# ⨳ simulating child's play ⨳

2012 Apr 02, Monday

⨳ 3 minute read ⨳ 678 words ⨳*array*⨳

*do loop*⨳

*sas*⨳

*simulation*⨳

as of late, the kids are enjoying simple, pre-risk board games. of these, i prefer chutes and ladders. for the most part, there’s no skill involved: you spin, you count, and maybe slide but hopefully climb. there’s no strategy involved. you just take what you get and hope to avoid the chutes. but i’ve been hesitant to start a game too close to bedtime (since it is essentially a game of luck, you could theoretically get stuck in a chute-ladder loop making no progress to the goal).

that got me wondering about the length of a ‘regular’ game.

since the game is essentially a game of chance (the haven’t mastered the art of ‘light’ spins), it’s pretty easy to simulate in SAS.

the first step was to create a 100-element array for the lookups (that’s one element for each position on the board). for this array, the index serves as the ‘landing’ position, and the value is the ‘destination’ position. if the index matches the value, there is no chute or ladder to worry about. if the index is greater than the position, you’ve landed at the base of a ladder; if the index is less than the position, you’ve landed at the start of a chute.

next, i simulate 1,000,000 games with up to 250 turns. all games start at index 0 (that is, the first spin will get them on the board) and are trying to end up on index 100 (without overshooting). the math is pretty easy: take your starting position and add the spin value to determine the landing value. then, check against the `cl`

array for any climbing or sliding needed before ending at the destination.

here’s my code:

```
data ChutesAndLadders;
format Game Turn Start Spin Land End 4.;
*** set up the wormhole array ***;
*** if cl[i] ne i, then a wormhole ***;
array cl(100) _temporary_ (
38, 2, 3, 14, 5, 6, 7, 8, 31, 10,
11, 12, 13, 14, 15, 6, 17, 18, 19, 20,
42, 22, 23, 24, 25, 26, 27, 84, 29, 30,
31, 32, 33, 34, 35, 44, 37, 38, 39, 40,
41, 42, 43, 44, 45, 46, 47, 26, 11, 50,
67, 52, 53, 54, 55, 53, 57, 58, 59, 60,
61, 19, 63, 60, 65, 66, 67, 68, 69, 70,
91, 72, 73, 74, 75, 76, 77, 78, 79,100,
81, 82, 83, 84, 85, 86, 24, 88, 89, 90,
91, 92, 73, 94, 75, 96, 97, 78, 99,100);
do game = 1 to 1000000;
GameOver=0;
do turn = 1 to 250 until (GameOver=1);
*** all games start at 0 ***;
if turn = 1 then do;
Start = 0;
Spin = 0;
Land = 0;
End = 0;
end;
*** starts where ended last turn ***;
Start = End;
*** spin the thing ***;
Spin = ceil(ranuni(8675309)*6);
*** last spin must be EXACTLY 100 ***;
if Start + Spin > 100 then Spin = 0;
*** landing spot (pre-wormhole)***;
Land = Start + Spin;
*** destination (post-wormhole) ***;
End = cl[Land];
*** game over? ***;
if End = 100 then GameOver=1;
output;
end;
end;
drop GameOver;
run;
```

a quick `proc univariate`

shows the following:

```
Variable: Turn
Moments
N 999896 Sum Weights 999896
Mean 39.576811 Sum Observations 39572695
Std Deviation 25.5080205 Variance 650.659108
Skewness 1.78522598 Kurtosis 4.7663735
Uncorrected SS 2216751859 Corrected SS 650590789
Coeff Variation 64.4519349 Std Error Mean 0.02550935
Basic Statistical Measures
Location Variability
Mean 39.57681 Std Deviation 25.50802
Median 33.00000 Variance 650.65911
Mode 22.00000 Range 243.00000
Interquartile Range 28.00000
```

one thing to note is that rarely (104 times out of 1000000 games) it takes a single player more than 250 spins to end on 100. that’s a long game. if all 4 players we simultaneously unlucky, that’d be a painfully long game.

**UPDATES**:

- I’ve spawned off a GitHub repo. This code can be found in the
`\ChutesAndLadders`

subdirectory. - A brief search online yielded this page which uses R for simulation and analysis.