Develop Reference

StackOverflowException on recursive anonymous functions

I'm trying to write a function to check whether a string is a palindrome, and using this example, I'm trying to reverse the string using a recursive anonymous function:
static Boolean checkPalindromeAnonRec(string str)
{
str = str.ToLower().Replace(" ", String.Empty);
Func<string, string> revStr = null;
revStr = delegate(string s)
{
if (s.Length > 1)
{ return revStr(s) + s[0]; }
else
{ return s; }
};
return (str == revStr(str));
}
But every time I run it I get a StackOverflowException. It's not obvious to me why, any ideas?

Well this is the problem:
if (s.Length > 1)
{ return revStr(s) + s[0]; }
Aside from the odd bracing style, that's just recursing with the original string - so it will keep going forever. I suspect you meant to use Substring somewhere so that it recursed using a shorter string...
I would actually try writing it as a simple non-anonymous (but still recursive) method to start with - so work out how you would recursively write:
static string Reverse(string input)
... then if you still want to inline that into your CheckPalindrome method, you can do so.

best way to convert collection to string

I need to convert a collection of <string,string> to a single string containing all the values in the collection like KeyValueKeyValue... But How do I do this effectively?
I have done it this way at the moment:
parameters = string.Join("", requestParameters.Select(x => string.Concat(x.Key, x.Value)));
But not sure it is the best way to do it, would a string builder be better? I guess the collection will contain a max of 10 pairs.

string.Join used to not really be the best option since it only accepted string[] or object[] parameters, requiring that any select-style queries needed to be completely evaluated and put into an array first.
.NET 4.0 brought with it an overload that accepts IEnumerable<string> -- which is what you are using -- and even an overload that accepts any IEnumerable<T>. These are definitely your best bet as they are now part of the BCL.
Incidentally, cracking open the source for the first overload in Reflector shows code that follows pretty closely to what davisoa suggested:
public static string Join(string separator, IEnumerable<string> values)
{
if (values == null)
{
throw new ArgumentNullException("values");
}
if (separator == null)
{
separator = Empty;
}
using (IEnumerator<string> enumerator = values.GetEnumerator())
{
if (!enumerator.MoveNext())
{
return Empty;
}
StringBuilder builder = new StringBuilder();
if (enumerator.Current != null)
{
builder.Append(enumerator.Current);
}
while (enumerator.MoveNext())
{
builder.Append(separator);
if (enumerator.Current != null)
{
builder.Append(enumerator.Current);
}
}
return builder.ToString();
}
}
So in other words, if you were to change this code to use a StringBuilder, you'd just be rewriting what MS already wrote for you.

With such a small collection, there isn't much of a performance concern, but I would probably just use a StringBuilder to Append all of the values.
Like this:
var sb = new Text.StringBuilder;
foreach (var item in requestParameters)
{
sb.AppendFormat("{0}{1}", item.Key, item.Value);
}
var parameters = sb.ToString();

String builder would be fine. Use append to add each a string to the string builder.
Basically the only reason why concat, replace, join, string+string , etc are considered not-the-best because they all tend to destroy the current string & recreate a new one.
So when you have adding strings like upto 10-12 time it really means you will destroy & recreate a string that many times.

Can all 'for' loops be replaced with a LINQ statement?

Is it possible to write the following 'foreach' as a LINQ statement, and I guess the more general question can any for loop be replaced by a LINQ statement.
I'm not interested in any potential performance cost just the potential of using declarative approaches in what is traditionally imperative code.
private static string SomeMethod()
{
if (ListOfResources .Count == 0)
return string.Empty;
var sb = new StringBuilder();
foreach (var resource in ListOfResources )
{
if (sb.Length != 0)
sb.Append(", ");
sb.Append(resource.Id);
}
return sb.ToString();
}
Cheers
AWC

Sure. Heck, you can replace arithmetic with LINQ queries:
http://blogs.msdn.com/ericlippert/archive/2009/12/07/query-transformations-are-syntactic.aspx
But you shouldn't.
The purpose of a query expression is to represent a query operation. The purpose of a "for" loop is to iterate over a particular statement so as to have its side-effects executed multiple times. Those are frequently very different. I encourage replacing loops whose purpose is merely to query data with higher-level constructs that more clearly query the data. I strongly discourage replacing side-effect-generating code with query comprehensions, though doing so is possible.

In general yes, but there are specific cases that are extremely difficult. For instance, the following code in the general case does not port to a LINQ expression without a good deal of hacking.
var list = new List<Func<int>>();
foreach ( var cur in (new int[] {1,2,3})) {
list.Add(() => cur);
}
The reason why is that with a for loop, it's possible to see the side effects of how the iteration variable is captured in a closure. LINQ expressions hide the lifetime semantics of the iteration variable and prevent you from seeing side effects of capturing it's value.
Note. The above code is not equivalent to the following LINQ expression.
var list = Enumerable.Range(1,3).Select(x => () => x).ToList();
The foreach sample produces a list of Func<int> objects which all return 3. The LINQ version produces a list of Func<int> which return 1,2 and 3 respectively. This is what makes this style of capture difficult to port.

In fact, your code does something which is fundamentally very functional, namely it reduces a list of strings to a single string by concatenating the list items. The only imperative thing about the code is the use of a StringBuilder.
The functional code makes this much easier, actually, because it doesn’t require a special case like your code does. Better still, .NET already has this particular operation implemented, and probably more efficient than your code1):
return String.Join(", ", ListOfResources.Select(s => s.Id.ToString()).ToArray());
(Yes, the call to ToArray() is annoying but Join is a very old method and predates LINQ.)
Of course, a “better” version of Join could be used like this:
return ListOfResources.Select(s => s.Id).Join(", ");
The implementation is rather straightforward – but once again, using the StringBuilder (for performance) makes it imperative.
public static String Join<T>(this IEnumerable<T> items, String delimiter) {
if (items == null)
throw new ArgumentNullException("items");
if (delimiter == null)
throw new ArgumentNullException("delimiter");
var strings = items.Select(item => item.ToString()).ToList();
if (strings.Count == 0)
return string.Empty;
int length = strings.Sum(str => str.Length) +
delimiter.Length * (strings.Count - 1);
var result = new StringBuilder(length);
bool first = true;
foreach (string str in strings) {
if (first)
first = false;
else
result.Append(delimiter);
result.Append(str);
}
return result.ToString();
}
1) Without having looked at the implementation in the reflector, I’d guess that String.Join makes a first pass over the strings to determine the overall length. This can be used to initialize the StringBuilder accordingly, thus saving expensive copy operations later on.
EDIT by SLaks: Here is the reference source for the relevant part of String.Join from .Net 3.5:
string jointString = FastAllocateString( jointLength );
fixed (char * pointerToJointString = &jointString.m_firstChar) {
UnSafeCharBuffer charBuffer = new UnSafeCharBuffer( pointerToJointString, jointLength);
// Append the first string first and then append each following string prefixed by the separator.
charBuffer.AppendString( value[startIndex] );
for (int stringToJoinIndex = startIndex + 1; stringToJoinIndex <= endIndex; stringToJoinIndex++) {
charBuffer.AppendString( separator );
charBuffer.AppendString( value[stringToJoinIndex] );
}
BCLDebug.Assert(*(pointerToJointString + charBuffer.Length) == '\0', "String must be null-terminated!");
}

The specific loop in your question can be done declaratively like this:
var result = ListOfResources
.Select<Resource, string>(r => r.Id.ToString())
.Aggregate<string, StringBuilder>(new StringBuilder(), (sb, s) => sb.Append(sb.Length > 0 ? ", " : String.Empty).Append(s))
.ToString();
As to performance, you can expect a performance drop but this is acceptable for most applications.

I think what's most important here is that to avoid semantic confusion, your code should only be superficially functional when it is actually functional. In other words, please don't use side effects in LINQ expressions.

Technically, yes.
Any foreach loop can be converted to LINQ by using a ForEach extension method,such as the one in MoreLinq.
If you only want to use "pure" LINQ (only the built-in extension methods), you can abuse the Aggregate extension method, like this:
foreach(type item in collection { statements }
type item;
collection.Aggregate(true, (j, itemTemp) => {
item = itemTemp;
statements
return true;
);
This will correctly handle any foreach loop, even JaredPar's answer. EDIT: Unless it uses ref / out parameters, unsafe code, or yield return.
Don't you dare use this trick in real code.
In your specific case, you should use a string Join extension method, such as this one:
///<summary>Appends a list of strings to a StringBuilder, separated by a separator string.</summary>
///<param name="builder">The StringBuilder to append to.</param>
///<param name="strings">The strings to append.</param>
///<param name="separator">A string to append between the strings.</param>
public static StringBuilder AppendJoin(this StringBuilder builder, IEnumerable<string> strings, string separator) {
if (builder == null) throw new ArgumentNullException("builder");
if (strings == null) throw new ArgumentNullException("strings");
if (separator == null) throw new ArgumentNullException("separator");
bool first = true;
foreach (var str in strings) {
if (first)
first = false;
else
builder.Append(separator);
builder.Append(str);
}
return builder;
}
///<summary>Combines a collection of strings into a single string.</summary>
public static string Join<T>(this IEnumerable<T> strings, string separator, Func<T, string> selector) { return strings.Select(selector).Join(separator); }
///<summary>Combines a collection of strings into a single string.</summary>
public static string Join(this IEnumerable<string> strings, string separator) { return new StringBuilder().AppendJoin(strings, separator).ToString(); }

In general, you can write a lambda expression using a delegate which represents the body of a foreach cycle, in your case something like :
resource => { if (sb.Length != 0) sb.Append(", "); sb.Append(resource.Id); }
and then simply use within a ForEach extension method. Whether this is a good idea depends on the complexity of the body, in case it's too big and complex you probably don't gain anything from it except for possible confusion ;)

int array to string

In C#, I have an array of ints, containing digits only. I want to convert this array to string.
Array example:
int[] arr = {0,1,2,3,0,1};
How can I convert this to a string formatted as: "012301"?

at.net 3.5 use:
String.Join("", new List<int>(array).ConvertAll(i => i.ToString()).ToArray());
at.net 4.0 or above use: (see #Jan Remunda's answer)
string result = string.Join("", array);

You can simply use String.Join function, and as separator use string.Empty because it uses StringBuilder internally.
string result = string.Join(string.Empty, new []{0,1,2,3,0,1});
E.g.: If you use semicolon as separator, the result would be 0;1;2;3;0;1.
It actually works with null separator, and second parameter can be enumerable of any objects, like:
string result = string.Join(null, new object[]{0,1,2,3,0,"A",DateTime.Now});

I realize my opinion is probably not the popular one, but I guess I have a hard time jumping on the Linq-y band wagon. It's nifty. It's condensed. I get that and I'm not opposed to using it where it's appropriate. Maybe it's just me, but I feel like people have stopped thinking about creating utility functions to accomplish what they want and instead prefer to litter their code with (sometimes) excessively long lines of Linq code for the sake of creating a dense 1-liner.
I'm not saying that any of the Linq answers that people have provided here are bad, but I guess I feel like there is the potential that these single lines of code can start to grow longer and more obscure as you need to handle various situations. What if your array is null? What if you want a delimited string instead of just purely concatenated? What if some of the integers in your array are double-digit and you want to pad each value with leading zeros so that the string for each element is the same length as the rest?
Taking one of the provided answers as an example:
result = arr.Aggregate(string.Empty, (s, i) => s + i.ToString());
If I need to worry about the array being null, now it becomes this:
result = (arr == null) ? null : arr.Aggregate(string.Empty, (s, i) => s + i.ToString());
If I want a comma-delimited string, now it becomes this:
result = (arr == null) ? null : arr.Skip(1).Aggregate(arr[0].ToString(), (s, i) => s + "," + i.ToString());
This is still not too bad, but I think it's not obvious at a glance what this line of code is doing.
Of course, there's nothing stopping you from throwing this line of code into your own utility function so that you don't have that long mess mixed in with your application logic, especially if you're doing it in multiple places:
public static string ToStringLinqy<T>(this T[] array, string delimiter)
{
// edit: let's replace this with a "better" version using a StringBuilder
//return (array == null) ? null : (array.Length == 0) ? string.Empty : array.Skip(1).Aggregate(array[0].ToString(), (s, i) => s + "," + i.ToString());
return (array == null) ? null : (array.Length == 0) ? string.Empty : array.Skip(1).Aggregate(new StringBuilder(array[0].ToString()), (s, i) => s.Append(delimiter).Append(i), s => s.ToString());
}
But if you're going to put it into a utility function anyway, do you really need it to be condensed down into a 1-liner? In that case why not throw in a few extra lines for clarity and take advantage of a StringBuilder so that you're not doing repeated concatenation operations:
public static string ToStringNonLinqy<T>(this T[] array, string delimiter)
{
if (array != null)
{
// edit: replaced my previous implementation to use StringBuilder
if (array.Length > 0)
{
StringBuilder builder = new StringBuilder();
builder.Append(array[0]);
for (int i = 1; i < array.Length; i++)
{
builder.Append(delimiter);
builder.Append(array[i]);
}
return builder.ToString()
}
else
{
return string.Empty;
}
}
else
{
return null;
}
}
And if you're really so concerned about performance, you could even turn it into a hybrid function that decides whether to do string.Join or to use a StringBuilder depending on how many elements are in the array (this is a micro-optimization, not worth doing in my opinion and possibly more harmful than beneficial, but I'm using it as an example for this problem):
public static string ToString<T>(this T[] array, string delimiter)
{
if (array != null)
{
// determine if the length of the array is greater than the performance threshold for using a stringbuilder
// 10 is just an arbitrary threshold value I've chosen
if (array.Length < 10)
{
// assumption is that for arrays of less than 10 elements
// this code would be more efficient than a StringBuilder.
// Note: this is a crazy/pointless micro-optimization. Don't do this.
string[] values = new string[array.Length];
for (int i = 0; i < values.Length; i++)
values[i] = array[i].ToString();
return string.Join(delimiter, values);
}
else
{
// for arrays of length 10 or longer, use a StringBuilder
StringBuilder sb = new StringBuilder();
sb.Append(array[0]);
for (int i = 1; i < array.Length; i++)
{
sb.Append(delimiter);
sb.Append(array[i]);
}
return sb.ToString();
}
}
else
{
return null;
}
}
For this example, the performance impact is probably not worth caring about, but the point is that if you are in a situation where you actually do need to be concerned with the performance of your operations, whatever they are, then it will most likely be easier and more readable to handle that within a utility function than using a complex Linq expression.
That utility function still looks kind of clunky. Now let's ditch the hybrid stuff and do this:
// convert an enumeration of one type into an enumeration of another type
public static IEnumerable<TOut> Convert<TIn, TOut>(this IEnumerable<TIn> input, Func<TIn, TOut> conversion)
{
foreach (TIn value in input)
{
yield return conversion(value);
}
}
// concatenate the strings in an enumeration separated by the specified delimiter
public static string Delimit<T>(this IEnumerable<T> input, string delimiter)
{
IEnumerator<T> enumerator = input.GetEnumerator();
if (enumerator.MoveNext())
{
StringBuilder builder = new StringBuilder();
// start off with the first element
builder.Append(enumerator.Current);
// append the remaining elements separated by the delimiter
while (enumerator.MoveNext())
{
builder.Append(delimiter);
builder.Append(enumerator.Current);
}
return builder.ToString();
}
else
{
return string.Empty;
}
}
// concatenate all elements
public static string ToString<T>(this IEnumerable<T> input)
{
return ToString(input, string.Empty);
}
// concatenate all elements separated by a delimiter
public static string ToString<T>(this IEnumerable<T> input, string delimiter)
{
return input.Delimit(delimiter);
}
// concatenate all elements, each one left-padded to a minimum length
public static string ToString<T>(this IEnumerable<T> input, int minLength, char paddingChar)
{
return input.Convert(i => i.ToString().PadLeft(minLength, paddingChar)).Delimit(string.Empty);
}
Now we have separate and fairly compact utility functions, each of which are arguable useful on their own.
Ultimately, my point is not that you shouldn't use Linq, but rather just to say don't forget about the benefits of creating your own utility functions, even if they are small and perhaps only contain a single line that returns the result from a line of Linq code. If nothing else, you'll be able to keep your application code even more condensed than you could achieve with a line of Linq code, and if you are using it in multiple places, then using a utility function makes it easier to adjust your output in case you need to change it later.
For this problem, I'd rather just write something like this in my application code:
int[] arr = { 0, 1, 2, 3, 0, 1 };
// 012301
result = arr.ToString<int>();
// comma-separated values
// 0,1,2,3,0,1
result = arr.ToString(",");
// left-padded to 2 digits
// 000102030001
result = arr.ToString(2, '0');

To avoid the creation of an extra array you could do the following.
var builder = new StringBuilder();
Array.ForEach(arr, x => builder.Append(x));
var res = builder.ToString();

string result = arr.Aggregate("", (s, i) => s + i.ToString());
(Disclaimer: If you have a lot of digits (hundreds, at least) and you care about performance, I suggest eschewing this method and using a StringBuilder, as in JaredPar's answer.)

You can do:
int[] arr = {0,1,2,3,0,1};
string results = string.Join("",arr.Select(i => i.ToString()).ToArray());
That gives you your results.

I like using StringBuilder with Aggregate(). The "trick" is that Append() returns the StringBuilder instance itself:
var sb = arr.Aggregate( new StringBuilder(), ( s, i ) => s.Append( i ) );
var result = sb.ToString();

string.Join("", (from i in arr select i.ToString()).ToArray())
In the .NET 4.0 the string.Join can use an IEnumerable<string> directly:
string.Join("", from i in arr select i.ToString())

I've left this here for posterity but don't recommend its use as it's not terribly readable. This is especially true now that I've come back to see if after a period of some time and have wondered what I was thinking when I wrote it (I was probably thinking 'crap, must get this written before someone else posts an answer'.)
string s = string.Concat(arr.Cast<object>().ToArray());

The most efficient way is not to convert each int into a string, but rather create one string out of an array of chars. Then the garbage collector only has one new temp object to worry about.
int[] arr = {0,1,2,3,0,1};
string result = new string(Array.ConvertAll<int,char>(arr, x => Convert.ToChar(x + '0')));

This is a roundabout way to go about it its not much code and easy for beginners to understand
int[] arr = {0,1,2,3,0,1};
string joined = "";
foreach(int i in arr){
joined += i.ToString();
}
int number = int.Parse(joined);

If this is long array you could use
var sb = arr.Aggregate(new StringBuilder(), ( s, i ) => s.Append( i ), s.ToString());

// This is the original array
int[] nums = {1, 2, 3};
// This is an empty string we will end up with
string numbers = "";
// iterate on every char in the array
foreach (var item in nums)
{
// add the char to the empty string
numbers += Convert.ToString(item);
}
// Write the string in the console
Console.WriteLine(numbers);

Good way to concatenate string representations of objects?

Ok,
We have a lot of where clauses in our code. We have just as many ways to generate a string to represent the in condition. I am trying to come up with a clean way as follows:
public static string Join<T>(this IEnumerable<T> items, string separator)
{
var strings = from item in items select item.ToString();
return string.Join(separator, strings.ToArray());
}
it can be used as follows:
var values = new []{1, 2, 3, 4, 5, 6};
values.StringJoin(",");
// result should be:
// "1,2,3,4,5,6"
So this is a nice extension method that does a very basic job. I know that simple code does not always turn into fast or efficient execution, but I am just curious as to what could I have missed with this simple code. Other members of our team are arguing that:
it is not flexible enough (no control of the string representation)
may not be memory efficient
may not be fast
Any expert to chime in?
Regards,
Eric.

Regarding the first issue, you could add another 'formatter' parameter to control the conversion of each item into a string:
public static string Join<T>(this IEnumerable<T> items, string separator)
{
return items.Join(separator, i => i.ToString());
}
public static string Join<T>(this IEnumerable<T> items, string separator, Func<T, string> formatter)
{
return String.Join(separator, items.Select(i => formatter(i)).ToArray());
}
Regarding the second two issues, I wouldn't worry about it unless you later run into performance issues and find it to be a problem. It's unlikely to much of a bottleneck however...

For some reason, I thought that String.Join is implemented in terms of a StringBuilder class. But if it isn't, then the following is likely to perform better for large inputs since it doesn't recreate a String object for each join in the iteration.
public static string Join<T>(this IEnumerable<T> items, string separator)
{
// TODO: check for null arguments.
StringBuilder builder = new StringBuilder();
foreach(T t in items)
{
builder.Append(t.ToString()).Append(separator);
}
builder.Length -= separator.Length;
return builder.ToString();
}
EDIT: Here is an analysis of when it is appropriate to use StringBuilder and String.Join.

Why don't you use StringBuilder, and iterate through the collection yourself, appending.
Otherwise you are creating an array of strings (var strings) and then doing the Join.

You are missing null checks for the sequence and the items of the sequence. And yes, it is not the fastest and most memory efficient way. One would probably just enumerate the sequence and render the string representations of the items into a StringBuilder. But does this really matter? Are you experiencing performance problems? Do you need to optimize?

this would work also:
public static string Test(IEnumerable<T> items, string separator)
{
var builder = new StringBuilder();
bool appendSeperator = false;
if(null != items)
{
foreach(var item in items)
{
if(appendSeperator)
{
builder.Append(separator)
}
builder.Append(item.ToString());
appendSeperator = true;
}
}
return builder.ToString();
}