Is There a Neat Way to Represent a Fraction as an Attributed String

Is there a neat way to represent a fraction as an attributed string?

If you want to have arbitrary fractions represented correctly, you should set the UIFontFeatureTypeIdentifierKey and UIFontFeatureSelectorIdentifierKey to kFractionsType and kDiagonalFractionsSelector respectively for UIFontDescriptorFeatureSettingsAttribute in a custom UIFontDescriptor. For example you can say something like:

let label = UILabel(frame: CGRect(x: 0.0, y: 0.0, width: 1000.0, height: 100.0))
let pointSize : CGFloat = 60.0
let systemFontDesc = UIFont.systemFontOfSize(pointSize,
    weight: UIFontWeightLight).fontDescriptor()
let fractionFontDesc = systemFontDesc.fontDescriptorByAddingAttributes(
    [
        UIFontDescriptorFeatureSettingsAttribute: [
            [
                UIFontFeatureTypeIdentifierKey: kFractionsType,
                UIFontFeatureSelectorIdentifierKey: kDiagonalFractionsSelector,
            ], ]
    ] )
label.font = UIFont(descriptor: fractionFontDesc, size:pointSize)
label.text = "The Fraction is: 23/271"

with the following result:

You can find more information here

Swift 3.0

extension UIFont
{
    static func fractionFont(ofSize pointSize: CGFloat) -> UIFont
    {
        let systemFontDesc = UIFont.systemFont(ofSize: pointSize).fontDescriptor
        let fractionFontDesc = systemFontDesc.addingAttributes(
            [
                UIFontDescriptorFeatureSettingsAttribute: [
                    [
                        UIFontFeatureTypeIdentifierKey: kFractionsType,
                        UIFontFeatureSelectorIdentifierKey: kDiagonalFractionsSelector,
                    ], ]
            ] )
        return UIFont(descriptor: fractionFontDesc, size:pointSize)
    }
}

let label = UILabel()
label.backgroundColor = .white
let pointSize: CGFloat = 45.0
let string = "This is a mixed fraction 312/13"
let attribString = NSMutableAttributedString(string: string, attributes: [NSFontAttributeName: UIFont.systemFont(ofSize: pointSize), NSForegroundColorAttributeName: UIColor.black])
attribString.addAttributes([NSFontAttributeName: UIFont.fractionFont(ofSize: pointSize)], range: (string as NSString).range(of: "12/13"))
label.attributedText = attribString
label.sizeToFit()

How to display fraction text number over another?

This is just Apple sample code from the Introducing the New System Fonts video from WWDC 2015 put into a playground and using a UILabel to render plain text fractions using font features. [Updated for Swift 4]

//: Playground - noun: a place where people can play

import UIKit
import CoreGraphics

let pointSize : CGFloat = 60.0
let systemFontDesc = UIFont.systemFont(ofSize: pointSize,
                                       weight: UIFont.Weight.light).fontDescriptor
let fractionFontDesc = systemFontDesc.addingAttributes(
    [
        UIFontDescriptor.AttributeName.featureSettings: [
            [
                UIFontDescriptor.FeatureKey.featureIdentifier: kFractionsType,
                UIFontDescriptor.FeatureKey.typeIdentifier: kDiagonalFractionsSelector,
                ],
        ]
    ] )

let label = UILabel(frame: CGRect(x: 0, y: 0, width: 200, height: 100))

label.font = UIFont(descriptor: fractionFontDesc, size:pointSize)
label.text = "12/48" // note just plain numbers and a regular slash

Just tap on the eye in the playground and you will see a beautiful fraction.

Introducing the New System Fonts (WWDC 2015 at 20:24)

Where to place miscellaneous widely used functions?

I'd extend UIFont with your fractionFont method:

extension UIFont {
    class func fractionFont() -> UIFont {
        /* Code here */
    }
}

Display fraction number in UILabel

I have created the following two helper functions which will return the unicode string for a given number (0-9) in subscript or superscript format:

-(NSString *)superscript:(int)num {

    NSDictionary *superscripts = @{@0: @"\u2070", @1: @"\u00B9", @2: @"\u00B2", @3: @"\u00B3", @4: @"\u2074", @5: @"\u2075", @6: @"\u2076", @7: @"\u2077", @8: @"\u2078", @9: @"\u2079"};
    return superscripts[@(num)];
}

-(NSString *)subscript:(int)num {

    NSDictionary *subscripts = @{@0: @"\u2080", @1: @"\u2081", @2: @"\u2082", @3: @"\u2083", @4: @"\u2084", @5: @"\u2085", @6: @"\u2086", @7: @"\u2087", @8: @"\u2088", @9: @"\u2089"};
    return subscripts[@(num)];
}

Once you have these declared, you can easily call something like this:

NSLog(@"%@/%@", [self superscript:5], [self subscript:6]);

Which would output the following:

⁵/₆

And even a screenshot for ya from my normal UILabel:

EDIT

Here's a function that will work with any fraction, including 37/100, for example:

-(NSString *)fraction:(int)numerator denominator:(int)denominator {

    NSMutableString *result = [NSMutableString string];

    NSString *one = [NSString stringWithFormat:@"%i", numerator];
    for (int i = 0; i < one.length; i++) {
        [result appendString:[self superscript:[[one substringWithRange:NSMakeRange(i, 1)] intValue]]];
    }
    [result appendString:@"/"];

    NSString *two = [NSString stringWithFormat:@"%i", denominator];
    for (int i = 0; i < two.length; i++) {
        [result appendString:[self subscript:[[two substringWithRange:NSMakeRange(i, 1)] intValue]]];
    }
    return result;
}

Calling the following:

NSLog(@"%@", [self fraction:37 denominator:100]);

Logs ³⁷/₁₀₀.

How to write fraction value using html?

Try the following:

1<sup>1</sup>⁄<sub>2</sub>

Performant algorithm to rationalize floats

There's a Stern–Brocot tree implementation shown here, but you'll have to profile to see which is better.

Addendum: I've had good results using org.jscience.mathematics.number.Rational in linear systems; org.apache.commons.math.fraction.BigFraction offers several constructors for double that may be useful. All throw suitable exceptions for undefined values.

Getting numerator and denominator of a fraction in R

A character representation of the fraction is stored in an attribute:

x <- fractions(0.175)
> strsplit(attr(x,"fracs"),"/")
[[1]]
[1] "7"  "40"

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