## List Slices

### October 27, 2020

During some recent personal programming, I needed a function to slice a list into pieces: Given a list of sub-list lengths, and an input list to be sliced, return a list of sub-lists of the requested lengths. For instance:, slicing the list (1 2 2 3 3 3) into sub-lists of lengths (1 2 3) returns the list of sub-lists ((1) (2 2) (3 3 3)). Extra list elements at the end of the input list are ignored, missing list elements at the end of the input list are returned as null lists.

Your task is to write a program that slices lists into sub-lists according to a specification of sub-list lengths. 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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## Doubled Pairs

### October 20, 2020

Today’s exercise is somebody’s homework problem:

You are given an array containing positive integers, all distinct. Write a program that determines if there exist in the array any numbers such that one is double the other. For instance, in the array [1,2,3,4], the pairs 1,2 and 2,4 are both doubles, so the program should return True; in the array [1,3,5,7,9] there is no such doubled pair, so the program should return False. For extra credit, return a pair that proves the result is True, if it exists. For extra-extra credit, return all such pairs. Your program must run in linear time.

The student who asked for help realized there was a simple nested-loop solution that runs in quadratic time, but that doesn’t meet the constraints of the homework problem.

Your task is to solve all three levels of 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.

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## Pandigital Squares, Faster And Smaller

### October 6, 2020

In a previous exercise, we wrote a program to compute pandigital squares, defined as ten-digit numbers with integral square roots in which each digit zero through nine appears exactly once. We remarked at the time that our solution, though not very fast, was fast enough.

Your task is to write a program that finds pandigital squares, reducing the time and space requirements of the naive solution. 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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## Non-Breaking Hyphen

### October 2, 2020

Today’s exercise tells the story of a problem I faced in my day job. A user apparently copy/pasted a field from Word, Excel or Outlook into our data processing system, which assumes plain ascii (the underlying database, Oracle, handles unicode properly, but the system on top of Oracle doesn’t). Unfortunately, although the field looked okay,

```10001366650-1
```

the dash was actually a non-breaking hyphen, unicode 201110, which broke the system in a rather large way. The field in question was the vendor invoice number in an accounts payable system, and when the check paying that invoice was written, the check-writing program dropped the remittance advice from that check, so every subsequent check had the wrong remittance advice attached. The error wasn’t discovered immediately, so some of the checks were already mailed, making recovery difficult (we couldn’t just void the check run and start over because some of the checks were already mailed, and restarting the check run meant the check numbers wouldn’t match). So it was a grand old mess. In case you’re curious, I demonstrated the error with this SQL statement

```select asciistr(fabinvh_vend_inv_code)
from   fabinvh
where  fabinvh_code = 'I2104519'```

which returned

`1000136650\20111`

Unicode is nearly thirty years old. Users have the right to expect that their systems handle unicode characters properly.

Your task is to write a program that detects unicode/ascii error; you might also tell us about any unicode horror stories you have faced. 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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