Reverse elements of a string array

Question

string[] myString = {"a","b","c","d"}
//Reverse string algorithm here
myString = {"d","c","b","a"}
I have been asked to do so in an interview without the help of any temporary variable or .NET class, string methods, etc to reverse the elements of the same string array. I was told to use basic programming constructs like loops. Since, I am up for another interview today, I am in a hurry to know whether this is actually possible because I could not find a solution to this.

Solution 1

Here you go, no support variables, no .net functions :) But it makes the assumptions that all the strings in the array have length 1 ( as they do in the code you posted).

string[] myString = {"a","b","c","d", "e"};
for(int i = 0; i < myString.Length/2; i++)
{
   myString[i] += myString[myString.Length -1 -i];
   myString[myString.Length -1 -i] = ""+myString[i][0];
   myString[i] = "" + myString[i][1];
}

Solution 2

Since you cannot use a temporary variable, the best I can think of is appending the strings first and then removing the appended part again:

// append last strings to first strings
for(int i = 0; i < myString.Length / 2; i++)
{
    myString[i] = myString[i] + myString[myString.Length - i - 1];
}

// copy first half to last half
for(int i = myString.Length / 2 + 1; i < myString.Length; i++)
{
    myString[i] = myString[myString.Length - i - 1]
                      .SubString(0, 
                              myString[myString.Length - i - 1].Length 
                               - myString[i].Length);
}

// remove useless part from first half
for(int i = 0; i < myString.Length / 2; i++)
{
    myString[i] = myString[i].SubString(
                                  myString[myString.Length - i - 1].Length 
                                   - myString[i].Length);
}

Stupid approach? Yes. But no additional variables are involved.

Solution 3

string[] myString = {"a","b","c","d"}

for(int = (myString.Length - 1); int >= 0; i--) {
    string rev =  myString[i];
    Console.Write(rev);
}

Solution 4

The right answer in your interview is "Why would you NOT use the class libraries?" But then say, "Well if I needed to write a customized method because the libraries don't support the need...". Then I would show both methods and argue when to use each method. If they had a problem with this explanation then I wouldn't want to work there anyway.

With Libraries:

string[] myString = {"a","b","c","d"};
List<string> list = myString.ToList();
list.Reverse();
myString = list.ToArray();

Without:

string[] myString = {"a","b","c","d"};
string[] newString = new string[myString.Length];
for (int i = 0, j = myString.Length - 1; i < myString.Length && j >= 0; i++, j--)
{
    newString[j] = myString[i];
}
myString = newString;

Solution 5

Sorry I posted a wrong answer... here's the verified one:

     int k = len - 1;
    for(int i = 0; i<len/2; i++)
    {
        myString[i] = myString[i]+"."+myString[k--];
    }
    for(int i = len/2; i<len; i++)
    {
        myString[i] = myString[k].substring(0, 1);
        myString[k] = myString[k--].substring(2,3);
    }

However, just consider this a pseudo-code... I did not check for .NET syntax.

View more solutions

Comments

[Magnus Grindal Bakken]

We're not here to do your job interview for you. At least show some things you've tried.

[Nisarg Shah]

You were asked to store the values in reverse or update the array?

[S_F]

I'm pretty sure there would be no downvotes if OP never metioned the job interview...

[Nisarg Shah]

@MagnusGrindalBakken We will not be solving this for his interview. It will be for our knowledge or if we have then we can share with him

[Magnus Grindal Bakken]

@NisargShah Note the second part of my comment. There's nothing wrong with asking for help about school problems, job interviews, or work projects, but you have to show that you've made an attempt.

[xanatos]

For ints (and similar types) you can use the xor trick, but for other types you'll need a temporary variable... Or use the stack as a temporary variable. You could cheat by using a parameter of a function as as temporary variable, but a parameter is still a variable.

[Nisarg Shah]

Can you resize array size in basic programming?

[Nisarg Shah]

If by any case you can resize array then first double the size of array. Later add values in reverse order and overwrite the first half of array with reversed values. Now resize the array with half size. Thats all...

[lbrahim]

@MagnusGrindalBakken I did this Needless to say it did not work so, I did not post it here.Technically, you would just be answering my previous encounter of the problem. There is no guarantee it will ever be asked later.

[Bernhard Barker]

@S_F In that case everyone might assume it's homework and it would get downvoted a whole lot more. StackOverflow guidelines dictates that you should show a sufficient attempt at solving the problem oneself before asking here, the lack of this is likely the only reason it was downvoted.

[Microsoft DN]

He wants to do it without using any existing methods

[Nisarg Shah]

OUt of question. It is from .net

[Hemario]

ah, without .NET classes. Ah well, I'll leave the answer for future reference.

[Nisarg Shah]

this is linq and also .net way of doing it

[Heslacher]

As stated: without the help of any temporary variable or .NET class, string methods.. -> SubString usage ;-(

[Vincent van der Weele]

@Heslacher that's no .NET, that's the SubString that I implicitly implemented myself but just did not show ;)

[xanatos]

@Heuster I don't think SubString is implementable without temporary variables :-)

[Vincent van der Weele]

@xanatos I'm afraid so too. But neither is string concatenation since strings are immutable. The question would make more sense to me for an array of integers (so the XOR trick could be used).

[Jean Logeart]

You use tmp variable in your solution. You just hide it making the code ugly and less efficient...

[Bernhard Barker]

That seems like a ridiculous assumption. If they're all length 1, the logical thing to do would be to have a char[] instead.

[Save]

@Dukeling If we were in the logic domain, you wouldn't be asked not to use .net funcions or accessory variables. I tried to solve the ask just as he presented it.

[Bernhard Barker]

The example is just that, an example. The description doesn't mention that we're limited to length 1 strings and it seems like a very strange constraint to put on strings, so my logical assumption is that the example only uses length 1 strings because that's less typing than longer strings. Yes, I'd be a lot happier with a wild-card-based approach.

[Bernhard Barker]

Is there a specific reason you didn't just put all 3 statements in a single loop?

[Vincent van der Weele]

@Dukeling stupidity :) I wondered the same after typing, but then I thought: maybe it's more clear this way (probably not)

[lbrahim]

Thanks @Save I was asked and therefore, asked for elements of length 1 in a string array. So it works and answers my question.

[DanielV]

cannot convert from 'System.Collections.Generic.List<string>' to 'System.Array'

[DanielV]

It will not work and throw: cannot convert from 'System.Collections.Generic.List<string>' to 'System.Array'

[Athena]

Please provide answers that do not require clarification from the asker.

[user1438038]

While your post might answer the question, please also provide a short explanation what your code actually does and how it solves the initial problem. Don't just drop a couple of poorly formatted lines of code.

[Koosh]

I agree with you here. I don't know why everyones makin it so difficult

[ProgrammingLlama]

There's a reason for that. The question states that you can't use a temporary variable, which I'd argue yours does.

[Eric Lippert]

The reason to say "without using libraries" is because someone had to write that library. If you are interviewing for a job that involves writing libraries, assuming that the library is already written is a non-starter.

[Benj Sanders]

@EricLippert correct, no contest. But, the specific assignment involved not using libraries available that are highly optimized. Unless you are removing dependencies, using the library is the correct method, otherwise why ever use a library, right?