I wrote a sample to convert string to float in Objective-C:
NSString *sampleFloatString = #"1.3";
float sampleFloatValue = [sampleFloatString floatValue];
But when I display sampleFloatValue, it shows '1.29999995'. I know it's equal to 1.3, but why is it not exactly '1.3'? Why do we need to format it explicitly? Is there any other way of doing this conversion?
Its called "Floating point error". The way that computers represent decimal numbers causes them to not be 100% accurate all the time:
http://docs.oracle.com/cd/E19957-01/806-3568/ncg_goldberg.html
Try this
float sampleFloatValue = (float) sampleFloatString;
Hope it helps.
if you just wants to show these value somewhere than u can do these and it shows 1.3 exact..
NSString *sampleFloatString = #"1.3";
float sampleFloatValue = [sampleFloatString floatValue];
NSLog(#"%.1f",sampleFloatValue);
Related
The solution of the square root of 11 in a normal calculator is roughly
3.31662479036
I am trying round to 3.32
So my problem I am having in iOS is;
In my viewDidLoad
int Mynumber = sqrt(11);
NSLog(#"%d", Mynumber);
I keep getting 3 when I should to get the first three integers.
Can anyone help me solve this?
int is for integers, not decimals. Use float and %f.
float myNumber = sqrtf(11);
NSLog(#"%.2f", myNumber);
float theFloat = sqrt(11);
NSLog(#"%.2f", theFloat);
// also u can use integer round figur like this...
int rounded = lroundf(theFloat); NSLog(#"%d",rounded);
int roundedUp = ceil(theFloat); NSLog(#"%d",roundedUp);
int roundedDown = floor(theFloat); NSLog(#"%d",roundedDown);
I'm currently parsing NSString values to NSNumbers and then adding them into a NSMutableArray called operands in an object called "data" like so:
NSNumberFormatter * f = [[NSNumberFormatter alloc] init];
[f setNumberStyle:NSNumberFormatterDecimalStyle];
NSNumber * myNumber = [f numberFromString:*operandString];
[data.operands addObject:myNumber];
I then retrieve those numbers, perform some math on them, then update the array:
double x = [[data.operands objectAtIndex: i]doubleValue];
double y = [[data.operands objectAtIndex: i + 1]doubleValue];
double answer = x * y;
[data.operands replaceObjectAtIndex:(i) withObject:[NSNumber numberWithDouble:answer]];
When I get the answer, everything looks fine eg: ( 3.33 * 5 = 16.65)
BUT, when I look in the debugger I'm seeing some crazy values for x and answer, such as:
x = 3.3300000000000001
answer = 16.649999999999999
Why is this happening? Am I loosing some precision with parsing these back and fourth? Is it how I've used the NSNumberFormatter to parse the string?
The reason I'm in trouble with this is because I'm trying to ensure there's no double overflow errors so I'm using this simple test to check the integrity:
if (answer / y != x){
//THROW OVERFLOW ERROR
}
With the above crazy numbers this is always inconsistent. When I NSLog the answer it comes out fine:
NSLog (#"%g", [[data.operands objectAtIndex:i]doubleValue]]);
Same for
NSLog (#"%f", [[data.operands objectAtIndex:i]doubleValue]]);
You are not losing any precision that you need to worry about. Those are the correct values. There are only about 2^60 different double numbers, that finite set has to try to approximately cover the infinite 'number of numbers' in the range that doubles cover.
In other words, there are no exact answers in computer land and your
if (answer / y != x){
//THROW OVERFLOW ERROR
}
Will not work. Or it may work much of the time, but fail if you push it. Instead you need to acknowledge the limited precision (which is pretty high precision) of doubles:
//Don't waste time worrying like this...
if (fabs(answer / y - x) > 1e-12*fabs(answer)){
//Not correct or useful thing to check don't use this - i did not check
}
// let the math package handle it:
if (isnan(answer)){
// we gots problems
}
if (!isnormal(answer)){
// we gots some other problems
}
Also don't forget that 10^300 is a very large number, doubles work pretty well. To use 32 bit floats you need to pay much more attention to order of execution, etc.
NSLog is likely outputting with fewer decimals of precision, and rounds to the nearest thing, so the answers look better.
in iOS Objetive-C I am trying to get the number typed by the user in a text field to set the upper bounder of a random number generation function in C.
- (IBAction)pushTheButton2:(id)sender {
u_int32_t upperBound = (u_int32_t) textField3.text;
textField4.text = [NSString stringWithFormat:#"%d", arc4random_uniform(upperBound)];
}
The output is a giant number that makes no sense. To test if function works, if I hardcode the actual upper bound in the arc4random_uniform function, such as arc4random_uniform(5), then it works!
I figured this could be some kind of literal conversion, so I tried to make this work with u_int32_t but still not outputting the right range.
Can someone help? Thanks
You are currently taking the memory reference pointer of the text and using that as the upper bound.
Try doing something like this instead...
NSInteger upperBound = [textfield.text intValue];
This will convert the string into an int that you can then use in the arc random function.
To parse string to integer you should do:
NSInteger upperBound = [textfield.text integerValue];
NSTimeInterval expirationTime = (secondsSinceUnixEpoch*1000)+120000;
expirationTime = ceil(expirationTime/2);
int expirationInt = (int)expirationTime;
NSLog(#"%d", expirationInt);
The log output is always negative, even though before I convert it to an int it's positive... I tried just multiplying it by -1 to make it positive again and it's just staying negative! I'm totally perplexed.... don't know much about C, am I just doing something silly??
The number (secondsSinceUnixEpoch*1000)+120000 looks to me like it's going to be way too large to fit in an int. Chances are the integer is overflowing and becoming negative.
Converting to long long is one solution. As you stated in a comment, you want to show a whole number for use in a URL. Just do this:
NSTimeInterval expirationTime = (secondsSinceUnixEpoch*1000)+120000;
expirationTime = ceil(expirationTime/2);
NSString *urlString = [NSString stringWithFormat:#"http://example.com?time=%.0f", expirationTime];
This will format the decimal number as a whole number.
I am trying to calculate a long value divided by an integer to give me what I would expect to be a double, although the result I am getting is 0. The code I am using...
double daysByYear = daysSinceBirthdayToService/365;
NSLog(#"%d", daysByYear);
In this code, daysSinceBirthdayToService variable is a Long Double which can be NSLogged using the following code (long)daysSinceBirthdayToService
It is declaired in the header file as a property of
#property (nonatomic) NSInteger daysSinceBirthdayToService;
Can anybody help me out with this, thanks!
The issue is that / between two longs will do an integral division.
To force a floating point division at least one of the operands needs to be cast to double.
e.g.
double daysByYear = daysSinceBirthdayToService/(double)365;
or if you have a literal make that a double by adding a decimal point
double daysByYear = daysSinceBirthdayToService/365.0;
double daysByYear = daysSinceBirthdayToService/365.0;
Can it be that %d outputs decimal number not a double?