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Answer to the C Puzzle

Posted on February 18, 2021February 15, 2021 by David

Solution — Image by Gino Crescoli from Pixabay

If you look back at the puzzle, you might think the for loop is a bit odd starting at -1 and that’s the snag. It exits the for loop immediately because -1 is actually more than 5. It’s a rather subtle bug. The type of d is obviously int, but the type of (TOTAL_ELEMENTS – 2) is unsigned int.

So what I believe is happening is the compiler is promoting d to an unsigned int which is 4294967295, and clearly that’s a bit larger than 5. To fix this just put (int) before the (TOTAL_ELEMENTS – 2) in the for loop and it will work.

A little C Puzzle

Posted on February 15, 2021February 12, 2021 by David

Question mark — Image by Gordon Johnson from Pixabay

The following C program doesn’t output anything. Why?

#include<stdio.h>

#define TOTAL_ELEMENTS (sizeof(array) / sizeof(array[0]))
int array[] = { 23,34,12,17,204,99,16 };

int main()
{
    int d;

    for (d = -1; d <= (TOTAL_ELEMENTS - 2); d++)
        printf("%d\n", array[d + 1]);

    return 0;
}

This came from this site by Gowri Kumar which has a lot more C puzzles. Answer in a day or two…

A challenging Puzzle – fitting words in a grid

Posted on December 14, 2020December 12, 2020 by David

$\scrabble words$ You are given a list of words. You have to fit them into a grid so that the grid is as small as possible. The words can go horizontal or vertical and can use letters from other words (like in Scrabble). But you can’t have words next to each other except where they share letters like in the picture above. MN BTW is not a valid Scrabble word so this is wrong! (I er borrowed it from somewhere!)

So words can be placed anywhere, in either orientation. Evaluating all the possibilities could probably be done by brute force. But for each word in each position on the board there are n-1 words to be fitted afterwards. Do you try and fit in each alternate word in a different orientation like horizontal then vertical then horizontal?

If there are ten words to fit in say in a starting grid of 20 x 20 and the first word is 6 characters long then it can fit in 15 different places along the top line and on each of the 20 lines so that’s 300 positions. Then if its vertical there another 300 positions. That’s 600 positions to try. And for each of those 600 there’s 9! different combinations of words to fit in. 9! = 362,880.

Of course if the first word and the second word have no letters in common then it may be as well to skip that 2nd word and go onto the 3rd and so on. We want to keep the grid from getting too large with unconnected words. Also We might limit the first word to start in th 6 positions in the top row and maybe only the first and 2nd rows. That vastly reduces the number of combinations to try out. But still it’s a tough thing to program.

C isn’t usually the best language to program anything with text in, but in this case, we’re not building sentences or searching, just looking at arrays of characters and so it might just be suitable.

I’m going to have a go at this and see if it can be done and if so I’ll publish my code but this may take a few days. If so then it could be used as a basis of a word search generator though the rules about words being next to each other could be relaxed plus words run forwards and backwards in both horizontal and vertical orientation and also diagonal.

Previous Puzzle Answer

This was “If I have the integers 4,5,6,7 and 8 and a special C function f so that f(4)=2, f(5) = 5, f(6)=10, f(7)=40 and f(8) = 92. What is the function f?”.Bit of a trick question this as the function is returning the number of unique Queens that could be fitted on a chess board of size n (4×4,5×5..8×8) without being able to attack each other.

Philip Jama has posted a C program on GitHub to arrange the number of Queens on a chessboard of the specified size. So the function wasn’t really a function…

Interesting number sequence

Posted on December 12, 2020December 12, 2020 by David

Chess Set — Image by Clker-Free-Vector-Images from Pixabay

Here’s an interesting puzzle (well for some people). If I have the integers 4,5,6,7 and 8 and a special C function f so that f(4)=2, f(5) = 5, f(6)=10, f(7)=40 and f(8) = 92. What is the function f? And no it’s not a simple array containing the numbers 2,5,10,40,92 at positions 4-8!

I also checked the OEIS (Online Encyclopaedia of Integer sequences) and it’s not in there either. Years ago I was fooled by a integer sequence that was the maximum number of electrons in orbital shells but its definitely not that.

Here’s a minor clue. It is game related. Answer in a day or two.