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?
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
This is normal for float values.
If you want to save initial (same) representation of float numbers in all places of your code, you can save them, for example, in NSString. When you will need float number you will just write [NSString floatValue];. But it is not effective if you have large amount of float values.
Related
I have two text fields and user entered the values. I can get the values of textFields like below
NSString *number1 = firstTextField.text;
NSString *number2 = secondTextField.text;
I want to multiply number1 and number2 without converting them into integer or number.I am doing like below
NSExpression *expression = [NSExpression expressionWithFormat:[NSString stringWithFormat:#"%#*%#",number1,number2]];NSLog(#"Multiplication result is----%#",[expression expressionValueWithObject:nil context:nil]);
I don't know if it is correct or not. If it is not correct please give me the suggestions how it can be possible.
By using NSExpression is one way to Multiply/Add/Subtract two number strings without converting them into integer or number.
NSExpression *expression = [NSExpression expressionWithFormat:expressionFormat];
You are using here expressionFormat as [NSString stringWithFormat:#"%#*%#",number1,number2];.
If you want to add or subtract number1 and number2 then replace * with + or -. In this way you have to proceed.
If the question was in an interview.
The interviewers were probably expecting you to write a method to go through both arrays and multiply the characters (converting one by one to integers) or (also identifying the represented character to know the equivalent integer number).
Searching on google there are some examples in different languages.
JAVA
http://csjobinterview.wordpress.com/2012/04/05/string-multiplication/
string multiplication
C++
Multiplying two number arrays
It is a common question in interviews.
I simply used textfields... and used the values that the user input into those textfields... here is one calculation example that has worked for me... in this example the user is trying to find the volume of beachstone they would need to order... the formula is pretty straight forward. Make sure you use the math brackets () to distinguish order of operations...
-(IBAction)calculate_beachstone:(id)sender {
float floatanswer =
([area1.text floatValue])+([area2.text floatValue])+([area3.text floatValue])+([area4.text floatValue])+([area5.text floatValue])+([area6.text floatValue])+([area7.text floatValue])+([area8.text floatValue])+([area9.text floatValue])+([area10.text floatValue]))
*([beachstone_depth.text floatValue])/12)/27);
NSString *stringRectResult=[[NSString alloc]
initWithFormat:#"%.1f",floatanswer];
answer_beachstone.text=stringRectResult;
}
I am adding the text found in the textfield (which user can only use numbers for input - custom keyboard)... in this example I have up to 10 fields which can be added together... then I use the * (multiply) to apply a depth in this example, and convert it back to a text string so I can display the result somewhere else...
In fact, if you write this part answer_beachstone to NSUserDefaults, you can use the result anywhere in different controllers, by calling it back.
If you're interested, here is how I did that...
-(IBAction)save_answer_beachstone:(id)sender {
save_answer_beachstone = [[NSString alloc] initWithString:answer_beachstone.text];
[answer_beachstone setText:save_answer_beachstone];
NSUserDefaults *save = [NSUserDefaults standardUserDefaults];
[save setObject:save_answer_beachstone forKey:#"save_answer_beachstone"];
}
Then I can use the resulting "answer" in any controller inside viewDidLoad...
[answer_beachstone setText:[[NSUserDefaults standardUserDefaults] objectForKey:#"save_answer_beachstone"]];
I know the question was asked 4 years ago, but this formula syntax works for me in a number of ways, in various apps...
You can't multiply two strings. It's not possible.
For this you have to convert it into Integer or int using NSNumber or using:
[secondTextField.text intValue].
NSInteger number = [firstTextField.text integerValue]*[secondTextField.text integerValue];
I'm trying to create NSDecimalNumber with simply format like: 123.00 with two fractional digits after dot, always. I tried use the NSFormatter and many other ways like converting float from string and creating then NSDecimalNumber from this string, but it's not working.
The problem is that I need only NSDecimalNumber in this format, not NSString or any other.
Thanks for any advice,
Paul
You may get idea from this.
float num = 123.1254546;
NSString *str = [NSString stringWithFormat:#"%.2f",num];
NSLog(#"%.2f %#",num,str);
I think you simply need to do Type Casting operation for Two times as bellow
float value = 123.562;
int myDigit = value;
it gives value 123 in myDigit variable
NSLog(#"MyDigit in Decimal = %d",myDigit);
Output is MyDigit in Decimal = 123
now if you want output like 123.000 then simply write as bellow
float valueWithZiro = myDigit;
NSLog(#"MyDigit with 000 == %3f",valueWithZiro);
Output is MyDigit in Decimal = 123.000
NSDecimalNumber, like NSNumber, cannot contain formatting information. The object structure simply doesn't support it. It represents a number, not the way the number is displayed.
You can convert it to a formatted NSString (which you say you don't want). But you can't do what you're asking.
You convert it to a formatted NSString using an NSNumberFormatter. It's the object that allows you to specify the decimal and thousands separators, number of decimal places to display, the currency symbol, etc.
Maybe you were looking to store a NSNumberDecimal with just two digits after the fraction?
If so NSDecimalNumberBehaviors is your friend.
I had a similar need and used the following:
self.decimalHandlingBehaviorForApp = [NSDecimalNumberHandler
decimalNumberHandlerWithRoundingMode:NSRoundUp
scale:2 raiseOnExactness:NO
raiseOnOverflow:NO raiseOnUnderflow:NO
raiseOnDivideByZero:YES];
Edit: added example of using it
// update the taxable total first
self.cartTaxableTotal = [self.cartTaxableTotal decimalNumberByAdding:itemAdded.priceOfItem
withBehavior:self.decimalHandlingBehaviorForApp];
I have a double that I need only the value of everything before the decimal point.
Currently I am using
NSString *level = [NSString stringWithFormat:#"%.1f",doubleLevel];
but when given a value of 9.96, this returns "10". So it is rounding. I need it to return only the "9". (note - when the value is 9.95, it correctly returns the "9" value.)
Any suggestions?
Thank You.
Simply assign the float/double value to a int value.
int intValue = doubleLevel;
Cast that baby as an int.
int castedDouble = doubleLevel;
Anything after the . in the double will be truncated.
9.1239809384 --> 9
123.90454980 --> 123
No rounding, simple truncation.
If you want to keep it as a float:
CGFloat f = 9.99;
f = floorf(f);
there are quite a variety of floor and round implementations.
they have been around since UN*X, and are actually part of those low-level libraries, be they BSD, Posix, or some other variety - you should make yourself familiar with them.
there are different versions for different "depths" of floating point variables.
NSString *level = [NSString stringWithFormat:#"%d",doubleLevel];
I have an iPhone app. I am storing a float value, (distance), in my sqlite3 db. (The field in the db is formatted to float) I am able to store float values correctly in the db no problem. However, I can't seem to figure out how to pull the value back out of the db the format and present it correctly. Here is my code for pulling the value out of my db and using it:
NSString *itemDistance = [NSString stringWithFormat:#"%f",[item distance]];
float questionDistance = [itemDistance floatValue];
[item distance] is a float value. I can't get this to work. I get a REALLY long value instead. What am I doing wrong? Any help would be greatly appreciated.
Thank you in advance for your help,
L.
Assuming your -distance method is returning the right value, then this sounds like basic misunderstanding of how floats work in C. Common mistake. Every developer falls into this trap at least once.
Floating point numbers can actually only represent a fairly limited number of values. Specifically, unless you happen to choose a value that is exactly representable as a float, you'll get the nearest value, which will often have many decimal places of data.
The reason for this is because a float is only 32 bits; 4 bytes. Now, how many numbers with decimal points are there between 0..1000000 or 0..1000 or, even, 0..1. Infinite. Floats implement a very finite subset of possible numeric values and do so in a way where the resulting possible values may have many decimal places.
Consider:
printf("%5.18f\n", (float) 2.05);
printf("%5.18f\n", (float) 2.45);
printf("%5.18f\n", (float) 4200.75);
printf("%5.18f\n", (float) 37.89);
printf("%5.18f\n", (float) 1.2);
printf("%5.18f\n", (float) -1.2);
This prints:
2.049999952316284180
2.450000047683715820
4200.750000000000000000
37.889999389648437500
1.200000047683715820
-1.200000047683715820
Those values are as close to the values in the bit of code that a float could represent.
If you need more precision, use a double. More precision than that? Use NSDecimalNumber.
And, if you can, use CoreData. It makes managing this kind of thing a bit more straightforward.
why go round in circles?
NSString *itemDistance = [NSString stringWithFormat:#"%.2f",[item distance]];
float questionDistance = [itemDistance floatValue];
Where the .2 is saying I want 2 decimal places.
Whats the type of of distance? If it is an instance of NSNumber then I dont think the code you wrote will work, try
NSString *itemDistance = [[item distance] stringValue];
float questionDistance = [itemDistance floatValue];
or even
float q=[[item distance] floatValue];
I have made a quadratic equation solver for the iPhone and when the text box is clicked, my own custom keypad appears. I have a button that changes whether the number is positive or negative. Right now I what happens is that when the button is pressed (0 - current value of text) is what is displayed in the text box, so if the number is positive, it will become negative and if it is negative it will become positive. I am having some problems doing this so what I wanted to is to put a minus sign at the beginning of the string if the number is positive and if the number is negative, the minus sign will be removed. Can anyone give me guidance on this?
Instead of negating using a mathematical function I assigned a NSMutableString to my UITextField then I inserted a "-" sign using insertString:atIndex: then I reassigned the changed string to my UITextField. To toggle between positive and negative, I created an if function so if the float value of my textfield is greater or equal to 0, then an "-" is inserted but if the float value of my text field is less than zero, the "-" is removed using deleteCharactersInRange. Here is my code as it stands:
- (IBAction)positivity{
NSMutableString *a = [NSMutableString stringWithString:aVal.text];
if([aVal.text floatValue]>=0){
[a insertString: #"-" atIndex: 0];
aVal.text = a;
}
else if([aVal.text floatValue]<0){
NSRange range = {0,1};
[a deleteCharactersInRange:range];
aVal.text = a;
}
}
aVal is the name of the UITextField that i am changing.
An alternative to the straight string approach is to not use a string. A while back I wrote a graphing calculator for iPhone that stored the equation internally in an NSMutableArray of NSStrings. Each slot in the array corresponded to one element in the equation, such as "x", "^", "sin(", etc.
When I needed to negate the equation, it was much easier to tell the array to insertObject:#"-" atIndex:0 than to try and insert it directly into the string. Then whenever the array was changed, I just remade the equation string like this:
NSString * newEquation = [equationElements componentsJoinedByString:#""];
While you could directly manipulate a string representation of a numeric value, such an approach is a bad idea. Not only is it less efficient than other alternatives, but potentially incorrect. (For example, #Ken's answer would result in two minus signs.)
What you probably want to do is negate the numeric value (just multiply it by -1, or subtract it from 0 as you suggested) and reflect that change in the interface (you mention a text box).
If you're using standard Cocoa controls (which inherit from NSControl, as NSTextField does) I suggest using -[NSControl setIntegerValue:] to change the text of the text field. If you (can) break up your UI well and have a text field for each variable in the quadratic equation, this should be fairly simple. (If you're using something other than an integer value, use something like -setDoubleValue: or -setFloatValue: instead.)
If you must create your own string beforehand, using an integer format specifier will display a "-" sign automatically if appropriate. Be sure to use %ld instead of %d (thanks, #Peter!) as the format specifier for NSInteger values to avoid possibly truncating values larger than 32-bit. For example:
NSString *result = [NSString stringWithFormat:#"%ld", nsIntegerValue];
In a more general sense, if you need to insert a dynamically-obtained string (not just something for which you can create a format string at compile time) you can also use an NSMutableString and its methods -appendString: and -insertString:atIndex: as well.