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Asked: 2026-05-25T10:16:18+00:00

I have an NSArray in which I would like to store double values. I

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I have an NSArray in which I would like to store double values. I defined it as follow

NSMutableArray *setOfDoubles;

I add the elements as follow

NSNumber *num;
num = [NSNumber numberWithDouble:someDouble];
[setOfDoubles addObject:num];

And I read the element as follow

num = [setOfDoubles lastObject];  
[setOfDoubles removeLastObject];
doubleValueToUse = [num doubleValue];

My problem is sometimes (Not always), for example when num (as an object) is 5.1, doubleValueToUse (as a double value) is 5.099999999999996. The way I figured num (as an object) is 5.1 is that I debug and when I am hovering the mouse on top of num on the line num = [setOfDoubles lastObject]; it shows 5.1 but after doubleValue conversion it becomes the number I mentioned. Does anybody know why is this happening?

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1 Answer

  1. Editorial Team

    Not every number can be accurately represented using a float variable. For example, you can’t precisely represent, say, 1/3 using a finite number of digits in our common decimal (base-10) system, but in ternary (base-3), it would be just 0.1. Similarly, the numbers you can write with a finite number of digits in decimal, may not necessarily have the finite number of digits in their binary representation, hence the error.

    A few links on the topic if you are interested:

    http://floating-point-gui.de/basic/

    http://www.mathworks.com/support/tech-notes/1100/1108.html

    http://download.oracle.com/docs/cd/E19957-01/806-3568/ncg_goldberg.html

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