The crossroads

This is part 7, the last numbered part. It does two things: shows you one program that uses everything parts 1–6 taught, so you can measure how much that now is — and then hands you the map, because from here the path fans out into themed tutorials that no single order serves.

The example is examples/Crossroads in the tutorial repository. It reads starting points from a file and walks each one by part 4’s halve-or-triple rule:

make -C examples/Crossroads run

Crossroads

  start      steps    highest
      7         16         52
     27        111       9232
  not a number: peak?
     97        118       9232

Read the table twice. The third line is a data line that was not a number, reported and skipped. And 97’s walk is neither 27’s prefix nor its scale — yet both funnel through the same summit, 9232. Different roads, one mountain pass; the walk is stranger than parts 4 and 5 let on.

What you can now read

Everything in this program is parts 1–6, plus one new thing:

type
    Walk = record
        start, steps, peak : int64;
    end;

A record gathers named facts under one name — exactly what part 5’s WalkFrom wanted when it had two facts to return and had to borrow your variable for the second. Now it answers all three as one value:

function WalkFrom(from : int64) : Walk;

and the value rule you already know extends whole: assigning a record copies all of it. The caller’s w is the function’s answer, nobody’s alias.

The rest you have seen: the while and the if are part 4’s; the function with its own locals is part 5’s; the Open … on fail caught do with Enoent answered by name, the defer Close, and the ReadLine loop are part 6’s. One new spelling — on fail use False — gives a fallible read a visible default instead of a handler: if the wire to the file breaks mid-loop, the loop simply ends. And notice the two kinds of “no” sitting side by side: a file that cannot be opened is a failure, with a channel and a name; a line that is not a number is just an answer — Val returns False and the program moves on. Mica keeps those apart on purpose, and the error philosophy page is where that line is drawn properly.

Try it

1. Feed it a monster. Add 6171 to starts.txt and re-run:

   6171        261     975400

Two hundred sixty-one steps, a peak near a million — from a four-digit start. The table format holds; the walk really is that strange.

2. Take the file away. Rename starts.txt and re-run:

starts.txt is not here — the walk needs its starting points

Part 6’s arrangement, doing its quiet work. Rename it back.

The map

The numbered path ends here; the themed tutorials each take one subject to the bottom. This is the reading order the series recommends when you want a particular thing:

Wherever you go next: every example lives in the tutorial repository, in learning order, each with a header that says what it teaches. And if the path left a gap the map does not cover, tell us: info@mica-dev.com.