First off, the column in the database table is a DECIMAL with a length of 10,4.
If I do this on the console or even in the controller, the result is correct:
rate = (1 / 0.7126) * 1.0883
but if I do this, it doesn't work:
rate = (1 / from_rate) * to_rate
The result is simply the value of to_rate.
from_rate and to_rate are the results of database queries and the values are correct, if I log the class using .class the result is BigDecimal, so I have no idea why this calculation doesn't work as intended.
I've tried explicitly converting using to_d and BigDecimal.new but they don't seem to make a difference.
From evidence, it looks like the integer division instance method (invoked by / as in (1 / from_rate) returns an integer result when the argument (the divisor) is BigDecimal, so in this case it's returning the integer value 1. This isn't a problem when the argument is a float as in those cases integer division does return a float.
You may be able to circumvent this problem by just using the float divisor... do that by specifying 1.0 (a float) as the dividend.
rate = (1.0 / from_rate) * to_rate
Or alternatively you can convert the from_rate value into a float.
rate = (1 / from_rate.to_f) * to_rate
Related
Given any number of the sort 78.689 or 1.12 for instance, what I'm looking for is to programmatically round the number to the nearest tenth place after the decimal.
I'm trying to do this in an environment where there is a math.floor() function that rounds to the lowest whole number, and as far as I can tell from documentation there's nothing like PHP's round() function.
There's simple snippet at: http://lua-users.org/wiki/SimpleRound
function round(num, numDecimalPlaces)
local mult = 10^(numDecimalPlaces or 0)
return math.floor(num * mult + 0.5) / mult
end
It will misbehave when numDecimalPlaces is negative, but there's more examples on that page.
You can use coercion to do this...
It work just like printf... You can try to do something like in this snippet.
value = 8.9756354
print(string.format("%2.1f", value))
-- output: 9.0
Considering that this is roblox, it would just be easier to make this a global variable instead of making a single module or creating your own gloo.
_G.round = function(x, factor)
local factor = (factor) and (10 ^ factor) or 0
return math.floor((x + 0.5) * factor) / factor
end
In my case, I was simply trying to make a string representation of this number... however, I imagine this solution could prove useful to others as well.
string.sub(tostring(percent * 100), 1, 4)
so to bring it back to a numerical representation, you could simply call tonumber() on the resulting number.
I am using lua 5.3.2 and the following piece of code gives me an error:
string.format("%d", 1.16 * 100)
whereas the following line works fine
string.format("%d", 1.25 * 100)
This is probably related to this question but the failure depends on the floating point value. Given that, in my case, a local variable (v) holds the float value, and is generated by an expresson that produces a value between 0 and 2 rounded to 2 decimal places.
How can I modify the code to ensure this doesn't fail for any possible value of v ?
You can use math.floor to convert to integer and add +0.5 if you need to round it: math.floor(1.16 * 100 + 0.5). Alternatively, "%.0f" should have the desired effect as well.
I have trouble with integer division in Dart as it gives me error: 'Breaking on exception: type 'double' is not a subtype of type 'int' of 'c'.'
Here's the following code:
int a = 500;
int b = 250;
int c;
c = a / b; // <-- Gives warning in Dart Editor, and throws an error in runtime.
As you see, I was expecting that the result should be 2, or say, even if division of 'a' or 'b' would have a result of a float/double value, it should be converted directly to integer value, instead of throwing error like that.
I have a workaround by using .round()/.ceil()/.floor(), but this won't suffice as in my program, this little operation is critical as it is called thousands of times in one game update (or you can say in requestAnimationFrame).
I have not found any other solution to this yet, any idea? Thanks.
Dart version: 1.0.0_r30798
That is because Dart uses double to represent all numbers in dart2js. You can get interesting results, if you play with that:
Code:
int a = 1;
a is int;
a is double;
Result:
true
true
Actually, it is recommended to use type num when it comes to numbers, unless you have strong reasons to make it int (in for loop, for example). If you want to keep using int, use truncating division like this:
int a = 500;
int b = 250;
int c;
c = a ~/ b;
Otherwise, I would recommend to utilize num type.
Integer division is
c = a ~/ b;
you could also use
c = (a / b).floor();
c = (a / b).ceil();
if you want to define how fractions should be handled.
Short Answer
Use c = a ~/ b.
Long Answer
According to the docs, int are numbers without a decimal point, while double are numbers with a decimal point.
Both double and int are subtypes of num.
When two integers are divided using the / operator, the result is evaluated into a double. And the c variable was initialized as an integer. There are at least two things you can do:
Use c = a ~/ b.
The ~/ operator returns an int.
Use var c;. This creates a dynamic variable that can be assigned to any type, including a double and int and String etc.
Truncating division operator
You can use the truncating division operator ~/ to get an integer result from a division operation:
4 ~/ 2; // 2 (int)
Division operator
The regular division operator / will always return a double value at runtime (see the docs):
for (var i = 4; i == 4; i = 3) {
i / 2; // 2 (double)
}
Runtime versus compile time
You might have noticed that I wrote a loop for the second example (for the regular division operator) instead of 4 / 2.
The reason for this is the following:
When an expression can be evaluated at compile time, it will be simplified at that stage and also be typed accordingly. The compiler would simply convert 4 / 2 to 2 at compile time, which is then obviously an int. The loop prevents the compiler from evaluating the expression.
As long as your division happens at runtime (i.e. with variables that cannot be predicted at compile time), the return types of the / (double) and ~/ (int) operators will be the types you will see for your expressions at runtime.
See this fun example for further reference.
Conclusion
Generally speaking, the regular division operator / always returns a double value and truncate divide can be used to get an int result instead.
Compiler optimization might, however, cause some funky results :)
NSLog(#"CEIL %f",ceil(2/3));
should return 1. However, it shows:
CEIL 0.000000
Why and how to fix that problem? I use ceil([myNSArray count]/3) and it returns 0 when array count is 2.
The same rules as C apply: 2 and 3 are ints, so 2/3 is an integer divide. Integer division truncates so 2/3 produces the integer 0. That integer 0 will then be cast to a double precision float for the call to ceil, but ceil(0) is 0.
Changing the code to:
NSLog(#"CEIL %f",ceil(2.0/3.0));
Will display the result you're expecting. Adding the decimal point causes the constants to be recognised as double precision floating point numbers (and 2.0f is how you'd type a single precision floating point number).
Maudicus' solution works because (float)2/3 casts the integer 2 to a float and C's promotion rules mean that it'll promote the denominator to floating point in order to divide a floating point number by an integer, giving a floating point result.
So, your current statement ceil([myNSArray count]/3) should be changed to either:
([myNSArray count] + 2)/3 // no floating point involved
Or:
ceil((float)[myNSArray count]/3) // arguably more explicit
2/3 evaluates to 0 unless you cast it to a float.
So, you have to be careful with your values being turned to int's before you want.
float decValue = (float) 2/3;
NSLog(#"CEIL %f",ceil(decValue));
==>
CEIL 1.000000
For you array example
float decValue = (float) [myNSArray count]/3;
NSLog(#"CEIL %f",ceil(decValue));
It probably evaluates 2 and 3 as integers (as they are, obviously), evaluates the result (which is 0), and then converts it to float or double (which is also 0.00000). The easiest way to fix it is to type either 2.0f/3, 2/3.0f, or 2.0f/3.0f, (or without "f" if you wish, whatever you like more ;) ).
Hope it helps
I'm trying to recalculate percentages in an after_update callback of my model.
def update_percentages
if self.likes_changed? or self.dislikes_changed?
total = self.likes + self.dislikes
self.likes_percent = (self.likes / total) * 100
self.dislikes_percent = (self.dislikes / total) * 100
self.save
end
end
This doesn't work. The percentage always comes out as a 100 or 0, which completely wrecks everything.
Where am I slipping up? I guarantee that self.likes and self.dislikes are being incremented correctly.
The Problem
When you divide an integer by an integer (aka integer division), most programming languages, including Ruby, assume you want your result to be an Integer. This is mostly due to History, because with lower level representations of numbers, an integer is very different than a number with a decimal point, and division with integers is much faster. So your percentage, a number between 0 and 1, has its decimal truncated, and so becomes either 0 or 1. When multiplied by 100, becomes either 0 or 100.
A General Solution
If any of the numbers in the division are not integers, then integer division will not be performed. The alternative is a number with a decimal point. There are several types of numbers like this, but typically they are referred to as floating point numbers, and in Ruby, the most typical floating point number is of the class Float.
1.0.class.ancestors
# => [Float, Precision, Numeric, Comparable, Object, Kernel]
1.class.ancestors
# => [Fixnum, Integer, Precision, Numeric, Comparable, Object, Kernel]
In Rails' models, floats are represented with the Ruby Float class, and decimal with the Ruby BigDecimal class. The difference is that BigDecimals are much more accurate (ie can be used for money).
Typically, you can "typecaste" your number to a float, which means that you will not be doing integer division any more. Then, you can convert it back to an integer after your calculations if necessary.
x = 20 # => 20
y = 30 # => 30
y.to_f # => 30.0
x.class # => Fixnum
y.class # => Fixnum
y.to_f.class # => Float
20 / 30 # => 0
20 / 30.0 # => 0.666666666666667
x / y # => 0
x / y.to_f # => 0.666666666666667
(x / y.to_f).round # => 1
A Solution For You
In your case, assuming you are wanting integer results (ie 42 for 42%) I think the easiest way to do this would be to multiply by 100 before you divide. That pushes your decimal point as far out to the right as it will ever go, before the division, which means that your number is as accurate as it will ever get.
before_save :update_percentages
def update_percentages
total = likes + dislikes
self.likes_percent = 100 * likes / total
self.dislikes_percent = 100 * dislikes / total
end
Notes:
I removed implicit self you only need them on assignment to disambiguate from creating a local variable, and when you have a local variable to disambiguate that you wish to invoke the method rather than reference the variable
As suggested by egarcia, I moved it to a callback that happens before the save (I selected before_save because I don't know why you would need to calculate this percentage on an update but not a create, and I feel like it should happen after you validate that the numbers are correct -- ie within range, and integers or decimal or whatever)
Because it is done before saving, we remove the call to save in the code, that is already going to happen
Because we are not explicitly saving in the callback, we do not risk an infinite loop, and thus do not need to check if the numbers have been updated. We just calculate the percentages every time we save.
Because likes/dislikes is an integer value and integer/integer = integer.
so you can do one of two things, convert to Float or change your order of operations.
self.likes_percent = (self.likes.to_f/total.to_f) * 100
Or, to keep everything integers
self.likes_percent = (self.likes * 100)/total
I'm not sure that this is the only problem that you have, but after_update gets called after the object is saved.
Try changing the update_percentages before - on a before_update or a before_validate instead. Also, remove the self.save line - it will be called automatically later on if you use one of those callbacks.