Awkward arc4random results

Question

I'm using this code, where 'length' value is '50'.

newX = (arc4random()%(lenght+1)) - (lenght/2);
newY = (arc4random()%(lenght+1)) - (lenght/2);
NSLog(@"Creature Move X:%f, Y:%f", newX, newY);

But in the debugger I get things like:

2012-01-02 21:10:50.794 Kipos[28833:207] Creature Move X:4294967296.000000, Y:4294967296.000000
2012-01-02 21:10:50.896 Kipos[28833:207] Creature Move X:4294967296.000000, Y:12.000000

What is happening?

newX and newY are floats:

float newX;
float newY;

Very strange... what's the type of '`lenght`' (typo btw), and can you NSLog it at the same time as this log? — joerick, Jan 02 '12 at 20:32
Length (typo *facepalm*) is an int. Already solved. But thanks! — David Da Silva Contín, Jan 02 '12 at 20:52

Paul R · Accepted Answer · 2012-01-02T20:49:50.953

5

arc4random returns an unsigned int (and presumably length is also unsigned). Change your code to e.g.

newX = (float)((int)(arc4random() % (length + 1))) - (length / 2));

in order to avoid overflow when you subtract.

Note that I have also added an explicit float cast for the result, which is not strictly necessary but it makes the code a little more self-explanatory.

edited Jan 02 '12 at 20:49

answered Jan 02 '12 at 20:39

Paul R

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Out of interest where did you look to find that out? My lazy googling couldn't find the return type of `%`. – Paul.s Jan 02 '12 at 20:43
So I guess here `length` was also unsigned? Otherwise the compiler should have converted the left operand to a signed int before doing the subtraction? Or do I have this wrong? – joerick Jan 02 '12 at 20:46
It worked! Thanks! It also worked Paul.s suggestion (change '2' for '2.00'): `newX = arc4random()%(length+1) - (length/2.00);` `newY = arc4random()%(length+1) - (length/2.00);` – David Da Silva Contín Jan 02 '12 at 20:48
1

@Paul.s: it's just the usual type rules for expressions in C - `unsigned % unsigned` will give an unsigned result – Paul R Jan 02 '12 at 20:53
Btw, instead of using %f in the NSLog, what would you suggest? – David Da Silva Contín Jan 02 '12 at 20:54
There's nothing wrong with `%f` but I prefer `%g` for logging and debug etc – Paul R Jan 02 '12 at 20:55
Thanks everyone for the help :) It was driving me crazy! – David Da Silva Contín Jan 02 '12 at 20:55

Awkward arc4random results

1 Answers1