Learn J with Real Code Examples

Updated Nov 20, 2025

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Code Sample Descriptions

J Counter and Theme Toggle

count =: 0
isDark =: 0

updateUI =: 3 : 0
    'Counter: ', ": count
    'Theme: ', (if. isDark do. 'Dark' else. 'Light')
)

increment =: 3 : 0
    count = count + 1
    updateUI ''
)

decrement =: 3 : 0
    count = count - 1
    updateUI ''
)

reset =: 3 : 0
    count = 0
    updateUI ''
)

toggleTheme =: 3 : 0
    isDark = 1 - isDark
    updateUI ''
)

'Simulate actions'
updateUI ''
increment ''
increment ''
toggleTheme ''
decrement ''
reset ''

Demonstrates a simple counter with theme toggling using J variables and verbs.

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J Fibonacci Sequence

fib =: 0 1
for_i. 8 do. fib =: fib , (+/@:1) fib ; end.
fib

Generates first 10 Fibonacci numbers using J array operations.

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J Factorial Calculator

n =: 5
! n

Calculates factorial using J's product/integers.

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J Prime Checker

isPrime =: 3 : 0
    n = y
    n > 1 &. >./ n | i.1 + n - 2
)

Checks if a number is prime using J modulo and all/any verbs.

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J Sum of Array

nums =: 1 2 3 4 5
+/ nums

Calculates the sum of an array.

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J Reverse String

s =: 'HELLO'
|: s

Reverses a string using J verbs.

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J Multiplication Table

n =: 5
(n * i.10) +: 'x ' , n

Generates multiplication table for a number using J.

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J Celsius to Fahrenheit

c =: 25
f =: c * 9 % 5 + 32
f

Converts Celsius to Fahrenheit using J expressions.

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J Simple Alarm Simulation

temp =: 80
thresh =: 75
if. temp > thresh do. 'Alarm: Temperature Too High!' else. 'Temperature Normal'

Simulates an alarm if temperature threshold is exceeded.

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J Random Walk Simulation

steps =: 10
pos =: 0
for_i. steps do. pos = pos + (0 < ?2) * 2 - 1 ; pos end.

Simulates a 1D random walk using J random verbs.

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Code Sample Descriptions

J Counter and Theme Toggle

count =: 0
isDark =: 0

updateUI =: 3 : 0
    'Counter: ', ": count
    'Theme: ', (if. isDark do. 'Dark' else. 'Light')
)

increment =: 3 : 0
    count = count + 1
    updateUI ''
)

decrement =: 3 : 0
    count = count - 1
    updateUI ''
)

reset =: 3 : 0
    count = 0
    updateUI ''
)

toggleTheme =: 3 : 0
    isDark = 1 - isDark
    updateUI ''
)

'Simulate actions'
updateUI ''
increment ''
increment ''
toggleTheme ''
decrement ''
reset ''

Demonstrates a simple counter with theme toggling using J variables and verbs.

Let’s Try →

J Fibonacci Sequence

fib =: 0 1
for_i. 8 do. fib =: fib , (+/@:1) fib ; end.
fib

Generates first 10 Fibonacci numbers using J array operations.

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J Factorial Calculator

n =: 5
! n

Calculates factorial using J's product/integers.

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J Prime Checker

isPrime =: 3 : 0
    n = y
    n > 1 &. >./ n | i.1 + n - 2
)

Checks if a number is prime using J modulo and all/any verbs.

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J Sum of Array

nums =: 1 2 3 4 5
+/ nums

Calculates the sum of an array.

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J Reverse String

s =: 'HELLO'
|: s

Reverses a string using J verbs.

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J Multiplication Table

n =: 5
(n * i.10) +: 'x ' , n

Generates multiplication table for a number using J.

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J Celsius to Fahrenheit

c =: 25
f =: c * 9 % 5 + 32
f

Converts Celsius to Fahrenheit using J expressions.

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J Simple Alarm Simulation

temp =: 80
thresh =: 75
if. temp > thresh do. 'Alarm: Temperature Too High!' else. 'Temperature Normal'

Simulates an alarm if temperature threshold is exceeded.

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J Random Walk Simulation

steps =: 10
pos =: 0
for_i. steps do. pos = pos + (0 < ?2) * 2 - 1 ; pos end.

Simulates a 1D random walk using J random verbs.

Let’s Try →