Haskell Program to Print Hollow Right Triangle Star Pattern

In Haskell we can use the replicate function and recursive function to create a hollow right triangle star pattern.

A hollow right triangle star pattern is a pattern made up of asterisks (*) that forms a right triangle shape with empty spaces in the middle as shown below.

**
* *
*  *
*   *
*    *
*     *
*      *
********

The shape is created by printing asterisks in a specific order, with the number of asterisks in each row increasing as the row number increases.

Algorithm

• Step 1 − The printRow function is defined using the replicate/recursion function,

• Step 2 − Program execution will be started from the main function. The main() function has whole control of the program. It is written as main = do. In the main function, a number is passed up to which the hollow right triangle star pattern is to be printed.

• Step 3 − The variable named, “n” is initialized. It will hold the integer up to which the hollow right triangle star pattern is to be printed.

• Step 4 − The result is printed to the console using the ‘putStrLn’ statement after the function is called.

Example 1

In this method, a function printPattern is defined that takes an integer n as an argument and returns a string representing a hollow right triangle star pattern of size n. The helper function printRow takes two integers n and i as arguments and returns a string representing a row of asterisks and spaces. The printRow function uses an if statement to determine whether the row should be filled with asterisks or whether it should be a hollow row with asterisks on the ends and spaces in the middle. The printPattern function uses list comprehension to generate n rows of asterisks and spaces.

module Main where

printRow :: Int -> Int -> String
printRow n i = if i == n then replicate i '*' ++ "
"
               else "*" ++ replicate (i - 1) ' ' ++ "*
"

printPattern :: Int -> String
printPattern n = concat [printRow n i | i <- [1..n]]

main :: IO ()
main = putStr (printPattern 8)

Output

**
* *
*  *
*   *
*    *
*     *
*      *
********

Example 2

In this example, the function is defined using the replicate function to print the hollow right triangle star pattern.

module Main where

printRow :: Int -> Int -> String
printRow n i = if i == 1 then "*
"
               else "*" ++ replicate (2 * i - 3) ' ' ++ "*
"

printPattern :: Int -> String
printPattern n = concat [printRow n i | i <- [1..n]]

main :: IO ()
main = putStr (printPattern 8)

Output

*
* *
*   *
*     *
*       *
*         *
*           *
*             *

Example 3

In this method, printRow is a recursive function that takes two arguments n and i representing the size of the pattern and the current row, respectively. If i is equal to 1, it returns a string with a single asterisk followed by a newline character. Otherwise, it returns a string with the first and last characters being asterisks and the middle part being a number of spaces equal to 2 * i - 3 (the number of spaces decreases as i increases), followed by a recursive call to printRow with i - 1 as the updated row number.

module Main where

printRow :: Int -> Int -> String
printRow n i
   | i == 1    = "*
"
   | otherwise = "*" ++ replicate (2 * i - 3) ' ' ++ "*
" ++ printRow n (i - 1)

printPattern :: Int -> String
printPattern n = printRow n n

main :: IO ()
main = putStr (printPattern 8)

Output:

*             *
*           *
*         *
*       *
*     *
*   *
* *
*

Conclusion

In Haskell, to print the hollow right triangle star pattern we can use replicate and recursive functions.