## One Step, Two Steps

### August 18, 2017

Today’s exercise is a simple matter of counting:

Suppose you can take either one or two steps at a time. How many ways can you travel a distance of

nsteps? For instance, there is 1 way to travel a distance of 1 step, 2 ways (1+1 and 2) to travel two steps, and 3 ways (1+1+1, 1+2 and 2+1) to travel three steps.

Your task is to write a program to count the number of ways to travel a distance of *n* steps, taking 1 or 2 steps at a time. 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.

## Phone Numbers

### August 15, 2017

Today’s task looks like homework:

Make a list of

n-digit phone numbers according to the following rules:1) First digit must not be zero.

2) First digit may be four, but no digits after the first may be four.

3) Any two consecutive digits must be different.

4) The user may specify any set of digits that may not appear in the phone number.For instance, if

n= 3 and the digits 1, 3, 5, 7, and 9 don’t appear in the output, the desired list of three-digit numbers starting with 2, 4, 6, or 8 followed by 0, 2, 6 or 8 without consecutive duplicates is: 202 206 208 260 262 268 280 282 286 402 406 408 420 426 428 460 462 468 480 482 486 602 606 608 620 626 628 680 682 686 802 806 808 820 826 828 860 862 868.

Your task is to write a program that generates phone numbers according to the rules 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.

## Ruth-Aaron Pairs

### August 11, 2017

We have an exercise today from the realm of recreational mathematics, based on a video from Numberphile. The two numbers 714 = 2 × 3 × 7 × 17 and 715 = 5 × 11 × 13 have, between them, the first seven prime numbers as their factors (baseball fans will understand the name of this exercise, others will have to watch the video). So the first exercise is to find other pairs of consecutive numbers that have as their factors all and only the first *n* primes, for some *n*.

Carl Pomerance, the speaker in the Numberphile video, credits one of his students for first noticing that the sums of the prime factors of 714 and 715 are equal: 2 + 3 + 7 + 17 = 5 + 11 + 13 = 29. So the second exercise is to find other pairs of consecutive numbers whose factors sum to the same number. This exercise can be divided into two parts: numbers where repeating factors are added in to the sum and numbers where only the distinct factors are added in to the sum.

Your task is to complete the three exercises; they will make much more sense if you watch the video. 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.

## Move Spaces To Beginning Of String

### August 8, 2017

Today’s exercise is a programming interview question from a programmer who very obviously didn’t get the job:

Given a C-style null-terminated string, move all the spaces in the string to the beginning of the string, in place, in one pass.

The candidate claimed it couldn’t be done, and apparently got into an argument with the interviewer, and took to the internet after the interview to vindicate himself about the “unfair question.”

Your task is to either write the requested program, or demonstrate that it cannot be done. 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.

## Last Name Comma First Name

### August 4, 2017

Today’s task is simple:

You are given a file containing one name per line in the format last name comma first name, optionally followed by another comma and a numeral (like Sr., or Jr., or IV), and are convert it to a file containing the names, one name per line, in the format first name, last name and numeral with no commas. You may assume the input is correctly formatted, with optional spaces after each of the two commas.

Your task is to convert a file of names to the correct format. When you are finished, you are welcome to read a suggested solution, or to post your own solution or discuss the exercise in the comments below.

## Find Target In A Matrix

### August 1, 2017

Consider a two-dimensional matrix containing integer entries in which all rows and all columns are sorted in ascending order; for example:

1 12 43 87 9 25 47 88 17 38 48 92 45 49 74 95

Your task is to write a program that takes a matrix as describe above, and a target integer, and determines if the target integer is present in the matrix. 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.