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0 pointsStavrosK14y ago0 comments

I'm sorry, my Java is rusty; is this the equivalent of the Python

    print("\n".join("%s: %s" % (key, value) for key, value in mydict.items()))

? If so, is it hard to do in Java?

0 comments

nitrogen14y ago

In 1997, before for-each and generics, it would've been very verbose. I learned Java in 2006 and haven't written much in a while, so I maybe slightly off, but I think you'd either get an Iterator for the Hashtable's keys and repeatedly call Iterator.next() and Hashtable.get(), or get the length of Hashtable.keys() and use a traditional C-style for loop.

Okay, I couldn't resist looking up the old docs, so this looks about right (Enumeration instead of Iterator -- two interfaces that do basically the same thing):

  void printTable(Hashtable table)
  {
    Enumeration e = table.keys();
    while(e.hasMoreElements()) {
      String key = (String)e.nextElement();
      String value = (String)table.get(key);
      System.out.println(key + ": " + value);
    }
  }

arethuza14y ago

Yeah - that's pretty much it. Except that being lazy I'd have probably not bothered casting the keys and values to Strings and just called toString() on them directly....

onemoreact14y ago

Did you expect people to be able to do this by hand? I mean infront of an IDE I could do this in 5-10 minutes. But on a blackboard or pen and paper there is little chance I would get anywhere close to a correct answer.

2 more replies

StavrosKOP14y ago

I see, thanks. It's more complicated than the Python version, which does the iteration implicitly, but I'm still surprised that nobody was able to do it.

phillytom14y ago

FYI - there's a potential defect there - keys are not required to be strings so you should lookup the value before you cast.

free14y ago

Ideally, whatever your key/value is would have the toString method over ridden to meaningfully represent it. Alternatively, I could write it as

for(Map.Entry<K, V> entry : map.entrySet()){

     System.out.println(entry.getKey());

     System.out.println(entry.getValue());

}

nitrogen14y ago

I was relying on the assumption from the original question that the keys and values were all Strings: "...a Hashtable containing String keys and values and printing them all out."

SoftwareMaven14y ago

Well, yours does iterate over the dictionary twice and generates a temporary list in the meantime. If you have a huge dictionary, this would explode. The equivalent Java code wouldn't do that (due to lack of list comprehensions).

kragen14y ago

At first glance, your statement is incorrect: there is no Python code to create a list here, and no list comprehensions. On further examination, though, string_join in stringobject.c invokes PySequence_Fast to convert its iterable argument into a sequence --- a temporary list, in this case --- so your statement that this code generates a temporary list is correct, although I suspect that this is more by accident than because you have a deep knowledge of the implementation of the Python standard library.

The temporary list of temporary strings probably will use less memory than the original dict did, though, so it's only a very mild sort of "explosion". It will, however, be several times bigger than the final output string, which also has to be huge if the dictionary is huge.

Python doesn't have anything like a StringBuffer. If it did, it would be reasonable for string_join to use it instead of generating a temporary list. The Python code above would look the same.

But hey, if you just want to print the values, you could say

    sys.stdout.printlines("%s: %s\n" % (k, v) for k, v in mydict.items())

but frankly I think I would write instead

    for k, v in mydict.items():
        print "%r: %r" % (k, v)

nyellin14y ago

Why doesn't StringIO count as a string buffer?

1 more reply

sur14y ago

He is using parentheses instead of square brackets, so the code is actually using a generator expression which will produce each value lazily.

eru14y ago

Is mydict.items() also lazy?

1 more reply

StavrosKOP14y ago

Aside from the fact that I'm using a generator, where does it iterate twice? I don't see it.

arethuza14y ago

No, it's really not - even with the java.util.Hashtable class that was all that Java had at the time. I think it was something that people from a C,C++ background didn't really expect to be able to do.

j / k navigate · click thread line to collapse

0 comments

nitrogen14y ago

Okay, I couldn't resist looking up the old docs, so this looks about right (Enumeration instead of Iterator -- two interfaces that do basically the same thing):

  void printTable(Hashtable table)
  {
    Enumeration e = table.keys();
    while(e.hasMoreElements()) {
      String key = (String)e.nextElement();
      String value = (String)table.get(key);
      System.out.println(key + ": " + value);
    }
  }

arethuza14y ago

Yeah - that's pretty much it. Except that being lazy I'd have probably not bothered casting the keys and values to Strings and just called toString() on them directly....

onemoreact14y ago

2 more replies

StavrosKOP14y ago

I see, thanks. It's more complicated than the Python version, which does the iteration implicitly, but I'm still surprised that nobody was able to do it.

phillytom14y ago

FYI - there's a potential defect there - keys are not required to be strings so you should lookup the value before you cast.

free14y ago

Ideally, whatever your key/value is would have the toString method over ridden to meaningfully represent it. Alternatively, I could write it as

for(Map.Entry<K, V> entry : map.entrySet()){

     System.out.println(entry.getKey());

     System.out.println(entry.getValue());

}

nitrogen14y ago

I was relying on the assumption from the original question that the keys and values were all Strings: "...a Hashtable containing String keys and values and printing them all out."

SoftwareMaven14y ago

kragen14y ago

Python doesn't have anything like a StringBuffer. If it did, it would be reasonable for string_join to use it instead of generating a temporary list. The Python code above would look the same.

But hey, if you just want to print the values, you could say

    sys.stdout.printlines("%s: %s\n" % (k, v) for k, v in mydict.items())

but frankly I think I would write instead

    for k, v in mydict.items():
        print "%r: %r" % (k, v)

nyellin14y ago

Why doesn't StringIO count as a string buffer?

1 more reply

sur14y ago

He is using parentheses instead of square brackets, so the code is actually using a generator expression which will produce each value lazily.

eru14y ago

Is mydict.items() also lazy?

1 more reply

StavrosKOP14y ago

Aside from the fact that I'm using a generator, where does it iterate twice? I don't see it.

arethuza14y ago

j / k navigate · click thread line to collapse