## O Tannenbaum

### December 24, 2013

Merry Christmas to all my readers.

Your task today is to write a program that draws a Christmas tree on the screen; it can be as simple or elaborate as you wish. When you are finished, you are welcome to read or run a suggested solution, or to post your own solution or discuss the exercise in the comments below.

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## Requiescat In Pace

### December 20, 2013

My mother died on Tuesday, December 17th, aged eighty-eight years, after a brief illness.

Requiem aeternam dona ei, Domine, et lux perpetua luceat ei.

I’ll be taking a short break from writing exercises. The Christmas exercise for next Tuesday is already written and loaded and will appear on schedule. Then I’ll be back to the normal Tuesday/Friday schedule after the first of the year.

## Remove Duplicates From A List

### December 17, 2013

We have today another exercise from our infinite supply of interview questions:

Return a list containing only one copy of any duplicates in an input list, with items in the output in the same order as their first appearance in the input.

Your task is to answer the interview question given above; you must provide at least two different solutions. When you are finished, you are welcome to read or run a suggested solution, or to post your own solution or discuss the exercise in the comments below.

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## Modular Factorial

### December 13, 2013

This question appears from time to time as an interview question or on the coding-challenge web sites.

Write a function that calculates n! (mod p) when p is prime. Then extend the function to calculate n! (mod m) when m is not prime. Can you calculate the factorials using fewer than n−1 modular multiplications?

For instance, 1000000! (mod 1001001779) is 744950559.

Your task is to write the indicated functions. When you are finished, you are welcome to read or run a suggested solution, or to post your own solution or discuss the exercise in the comments below.

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## Rock Paper Scissors

### December 10, 2013

Today’s exercise is our five hundredth! It’s hard to believe. I remember writing the fiftieth exercise and thinking I would never get to a hundred. Even after all this practice it is hard to write two exercises every week, but your comments and private emails and referrals from other web sites sustain me. Many thanks to all my readers.

Our tradition for these milestone exercises is to have a party, which means we need a game: so we write one. Today’s exercise is to write an interactive rock-paper-scissors game: rock blunts scissors, paper wraps rock, scissors cut paper.

Your task is to write a program to play rock-paper-scissors with a human player, keeping score as you go. When you are finished, you are welcome to read or run a suggested solution, or to post your own solution or discuss the exercise in the comments below.

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## Left-Handed Words

### December 6, 2013

On a standard English keyboard, the letters that a touch-typist strikes with the fingers of the left hand are Q, W, E, R, T on the top row, A, S, D, F, G on the home row, and Z, X, C, V, B on the bottom row. For instance, words like FAST and ZEBRAS are left-handed words; PACKAGE and SOUTH are not.

Your task is to write a program that finds words that can be spelled using only letters that are struck with the fingers of the left hand; perhaps you are writing an exercise for a typing book. When you are finished, you are welcome to read or run a suggested solution, or to post your own solution or discuss the exercise in the comments below.

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## Reversing Parts Of A List

### December 3, 2013

This exercise is intended for beginning programmers who need to strengthen their understanding of linked lists. It comes in three parts:

First, write a function that reverses the elements of a linked list pairwise; for instance, given the list (1 2 3 4 5 6) the pairwise reversal is (2 1 4 3 6 5). If the list has an odd number of elements, keep the last element at the end of the list; for instance, given the list (1 2 3 4 5) the pairwise reversal is (2 1 4 3 5).

Second, write a function the reverses the elements of a linked list k-wise; for instance, given the list (1 2 3 4 5 6) the 3-wise reversal is (3 2 1 6 5 4) and the 4-wise reversal is (4 3 2 1 6 5).

Third, write a function that reverses the elements of a linked list by halves; for instance, given the list (1 2 3 4 5 6) the half-wise reversal is (3 2 1 6 5 4), with each half reversed independently. If the list has an odd number of elements, the middle element may be assigned to either half at the discretion of the programmer.

Your task is to write the three functions given above. When you are finished, you are welcome to read or run a suggested solution, or to post your own solution or discuss the exercise in the comments below.

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