4-rings or 4-squares puzzle: Difference between revisions
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This uses the facility standardised in F90 whereby DO-loops can have text labels attached (not in the usual label area) so that the END DO statement can have the corresponding label, and any CYCLE statements can use it also. Similarly, the subroutine's END statement bears the name of the subroutine. This is just syntactic decoration. Rather more useful is extended syntax for dealing with arrays and especially the function ALL for making multiple tests without having to enumerate them in the code. To gain this convenience, the EQUIVALENCE statement makes variables A, B, C, D, E, F, and G occupy the same storage as <code>INTEGER I(7)</code>, an array.
One could abandon the use of the named variables in favour of manipulating the array equivalent, and indeed develop code which performs the nested loops via messing with the array, but for simplicity, the individual variables are used. However, tempting though it is to write a systematic sequence of seven nested DO-loops, the variables are not in fact all independent: some are fixed once others are chosen. Just cycling through all the notional possibilities when one only is in fact possible is a bit too much brute-force-and-ignorance, though other problems with other constraints, may encourage such exhaustive stepping. As a result, the code is more tightly bound to the specific features of the problem.
Also standardised in F90 is the $ format code, which specifies that the output line is not to end with the WRITE statement. The problem here is that Fortran does not offer an IF ...FI bracketing construction inside an expression, that would allow something like <lang Fortran>WRITE(...) FIRST,LAST,IF (UNIQUE) THEN "Distinct values only" ELSE "Repeated values allowed" FI // "."</lang> so that the correct alternative will be selected. Further, an array (that would hold those two texts) can't be indexed by a LOGICAL variable, and playing with EQUIVALENCE won't help, because the numerical values revealed thereby for .TRUE. and .FALSE. may not be 1 and 0. And anyway, parameters are not allowed to be accessed via EQUIVALENCE to another variable.
So, a two-part output, and to reduce the blather, two IF-statements. <lang Fortran> SUBROUTINE FOURSHOW(FIRST,LAST,UNIQUE) !The "Four Rings" or "Four Squares" puzzle.
Choose values such that A+B = B+C+D = D+E+F = F+G, all being integers in FIRST:LAST...
INTEGER FIRST,LAST !The range of allowed values.
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END </lang>
Output: not in a neat order because the first variable is not
<pre>
The Four Rings puzzle, over 1 to 7. Distinct values only.
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