Blog Archives

null vs. undefined (javascript)

Aside

how well do you know your “null”s and “undefined”s?

write down what you expect the result of each operation to be. run them in a javascript console (Chrome, Firefox/Firebug, Safari). compare, verify, contrast, and, for fun, give yourself a score out of “3/4 donut”.

// Equality
null == undefined
null === undefined
!null == !undefined
!null === !undefined

// addition and concatenation
null + ''
undefined + ''
null + 1
undefined + 1

// fun with enumerations
test = []; test.push(null); console.log(test)
test = []; test.push(undefined); console.log(test)
{null:2}
{undefined:2}
{a:null}
{a:undefined}
[More programming riddles]

wtf javascript

Aside

javascript riddle time.

say that you’ve ventured into the land of XmlHTTPRequest ( $.ajax calls for the jquery people), and you’ve heard through the grapevine that (gasp!) it may be possible to make cross-domain http calls with this tool.

excitement! if you’re like me, you’ll see a chance to hack in fixes for a myriad of small bits of javascript you have lying around. so, off you’ll go, merrily trying anything that might work – your initial code might even look something like this, if you’re used to jquery:

  request = $.ajax({
        type: "POST", // !? - request sent through as HTTP GET
        url: 'http://someotherdomain.com/,
        data: 'some data',
        async: true,
        cache: false,
        dataType: 'jsonp',
        crossDomain: true,
        xhrFields: {
	       withCredentials: true //share cookies across domains!
	},
  });
  request.abort(); // !? - does nothing

confused

ok, so, technically this is a wtf jquery, more so than a wtf javascript. ‘jsonp’ crossdomain requests aren’t currently supported, natively, by most browsers – a native implementation would take significant care to avoid being a security risk (i’m not entirely sure it’s possible, even). there is, though, a fairly generic hack for implementing them. take a look at jquery-jsonp, which is close to the state of the art of what’s possible nowadays, and you’ll even see it there:

  // Create the script tag
  script = $( STR_SCRIPT_TAG )[ 0 ];
  script.id = STR_JQUERY_JSONP + count++;
  
  ...

  // Set source
  script.src = url;

create a new script tag, with the source set to be the target of your jsonp request, dynamically add it to the document tree, and wrap it in event handlers to handle the response or any errors.

since it’s a html script tag, it always leads to a browser HTTP GET request; and, since the request is initiated indirectly, by adding the tag to the document tree, some of the usual niceties for XmlHTTPRequest aren’t available – eg, the ability to interrupt.

mystery demystified – the behaviour is correct, though still unpleasantly surprising to me

[More programming riddles]

project euler problem #8

Aside

Find the greatest product of five consecutive digits in the following 1000-digit number:

73167176531330624919225119674426574742355349194934
96983520312774506326239578318016984801869478851843
85861560789112949495459501737958331952853208805511
12540698747158523863050715693290963295227443043557
66896648950445244523161731856403098711121722383113
62229893423380308135336276614282806444486645238749
30358907296290491560440772390713810515859307960866
70172427121883998797908792274921901699720888093776
65727333001053367881220235421809751254540594752243
52584907711670556013604839586446706324415722155397
53697817977846174064955149290862569321978468622482
83972241375657056057490261407972968652414535100474
82166370484403199890008895243450658541227588666881
16427171479924442928230863465674813919123162824586
17866458359124566529476545682848912883142607690042
24219022671055626321111109370544217506941658960408
07198403850962455444362981230987879927244284909188
84580156166097919133875499200524063689912560717606
05886116467109405077541002256983155200055935729725
71636269561882670428252483600823257530420752963450

http://projecteuler.net/problem=8

highlight below for my solution:


a_string = "7316717653133062491922511967442657474235534919493496983520312774506326239578318016984801869478851843858615607891129494954595017379583319528532088055111254069874715852386305071569329096329522744304355766896648950445244523161731856403098711121722383113622298934233803081353362766142828064444866452387493035890729629049156044077239071381051585930796086670172427121883998797908792274921901699720888093776657273330010533678812202354218097512545405947522435258490771167055601360483958644670632441572215539753697817977846174064955149290862569321978468622482839722413756570560574902614079729686524145351004748216637048440319989000889524345065854122758866688116427171479924442928230863465674813919123162824586178664583591245665294765456828489128831426076900422421902267105562632111110937054421750694165896040807198403850962455444362981230987879927244284909188845801561660979191338754992005240636899125607176060588611646710940507754100225698315520005593572972571636269561882670428252483600823257530420752963450"

print max([reduce(lambda x, y : x * y, [int(x) for x in a_string[i:i+5]]) 
           for i in range(len(a_string) - 5)
          ])

[More programming riddles]

project euler problem #6

Aside

The sum of the squares of the first ten natural numbers is,

12 + 22 + … + 10^2 = 385
The square of the sum of the first ten natural numbers is,

(1 + 2 + … + 10)^2 = 552 = 3025
Hence the difference between the sum of the squares of the first ten natural numbers and the square of the sum is 3025 − 385 = 2640.

Find the difference between the sum of the squares of the first one hundred natural numbers and the square of the sum.

http://projecteuler.net/problem=6

highlight below for my solution:


# A = (sum[1..n])^2 = [n(n+1)/2] ^ 2
# B = sum[1^2..n^2] = n(n+1)(2n+1)/6
# A-B = n(n+1)(3n^2-n-2)/12


def delta_sum_products_product_sum(n):
    return n * (n + 1) * (3 * (n ** 2) - n - 2) / 12


print delta_sum_products_product_sum(100)

[More programming riddles]

project euler problem #7

Aside

By listing the first six prime numbers: 2, 3, 5, 7, 11, and 13, we can see that the 6th prime is 13.

What is the 10 001st prime number?

http://projecteuler.net/problem=7

highlight below for my solution:


#using a prime number set datastructure - https://gist.github.com/aausch/6709819
p =  PrimeSet()

i = 10001

while (len(p)<10001):
    i in p
    i += i

print sorted(list(p))[10000]

(see also my solution to problem #5)

[More programming riddles]

project euler problem #5

Aside

2520 is the smallest number that can be divided by each of the numbers from 1 to 10 without any remainder.

What is the smallest positive number that is evenly divisible by all of the numbers from 1 to 20?

http://projecteuler.net/problem=5

highlight below for my solution:


#using a prime number set datastructure - https://gist.github.com/aausch/6709819
p = PrimeSet()

def min_product(n):
    n in p #initialize the PrimeSet with all primes less than n
    product = 1
    for prime in p:
        product = product * (prime ** (int(n ** (1.0/prime))))
    return product

print min_product(20)

(see also my solution to problem #3)

[More programming riddles]