- Type detection

### U

- Ulam spiral (for primes)
- Unbias a random generator
- Undefined values
- Unicode strings
- Unicode variable names
- Unique characters
- Universal Turing machine
- Unix/ls
- Unprimeable numbers
- Untouchable numbers
- Untrusted environment
- UPC
- Update a configuration file
- URL decoding
- URL encoding
- URL parser
- Use another language to call a function
- User defined pipe and redirection operators
- User input/Graphical
- User input/Text
- UTF-8 encode and decode

### V

- Validate International Securities Identification Number
- Vampire number
- Van der Corput sequence
- Van Eck sequence
- Variable declaration reset
- Variable size/Get
- Variable size/Set
- Variable-length quantity
- Variables
- Variadic function
- Vector
- Vector products
- Verhoeff algorithm
- Verify distribution uniformity/Chi-squared test
- Verify distribution uniformity/Naive
- Vibrating rectangles
- Vigenère cipher
- Visualize a tree
- Vogel's approximation method
- Voronoi diagram

### W

- Walk a directory/Non-recursively
- Walk a directory/Recursively
- Water collected between towers
- Web scraping
- Weird numbers
- Welch's t-test
- Window creation
- Wireworld
- Word break problem
- Word frequency
- Word search
- Word wrap
- Wordle comparison
- World Cup group stage
- Write entire file
- Write float arrays to a text file
- Write language name in 3D ASCII

- Pages using duplicate arguments in template calls
- Execution method/Interpreted
- Typing/Safe
- Typing/Strong (but regular)
- Typing/Checking/Dynamic
- Programming Languages
- Programming paradigm/Dynamic
- Programming paradigm/Functional
- Programming paradigm/Imperative
- Programming paradigm/Procedural
- Programming paradigm/Reflective
- Programming paradigm/Tacit