## Binary Search

### April 29, 2016

I goofed.

While writing a program (it may appear in a future exercise) I needed to search a sorted array for a target value. I should have copied an existing binary search, but instead I wrote my own, since I’m a good programmer and can certainly write a simple function like that. You won’t have any trouble guessing what happened next.

Your task is to write a binary search function; do it yourself, without looking at any library implementations or searching the internet. You might also want to write a test script to give you confidence in your function. 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.

## An Integer Formula For Fibonacci Numbers

### April 26, 2016

Today’s exercise isn’t really an exercise but an astonishing integer formula for computing the *n*th Fibonacci number; here it is in Python:

def fib(n): return (4 << n*(3+n)) // ((4 << 2*n) − (2 << n) − 1) & ((2 << n) − 1)

You can see an explanation here and discussion here.

Your task is to translate the program to your favorite language. 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.

## GCD Sum

### April 22, 2016

Today’s exercise is inspired by A018804: Find the sum of the greatest common divisors gcd(*k*, *n*) for 1 ≤ *k* ≤ *n*.

Your task is to write a function that finds the sum of the greatest common divisors. 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.

## Interview Timing

### April 19, 2016

I came across an interesting interview question recently. I’ll tell you the question shortly. What made it interesting was that the same question was given to all candidates, and they were timed in getting a solution; candidates with shorter times were given higher scores than candidates with longer times.

Your task is to write the requested program, which you can access HERE after you are set up for timing.

## Ten-Digit Pandigital Numbers Divisible By 1 Through 9

### April 15, 2016

Today’s exercise solves somebody’s homework problem — but not too much, since he had to write his program in Java:

Find the two smallest ten-digit pandigital numbers (numbers that contain all the digits from 0 through 9) that are divisible by the numbers 1 through 9. Then find the two largest pandigital numbers that are divisible by the numbers 1 through 9.

Your task is to solve the homework problem. 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.

## Titlecase

### April 12, 2016

A string is titlecased when the first letter of each word is capitalized and the remaining letters are lower case. For instance, the string “programming PRAXIS” becomes “Programming Praxis” when titlecased.

Your task is to write a function that takes a string and returns it in titlecase. 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.

## Google Interview Question

### April 8, 2016

Today’s exercise is an interview question from Google:

Given a list of words, find the maximum value of the product of the lengths of two words from the list, subject to the constraint that the two words cannot share any letters. For instance, given the words ABCW, BAZ, FOO, BAR, XTFN, and ABCDEF, the pair FOO BAR has a product of 9, the pair BAZ XFTN has a product of 12, and the pair ABCW XTFN has a product of 16, which is the maximum. Note that the pair ABCW ABCDEF doesn’t work because the two words share three letters.

Your task is to write a program to solve Google’s interview question. 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.

## Java Interview Question

### April 5, 2016

We have an interview question today:

Input comes from a file containing pipe-delimited records with three fields: student id (a positive integer), course title (a string), and score (a positive integer). You may assume that any combination of student id and course is unique. Here’s an example input file:

22|Math|45 23|English|52 22|English|51 26|Math|72 23|Math|61 21|English|81The file may have any number of records, and there is no limit on the number of unique courses. You should write a program to read the file and write a list of all courses in the file, combined with the score of the lowest-numbered student in the course. Thus, the correct output for the input shown above is:

Math 45 English 81

Your task is to write a program to solve the interview question; the original question specified a Java solution, but you are free to use any language. 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.

## SUM And XOR

### April 1, 2016

I don’t know the origin of it, but today’s exercise must be either a homework problem or an interview question:

Assume there are two positive integers

aandbthat have sumsand XORx. Given anson the range [2, 10^{12}] andxon the range [0, 10^{12}], find a list of possible values of the ordered pairs (a,b). For instance, givens= 9 andx= 5, there are four possible pairs: (2, 7), (7, 2), (3, 6) and (6, 3).

Your task is to write a program that finds the pairs. 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.