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Permutations of ArrayList<Integer>

im working on a problem where i have to obtain all permutations of an arraylist of numbers. The only restriction is that any number cant start with 0, so if we have [0,1,2] we would obtain
[1,2,0]
[1,0,2]
[2,0,1]
[2,1,0]
i know how to do this with 3 loops but the thing is that i have to repeat this to different sets of numbers with differentes sizes, so i need one method that i can apply to different sets of numbers but i have no clue on how to do this. I imagine i have to used some kind of recursive function but i dont know how to implement it so the numbers cant start with a 0. Any ideas? please dont just post the code i want to understand the problem, thank you in advantage!!

Curious question! Interesting code kata.
I naively think I would have a recursive method that takes:
a list of the items currently chosen by the caller
a set of the items available for the callee
The method would iterate over the set to chose 1 more item and call itself with the list extended by this item, and the set reduced by this item. Upon return, remove from list, add back to set and go on with next item (take a defensive copy of the set of course).
If the current list is empty, the selected first item cannot be 0, as per your rules. If you must collect the permutations somewhere (not just print), a 3rd argument would be required for a collection or an observer.
The recursion obvioulsy stops when the available set is empty, at which point the permutation is sent to the collection or observer.
If items can repeat, you may have benefit from sorting them first in order to skip electing the same item again at a given position.
Beware this quires a recursion depth of N, for N items. But the danger is minimal because even with N=10000, it may not stackoverflow, but the CPU time to complete would be order(N!) (probably end of universe...)

You could solve this recursively as described here: Permutation of an ArrayList of numbers using recursion.
The only thing that is missing there is your restriction with the zeros, which could be solved somehow like this (the loop is taken from the example above):
for (List<Integer> al : myLists) {
// The part you need to add:
if (al.get(0) == 0) {
continue;
}
String appender = "";
for (Integer i : al) {
System.out.print(appender + i);
appender = " ";
}
System.out.println();
}
You basically check the first element of each permutation and skip the ones with a leading zero. The continue jumps to the next iteration of the loop and therefore to the next permutation.

Trying to use an assert command in conjuntion with arraylist using selenium, but it breaks if the elements are not in the proper order

I have a string that needs to be compared to the names that are on the website. So the first thing I do is get the number of rows (because some arrays have more or fewer than 2 people in them) and then put that size into an int. String[] names come from the names that selenium is supposed to find when it goes to the website to execute this statement assertTrue(assertion.getText().contains(names[i-1])); The problem is: if the names do not appear in the order in which they appear in the array it breaks. In other words, if Mick Jagger is in li[1] and Keith Richards is in li[2], everything runs as expected. But if Keith Richards appears in li[1] it breaks. Furthermore, I am supposed to use the assertTrue command to do this. I have tried sorting, pushing whats on the web into a new ArrayList and I keep getting errors. Anyone know a good way to ensure the order isn't important and still use the assertTrue command?
Thanks,
Scott
WebElement assertion = null;
List<WebElement> assignees = driver.findElements(By.xpath(".//*[#id='assignee']/li"));
int count = assignees.size();
String[] names = {"Mick", "Keith"};
for (int i = 1; i < count; i++)
{
assertion = driver.findElement(By.xpath(".//*[#id='assignee']/li["+i+"]"));
assertTrue(assertion.getText().contains(names[i-1]));

If names represents the full string, you can just flip it. Make sure the text in your assertion (probably should be named something like assignee instead of assertion) is contained in your collection:
assertTrue(Arrays.asList(names).contains(assertion.getText());
Let me know if this won't work because a name is actually a subset of the text in assertion and I'll adjust the answer.
If they don't exactly match (which you have indicated they don't), you could use linq in c# to match this. Since you're using java you can use an additional loop. There may be a more efficient way to do this in java that I'm not aware of.
String assigneeText = assertion.getText();
boolean nameFound = false;
for(String name: names)
{
nameFound = assigneeText.contains(name);
if(nameFound)
{
break;
}
}
assertTrue(nameFound, "None of the expected names were found in the following assignee text: " + assigneeText);

about java recursion to create combination of string

The question was asking me to return set containing all the possible combination of strings made up of "cc" and "ddd" for given length n.
so for example if the length given was 5 then set would include "ccddd" and "dddcc".
and length 6 would return set containing "cccccc","dddddd"
and length 7 would return set contating "ccdddcc","dddcccc","ccccddd"
and length 12 will return 12 different combination and so on
However, set returned is empty.
Can you please help?
"Please understand extremeply poor coding style"
public static Set<String> set = new HashSet<String>();
public static Set<String> generateset(int n) {
String s = strings(n,n,"");
return set; // change this
}
public static String strings(int n,int size, String s){
if(n == 3){
s = s + ("cc");
return "";}
if(n == 2){
s = s + ("ddd");
return "";}
if(s.length() == size)
set.add(s);
return strings(n-3,size,s) + strings(n-2,size,s);
}

I think you'll need to rethink your approach. This is not an easy problem, so if you're extremely new to Java (and not extremely familiar with other programming languages), you may want to try some easier problems involving sets, lists, or other collections, before you tackle something like this.
Assuming you want to try it anyway: recursive problems like this require very clear thinking about how you want to accomplish the task. I think you have a general idea, but it needs to be much clearer. Here's how I would approach the problem:
(1) You want a method that returns a list (or set) of strings of length N. Your recursive method returns a single String, and as far as I can tell, you don't have a clear definition of what the resulting string is. (Clear definitions are very important in programming, but probably even more so when solving a complex recursive problem.)
(2) The strings will either begin with "cc" or "ddd". Thus, to form your resulting list, you need to:
(2a) Find all strings of length N-2. This is where you need a recursive call to get all strings of that length. Go through all strings in that list, and add "cc" to the front of each string.
(2b) Similarly, find all strings of length N-3 with a recursive call; go through all the strings in that list, and add "ddd" to the front.
(2c) The resulting list will be all the strings from steps (2a) and (2b).
(3) You need base cases. If N is 0 or 1, the resulting list will be empty. If N==2, it will have just one string, "cc"; if N==3, it will have just one string, "ddd".
You can use a Set instead of a list if you want, since the order won't matter.
Note that it's a bad idea to use a global list or set to hold the results. When a method is calling itself recursively, and every invocation of the method touches the same list or set, you will go insane trying to get everything to work. It's much easier if you let each recursive invocation hold its own local list with the results. Edit: This needs to be clarified. Using a global (i.e. instance field that is shared by all recursive invocations) collection to hold the final results is OK. But the approach I've outlined above involves a lot of intermediate results--i.e. if you want to find all strings whose length is 8, you will also be finding strings whose length is 6, 5, 4, ...; using a global to hold all of those would be painful.

The answer to why set is returned empty is simply follow the logic. Say you execute generateset(5); which will execute strings(5,5,"");:
First iteration strings(5,5,""); : (s.length() == size) is false hence nothing added to set
Second iteration strings(2,5,""); : (n == 2) is true, hence nothing added to set
Third iteration strings(3,5,""); : (n == 3) is true, hence nothing added
to set
So set remains un changed.

Error out of bounds exception when running recursive program in Java

I'm learning about recursion as part of a Java tutorial and I am looking for a little help.
We need to make a recursive Java program which will work out how to get from one city to the other when there is no direct flight.
My latest issue is Im getting an error out of bounds exception after the code has 2 cities in the flightRoute array List. it gives the error "IndexOutOfBoundsException Index 2 Size 2"
The connection value is an arrayList which takes all the cities that the city connects with and the flightRoute is also an arrayList which keeps track of the cities we have had to travel to in order to reach our destination.
I just cannot work out why it will not proceed.
I would appreciate some help on this if you could.
I do not want to overflow you guys with code so i'll put up the method that you should need. If you need more I will happily add some more code.
public boolean determineRoute(City from, City to, ArrayList<City> flightRoute)
{
//the Connections value takes all the connecting cities we can travel to from a departure point
Connections = from.getConnections();
City theCity = Connections.get(i);
//searches in the connecting cities from the current city as to if it contains the city we wish to travel to
if (flightRoute.contains(to)|| 7 >8)
{
System.out.println("Congrats you can go their cause one of its connecting cities is the to city that u wanna go to");
return true;
}
System.out.println("the City name "+theCity);
if(flightRoute.contains(theCity))
{
System.out.println("Sorry it cannot be added "+Connections.get(i));
}
else
{
//add connecting city to list for future reference
flightRoute.add(Connections.get(i));
//takes the lates connection and uses it for recursion (below)
from = Connections.get(i);
i++;
//recursive part which sends a new from city for analysis until the city we want to travel to arises
determineRoute(from, to, flightRoute);
}
return true;
}

Where do you set the i value? Anyway, the only get() you do use i as index, so it is clear that Connections does not have as much items as i.
BTW:
a) Next time mark at which line the exception is thrown (from what I see, probaly the first get()
b) use lowercase for variable names: connections instead of Connections

The problem is that i is non declared locally, so you're forever incrementing some instance variable. Declare it locally and set it to zero.
Before you can make any real progress fixing this code, I advise you clean it up:
Name variables with a leading lowercase - ie connections not Connections
Use local variables where it makes sense - like connections
Remove nonsense code, such as the test if 7 > 8 is true - of course it's always true!
Use proper indentation of blocks
Iterate over collections using a "foreach" loop: for (City city : from.getConnections())
Tend to name your variables the same as the class name, but with a lowercase letter, so city, not theCity

Help me understand question related to HashMap in Java

Im given a task which i am a little confused to understand. Here is the question statement:
The following program should read a file and store all its tokens in a member variable.
Your task is to write a single method that returns the number of items in tokenMap, the average length (as double value) of the elements in tokenMap, and the number of tokens starting with character "a".
Here the tokenMap is an object of type HashMap<String, Integer>;
I do have some idea about HashMap but what i want to know the "key value" for HashMap required is a single character or the whole word?? that i should store in tokenMap.
Also how can i compute the average length?

Looks like you have to use the entire word as the key.
The average length of tokens can be computed by summing the lengths of each token and dividing by the number of tokens.
In Java, you can find the number of tokens in the HashMap by tokenMap.size().
You can write loops that visit each member of the map like this:
for(String t: tokenMap.values()){
//t is a token
}
and if you look up String in the Java API docs you will see that it is easy to find the length of a String.

To compute the average length of the items in a hash map, you'll have to iterate over them all and count the length and calculate the average.
As for your other question about what to use for a key, how are we supposed to know? A hashmap can use practically any* value for a key.
*The value must be hashable, which is defined differently for different languages.

Reading the question closely, it seems that you have to read a file, extract each word and use it as the key value, and store the length of each key as the integer:
an example line
leads to a HashMap like this
an : 2
example : 7
line : 4
After you've built your map (made of keys mapping to entries, or seemingly elements in the question), you'll need to run some statistics over it to find
the number of keys (look at HashMap)
the average length of all keys (again, simple enough)
the number beginning with "a" (just look at the String)
Then make a value object containing these values and return it from the method that does the statistics.
I know I've given more information that you require, but someone else may benefit from a little extra help.

Guys there is some confusion. Im not asking for a solution. Im just confused for one thing.
For the time being, im gonna use String type as the key type.
The only confusion i have is once i read the file line by line, should i split it based upon words or based upon each character. So that the key value should be a single character type string or a String of whole word.
If you can go through the question statement, what do you suggest. That's all im asking.

should i split it based upon words or
based upon each character
The requirement is to make tokens, so you should split them based on words. Each word becomes a unique String key. It would make sense for the value to be the count of each token.
If the file you are reading has these three lines:
int alpha;
int beta;
float delta;
Then you should have something like
<"int", 2>
<";", 3>
<"alpha", 1>
<"beta", 1>
<"float", 1>
<"delta", 1>
(The semicolon may or may not be considered a token.)
Your average length would be ( 3x2 + 3x1 + 5 + 4 + 5 + 5) / 6.
Your length of tokens starting with "a" would be 5.0.
Look elsewhere on this forum for keySet and you should be good to go.