CODEWITHSUNDEEP
iosobjective-cc
Takuya Takahashi
Takuya Takahashi

Reputation: 453

Strange value of calculated float in Objective-C

My app calculated like this.

float containTaxValue = 840;
float taxRate = 5;
float divisionedNum = (100 + taxRate)/100;
float removedTaxValue = containTaxValue/divisionedNum;
float taxValue = containTaxValue - removedTaxValue;

finally the answer is 

NSLog(@"%f",removedTaxValue); => 800.000061
NSLog(@"%f",containTaxValue); => 840.000000
NSLog(@"%f",taxValue); => 39.999939

I would like to get "taxValue == 40.000000" in this code.

I couldn't make sense what's issue. Let me know any advise please.

Upvotes: 0

Views: 870

Answers (4)

Hemang
Hemang

Reputation: 27072

If you're not sure how long your number after dot, you can use %.6f instead simple %f to print exact 6 digits after dot, to round up the values you can use ceil(double) function, It will round up the values.

with %f only,

float taxValue = 39.999999; // you've 6 digits after dot
NSLog(@"%f",ceil(taxValue));

Output, 40.000000

with %.6f,

float taxValue = 39.99; // you've only 2 digits after dot but still you want to show 6 digit
NSLog(@"%.6f",ceil(taxValue));

Output, 40.000000

Upvotes: 0

Abhishek Singh
Abhishek Singh

Reputation: 6166

The IEEE 754 standard is a way of storing floating-point numbers in an easy to manipulate way by the machine. This method is used by the INtel and mot of the processors.

IEEE 754 specifies that numbers be stored in binary format to reduce storage requirements and allow the built-in binary arithmetic instructions that are available on all microprocessors to process the data in a relatively rapid fashion. However, some numbers that are simple, non-repeating decimal numbers are converted into repeating binary numbers that cannot be stored with perfect accuracy.

1/10 can be represented in Decimal form .1

But In Binary form it Becomes: .0001100011000111000111 (and so on) 

And Hence the rounding-off error occurs.

You have to convert it to int to round it off.

The Binary Conversion of 1.05 also goes on 00111111 10000110 01100110 01100110....

Upvotes: 2

Vishnu
Vishnu

Reputation: 2243

To round up float Values you can use following code

float A,B; // this variables have to be floats!
int roundDown = floor(A/B); // rounded down
int roundUp = ceil(A/B); // rounded Up
int rounded = lroundf(theFloat); //rounded

The result int value is converted again to float

Hope this Helps !!!

Upvotes: 0

Nikolai Ruhe
Nikolai Ruhe

Reputation: 81878

float cannot exactly represent many values, for example 1.05. Rounding errors occur and carry forward to the final result.

Upvotes: 1

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