Steve Yegge’s Phone-Screen Coding Exercises

June 30, 2009

All of these are easy, and very much shorter than the Java versions that Yegge provides:

Most Scheme systems provide a string-reverse function, although it is not required by the Standard. Here is a purely-Standard version:
(define (string-reverse s) (list->string (reverse (string->list s))))
Yegge gives this function to compute fibonacci numbers:
(define (fib n) (if (<= n 1) n (+ (fib (- n 1)) (fib (- n 2)))))

That function takes exponential time to compute, due to the constant re-computation of previously-computed results. If Yegge gave me that function during a phone screen, he wouldn’t pass to the next stage of the hiring process. Here is a linear-time version of the fibonacci function:

(define (fib n) (let fib ((n n) (f1 1) (f2 0)) (if (zero? n) f2 (fib (- n 1) (+ f1 f2) f1))))
Standard Scheme doesn’t provide formatted output, but most Scheme systems provide a version of Lisp’s format function, which provides a wealth of options:
(define (times-table n) (do ((i 1 (+ i 1))) ((> i n)) (do ((j 1 (+ j 1))) ((> j n) (newline)) (format #t "~4d" (* i j)))))
Read gets the next object from the input; if the input consists of integers separated by newlines, each call to read will return the next number on the input:
(define (sum-file file-name) (with-input-from-file file-name (lambda () (let loop ((n (read)) (sum 0)) (if (eof-object? n) sum (loop (read) (+ n sum)))))))

If you prefer, you can use the fold-input function from the Standard Prelude:

(define (sum-file file-name) (fold-input read + 0 file-name))
Things don’t get much simpler than this:
(define (print-odds) (do ((i 1 (+ i 2))) ((> i 100)) (display i) (newline)))
Well, maybe they do:
(define (largest xs) (apply max xs))
Standard Scheme provides number->string, which takes an optional radix argument but doesn’t write leading zeros, so we make our own to-hex function:
(define (format-rgb r g b) (define (to-hex n) (string (string-ref "0123456789ABCDEF" (quotient n 16)) (string-ref "0123456789ABCDEF" (modulo n 16)))) (string-append (to-hex r) (to-hex g) (to-hex b)))

If you like format, it has an option to print hexadecimal numbers.

It’s hard to imagine that coding exercises like this would be a barrier for any practicing programmer, but apparently they are; Yegge got the fibonacci question wrong, and if you read the comments on his original article, he also got the print-odds program wrong initially (he since changed his article, claiming the mistake was due to a failed optimization), printing even numbers instead of odd numbers. Of course, that’s the reason this blog exists: to give savvy programmers a chance to practice their skills, so the don’t make dumb coding mistakes, either during a phone screening or in their working programs.

Posted by programmingpraxis

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10 Responses to “Steve Yegge’s Phone-Screen Coding Exercises”

Programming Praxis – Steve Yegge’s Phone-Screen Coding Exercises « Bonsai Code said
June 30, 2009 at 1:27 PM
[…] Praxis – Steve Yegge’s Phone-Screen Coding Exercises By Remco Niemeijer Today’s Programming Praxis problem is an easy one. In 2005, Steve Yegge posted an article about […]

Remco Niemeijer said

June 30, 2009 at 1:27 PM

My Haskell solution (see http://bonsaicode.wordpress.com/2009/06/30/programming-praxis-steve-yegge%E2%80%99s-phone-screen-coding-exercises/ for a version with comments):

import Text.Printf
import Data.Word

reverse' :: String -> String
reverse' = foldr (:) []

fib :: (Num a) => Int -> a
fib n = fibs !! n where fibs = 0 : 1 : zipWith (+) fibs (tail fibs)

timesTable :: IO ()
timesTable = mapM_ putStrLn [concat
    [printf "%4d" (a * b) | a <- &#91;1..12&#93;&#93; | b <- &#91;1..12 :: Int&#93;&#93;

sumFile :: FilePath -> IO ()
sumFile path = print . sum . map read . lines =<< readFile path

printOdds :: IO ()
printOdds = print $ filter odd &#91;1..99&#93;

largest :: &#91;Int&#93; -> Int
largest = foldl max 0

toHex :: Word8 -> Word8 -> Word8 -> String
toHex = printf "%02x%02x%02x"

Scott Pedersen said
June 30, 2009 at 10:56 PM
C# Solution

I couldn’t resist writing smartass responses for questions three and five to just return fixed strings. I also wrote more general answers that I thought were more in keeping with the spirit of the exercise.

The basic recursive implementation for Fibonacci numbers is a classic, but is terribly slow. It’s the answer I’d expect out of most interview candidates as a first pass since it is taught so regularly as an example of recursion. However, they’d be in trouble if they couldn’t quickly identify the problems with such an implementation. I wrote out a couple of alternatives that are a bit more efficient.
Remco Niemeijer said
July 1, 2009 at 2:34 PM
Thanks to nonowarn for catching a bug:

The correct code for exercise 1 should be
```
reverse' :: String -> String
reverse' = foldl (flip (:)) []
```
liesen said
July 1, 2009 at 5:21 PM
I think you’ve got them all, Remco. My Haskell solutions are similar. Here is one simpler solution to #5:

printOdds = print [1,3..99]

There’s one more bug in Remco’s code, though: largest assumes positive integers, but it should read:

largest = foldl1 max
Remco Niemeijer said
July 1, 2009 at 10:33 PM
Yeah, I noticed that one too, but didn’t get round to fixing it. However, if you treat Steve Yegge’s solution as the spec then the correct implementation is actually

largest = foldl max minBound

Connochaetes said

July 5, 2009 at 12:51 PM

def reverse(str)
  # 1. Write a function to reverse a string.
  len = str.length - 1
  len.downto(0).collect {|n| str[n].chr}.to_s
end

def nth_fib(n)
  # 2. Write a function to compute the Nth fibonacci number.
  # NB. assumes fibonacci numbers are zero-indexed.
  x,y = 0,1
  n.times {x,y = y,x+y}
  y
end

def multi_table
  # 3. Print out the grade-school multiplication table up to 12 x 12.
  size = 12
  1.upto(size).each do |x|
    1.upto(size).each do |y|
      printf "%4d", x*y
    end
    printf "\n"
  end
end

def sum_ints_from_file(filename)
  # 4. Write a function that sums up integers from a text file, one per line.
  sum = 0
  File.open(filename).each {|line| sum += line.to_i}
  sum
end

def print_odd_numbers
  # 5. Write a function to print the odd numbers from 1 to 99.
  (1..99).step(2).each {|n| puts n} ; nil
end

def find_largest_value(arr)
  # 6. Find the largest int value in an int array.
  arr.inject {|x,y| x > y ? x : y}
end

def format_RGB(r,g,b)
  # 7. Format an RGB value (three 1-byte numbers) as a 6-digit hexadecimal string.
  format "%02X%02X%02X", r,g,b
end

Programming Praxis – Learn A New Language « Bonsai Code said
June 25, 2010 at 1:07 PM
[…] for the exercise, I picked Steve Yegge’s Phone-Screen Coding Exercises, figuring seven different small problems might cover slightly more ground than one bigger […]
Graham said
June 28, 2011 at 10:17 PM
I’m never sure how much of a language’s built-in functionality we’re supposed to use for a question.
Python solution
Common Lisp solution

brooknovak said

June 18, 2013 at 5:51 PM

For the RGB => Hex conversion you can always take advantage API knowledge (such as string formatting features), but its not as fun!

public static string ConvertToHex(byte r, byte g, byte b) {
	char[] buffer = new char[6];
	int rgb = (r << 16) | (g << 8) | b;
	for (int i = 0; i < 6; i++) {
		int val = rgb & 15;
		if (val <= 9)
			buffer [5 - i] = (char)('0' + val);
		else 
			buffer [5 - i] = (char)('A' + (val - 10));
		rgb >>= 4;
	}
	return new string(buffer);
}

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