Four 4's problem
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Four4s ( problem can be used to learn/teach many of the concepts in computer science:

1) Writing a program to generate expressions for all numbers in a range (for example 1-50) turned out to be an interesting exercise. We need to build up complex expressions from the simpler ones as follows:

   a.  Simplest expressions: 

       First create all expressions of the form: B(1) = { .4, 4.}, B(2) = { .44, 4.4, 44. } etc., B(4)  = { .4444, ... , 4444.} 

       Here B(n) stands for an expressions with no operations and no functions applied to a list of n 4?s. Only decimal point is allowed.

   b.  Next simple expressions: 

       Create all expressions that apply unary functions (like factorial, sqrt) to B(n)  as well as to the results of first application so as to get expressions like sqrt(fact(4)) etc.,
       i.   U(n) = B(n) UNION { OP1(U(n)) | OP1 is one of factorial, 
sqrt etc.,}
       ii.  Here U(n) represents all B(n) and other expressions we can get by applying unary operations (like !, sqrt) and having n 4?s in the expression
       iii. Note that this leads to infinite recursion: for example :
            fact(fact(fact(???.(4)))). We can stop after x number of applications of OP1 to any U(n)

   c.  Finally Complex expressions: 
       Create expressions that apply binary operations (+, *, /, -) to U(n) and to the results of first application. Call them S(opc, n). Each S has opc number of binary operators (like +, /) and a total of n 4?s in the expression. An example from the set S(2, 4) is 44.+4.-4. Note that opc < n to create a valid expression.
       i.   S(0, n) = U(n)
       ii.  S(1, n) = { S(0, k1) OP2 S(0, k2) | for all k1, k2 such that k1+k2 = n and n > 1 and OP2 is one of +, -, /, *, exponent etc.,}
       iii. For all opc > 1:  S(opc, n) = { S(m, k1) OP2 S(opc-1-m, k2) | m = 0, 1, ? opc-1 and opc >= 2 and OP2 is one of +, -, /, *, etc.,}
       iv.  We again have to stop after x applications to avoid into infinite recursion.
       v.   If we want to allow parentheses to change the default order of computation, then we need to consider those expressions also in the above S(opc, n) computation.  I could not generate expressions for values 35, 41 (in the range 1-100) without using parenthesized expressions.

2) The above program is in general going to be slow. So, here the optimization is must before we can make use of it (after a subset of optimizations, the program ran lot faster - under 2 minutes for range 1-50.  Of course, 2 minutes is still slow). Few things we can do:

   a. Stop application of unary/binary operations after 1 application (x=1)
   b. Write custom factorial and sqrt functions instead of depending on the compiler provided ones: So that you can stop evaluating an expression when the factorial value exceeds a set limit or if the argument for factorial is not integer or when the argument to sqrt is ?ve reject the corresponding expression
   c. Do not use recursive function to compute factorial: fact(n) { if n == 1: return 1 else return n * fact(n-1)}. Here you will be computing fact(x) again and again. Use a table of pre computed factorials.
   d. When building complex expressions from simpler ones, reuse the simpler expressions instead of creating them again and again
   e. Create custom expression evaluator to stop as soon as we know that the value is going to be too big for the desired range (in this example, 1-50).
   f. Instead of trying all generated expressions to find the possible expressions for a given number (say 34) -
      -  Evaluate each expression and maintain a hashtable of (key, value) = (value of expression, array of expressions that produce this value)

3) Some enhancements: Each value can have a lot of expressions that can generate it. By giving a complexity score to each of unary/binary operations and decimal point, we can assign an overall complexity score to each expression and keep only the ones with low score. For example for value 1 we prefer expression 44/44 over expression 4.4/4.4 or factorial(44)/ factorial(44).

4) Interesting CS things:
   a. Recursive functions ? recursive application of operators to simple expressions to generate complex ones ? parse trees that get generated out of expressions, etc.,
   b. Recursive function theory: Without stopping repeated application of operators, each of the sets U, S will keep on generating until infinity - the theory of recursive functions and recursively enumerable functions.
   c. Dynamic programming: Reuse of generated values (for example in computing a table of factorials) and expressions (while building complex ones from simple ones) ? many of dynamic programming techniques can be taught just from this.
   d. Algorithm design: Need for brute-force algorithms (until a mathematician finds a formula to auto-generate the expression(s) for a given value), optimization, back-tracking etc
   e. Practical aspects of developing complex software applications ?for example need to assign complexity score to answers and pick the best ones based on prioritization
A python script that demonstrates some of above things (it is ugly ? wrote just to validate above points) is attached. Here is some excerpt from the output:
1 --->
            .44/.4 4
35 --->
41 --->
47 --->