I have some code and when it executes, it throws a IndexOutOfRangeException, saying,
Index was outside the bounds of the array.
What does this mean, and what can I do about it?
Depending on classes used it can also be ArgumentOutOfRangeException
An exception of type 'System.ArgumentOutOfRangeException' occurred in mscorlib.dll but was not handled in user code Additional information: Index was out of range. Must be non-negative and less than the size of the collection.
What Is It?
This exception means that you're trying to access a collection item by index, using an invalid index. An index is invalid when it's lower than the collection's lower bound or greater than or equal to the number of elements it contains.
When It Is Thrown
Given an array declared as:
byte[] array = new byte[4];
You can access this array from 0 to 3, values outside this range will cause IndexOutOfRangeException to be thrown. Remember this when you create and access an array.
Array Length
In C#, usually, arrays are 0-based. It means that first element has index 0 and last element has index Length - 1 (where Length is total number of items in the array) so this code doesn't work:
array[array.Length] = 0;
Moreover please note that if you have a multidimensional array then you can't use Array.Length for both dimension, you have to use Array.GetLength():
int[,] data = new int[10, 5];
for (int i=0; i < data.GetLength(0); ++i) {
for (int j=0; j < data.GetLength(1); ++j) {
data[i, j] = 1;
}
}
Upper Bound Is Not Inclusive
In the following example we create a raw bidimensional array of Color. Each item represents a pixel, indices are from (0, 0) to (imageWidth - 1, imageHeight - 1).
Color[,] pixels = new Color[imageWidth, imageHeight];
for (int x = 0; x <= imageWidth; ++x) {
for (int y = 0; y <= imageHeight; ++y) {
pixels[x, y] = backgroundColor;
}
}
This code will then fail because array is 0-based and last (bottom-right) pixel in the image is pixels[imageWidth - 1, imageHeight - 1]:
pixels[imageWidth, imageHeight] = Color.Black;
In another scenario you may get ArgumentOutOfRangeException for this code (for example if you're using GetPixel method on a Bitmap class).
Arrays Do Not Grow
An array is fast. Very fast in linear search compared to every other collection. It is because items are contiguous in memory so memory address can be calculated (and increment is just an addition). No need to follow a node list, simple math! You pay this with a limitation: they can't grow, if you need more elements you need to reallocate that array (this may take a relatively long time if old items must be copied to a new block). You resize them with Array.Resize<T>(), this example adds a new entry to an existing array:
Array.Resize(ref array, array.Length + 1);
Don't forget that valid indices are from 0 to Length - 1. If you simply try to assign an item at Length you'll get IndexOutOfRangeException (this behavior may confuse you if you think they may increase with a syntax similar to Insert method of other collections).
Special Arrays With Custom Lower Bound
First item in arrays has always index 0. This is not always true because you can create an array with a custom lower bound:
var array = Array.CreateInstance(typeof(byte), new int[] { 4 }, new int[] { 1 });
In that example, array indices are valid from 1 to 4. Of course, upper bound cannot be changed.
Wrong Arguments
If you access an array using unvalidated arguments (from user input or from function user) you may get this error:
private static string[] RomanNumbers =
new string[] { "I", "II", "III", "IV", "V" };
public static string Romanize(int number)
{
return RomanNumbers[number];
}
Unexpected Results
This exception may be thrown for another reason too: by convention, many search functions will return -1 (nullables has been introduced with .NET 2.0 and anyway it's also a well-known convention in use from many years) if they didn't find anything. Let's imagine you have an array of objects comparable with a string. You may think to write this code:
// Items comparable with a string
Console.WriteLine("First item equals to 'Debug' is '{0}'.",
myArray[Array.IndexOf(myArray, "Debug")]);
// Arbitrary objects
Console.WriteLine("First item equals to 'Debug' is '{0}'.",
myArray[Array.FindIndex(myArray, x => x.Type == "Debug")]);
This will fail if no items in myArray will satisfy search condition because Array.IndexOf() will return -1 and then array access will throw.
Next example is a naive example to calculate occurrences of a given set of numbers (knowing maximum number and returning an array where item at index 0 represents number 0, items at index 1 represents number 1 and so on):
static int[] CountOccurences(int maximum, IEnumerable<int> numbers) {
int[] result = new int[maximum + 1]; // Includes 0
foreach (int number in numbers)
++result[number];
return result;
}
Of course, it's a pretty terrible implementation but what I want to show is that it'll fail for negative numbers and numbers above maximum.
How it applies to List<T>?
Same cases as array - range of valid indexes - 0 (List's indexes always start with 0) to list.Count - accessing elements outside of this range will cause the exception.
Note that List<T> throws ArgumentOutOfRangeException for the same cases where arrays use IndexOutOfRangeException.
Unlike arrays, List<T> starts empty - so trying to access items of just created list lead to this exception.
var list = new List<int>();
Common case is to populate list with indexing (similar to Dictionary<int, T>) will cause exception:
list[0] = 42; // exception
list.Add(42); // correct
IDataReader and Columns
Imagine you're trying to read data from a database with this code:
using (var connection = CreateConnection()) {
using (var command = connection.CreateCommand()) {
command.CommandText = "SELECT MyColumn1, MyColumn2 FROM MyTable";
using (var reader = command.ExecuteReader()) {
while (reader.Read()) {
ProcessData(reader.GetString(2)); // Throws!
}
}
}
}
GetString() will throw IndexOutOfRangeException because you're dataset has only two columns but you're trying to get a value from 3rd one (indices are always 0-based).
Please note that this behavior is shared with most IDataReader implementations (SqlDataReader, OleDbDataReader and so on).
You can get the same exception also if you use the IDataReader overload of the indexer operator that takes a column name and pass an invalid column name.
Suppose for example that you have retrieved a column named Column1 but then you try to retrieve the value of that field with
var data = dr["Colum1"]; // Missing the n in Column1.
This happens because the indexer operator is implemented trying to retrieve the index of a Colum1 field that doesn't exist. The GetOrdinal method will throw this exception when its internal helper code returns a -1 as the index of "Colum1".
Others
There is another (documented) case when this exception is thrown: if, in DataView, data column name being supplied to the DataViewSort property is not valid.
How to Avoid
In this example, let me assume, for simplicity, that arrays are always monodimensional and 0-based. If you want to be strict (or you're developing a library), you may need to replace 0 with GetLowerBound(0) and .Length with GetUpperBound(0) (of course if you have parameters of type System.Array, it doesn't apply for T[]). Please note that in this case, upper bound is inclusive then this code:
for (int i=0; i < array.Length; ++i) { }
Should be rewritten like this:
for (int i=array.GetLowerBound(0); i <= array.GetUpperBound(0); ++i) { }
Please note that this is not allowed (it'll throw InvalidCastException), that's why if your parameters are T[] you're safe about custom lower bound arrays:
void foo<T>(T[] array) { }
void test() {
// This will throw InvalidCastException, cannot convert Int32[] to Int32[*]
foo((int)Array.CreateInstance(typeof(int), new int[] { 1 }, new int[] { 1 }));
}
Validate Parameters
If index comes from a parameter you should always validate them (throwing appropriate ArgumentException or ArgumentOutOfRangeException). In the next example, wrong parameters may cause IndexOutOfRangeException, users of this function may expect this because they're passing an array but it's not always so obvious. I'd suggest to always validate parameters for public functions:
static void SetRange<T>(T[] array, int from, int length, Func<i, T> function)
{
if (from < 0 || from>= array.Length)
throw new ArgumentOutOfRangeException("from");
if (length < 0)
throw new ArgumentOutOfRangeException("length");
if (from + length > array.Length)
throw new ArgumentException("...");
for (int i=from; i < from + length; ++i)
array[i] = function(i);
}
If function is private you may simply replace if logic with Debug.Assert():
Debug.Assert(from >= 0 && from < array.Length);
Check Object State
Array index may not come directly from a parameter. It may be part of object state. In general is always a good practice to validate object state (by itself and with function parameters, if needed). You can use Debug.Assert(), throw a proper exception (more descriptive about the problem) or handle that like in this example:
class Table {
public int SelectedIndex { get; set; }
public Row[] Rows { get; set; }
public Row SelectedRow {
get {
if (Rows == null)
throw new InvalidOperationException("...");
// No or wrong selection, here we just return null for
// this case (it may be the reason we use this property
// instead of direct access)
if (SelectedIndex < 0 || SelectedIndex >= Rows.Length)
return null;
return Rows[SelectedIndex];
}
}
Validate Return Values
In one of previous examples we directly used Array.IndexOf() return value. If we know it may fail then it's better to handle that case:
int index = myArray[Array.IndexOf(myArray, "Debug");
if (index != -1) { } else { }
How to Debug
In my opinion, most of the questions, here on SO, about this error can be simply avoided. The time you spend to write a proper question (with a small working example and a small explanation) could easily much more than the time you'll need to debug your code. First of all, read this Eric Lippert's blog post about debugging of small programs, I won't repeat his words here but it's absolutely a must read.
You have source code, you have exception message with a stack trace. Go there, pick right line number and you'll see:
array[index] = newValue;
You found your error, check how index increases. Is it right? Check how array is allocated, is coherent with how index increases? Is it right according to your specifications? If you answer yes to all these questions, then you'll find good help here on StackOverflow but please first check for that by yourself. You'll save your own time!
A good start point is to always use assertions and to validate inputs. You may even want to use code contracts. When something went wrong and you can't figure out what happens with a quick look at your code then you have to resort to an old friend: debugger. Just run your application in debug inside Visual Studio (or your favorite IDE), you'll see exactly which line throws this exception, which array is involved and which index you're trying to use. Really, 99% of the times you'll solve it by yourself in a few minutes.
If this happens in production then you'd better to add assertions in incriminated code, probably we won't see in your code what you can't see by yourself (but you can always bet).
The VB.NET side of the story
Everything that we have said in the C# answer is valid for VB.NET with the obvious syntax differences but there is an important point to consider when you deal with VB.NET arrays.
In VB.NET, arrays are declared setting the maximum valid index value for the array. It is not the count of the elements that we want to store in the array.
' declares an array with space for 5 integer
' 4 is the maximum valid index starting from 0 to 4
Dim myArray(4) as Integer
So this loop will fill the array with 5 integers without causing any IndexOutOfRangeException
For i As Integer = 0 To 4
myArray(i) = i
Next
The VB.NET rule
This exception means that you're trying to access a collection item by index, using an invalid index. An index is invalid when it's lower than the collection's lower bound or greater than equal to the number of elements it contains. the maximum allowed index defined in the array declaration
Simple explanation about what a Index out of bound exception is:
Just think one train is there its compartments are D1,D2,D3.
One passenger came to enter the train and he have the ticket for D4.
now what will happen. the passenger want to enter a compartment that does not exist so obviously problem will arise.
Same scenario: whenever we try to access an array list, etc. we can only access the existing indexes in the array. array[0] and array[1] are existing. If we try to access array[3], it's not there actually, so an index out of bound exception will arise.
To easily understand the problem, imagine we wrote this code:
static void Main(string[] args)
{
string[] test = new string[3];
test[0]= "hello1";
test[1]= "hello2";
test[2]= "hello3";
for (int i = 0; i <= 3; i++)
{
Console.WriteLine(test[i].ToString());
}
}
Result will be:
hello1
hello2
hello3
Unhandled Exception: System.IndexOutOfRangeException: Index was outside the bounds of the array.
Size of array is 3 (indices 0, 1 and 2), but the for-loop loops 4 times (0, 1, 2 and 3). So when it tries to access outside the bounds with (3) it throws the exception.
A side from the very long complete accepted answer there is an important point to make about IndexOutOfRangeException compared with many other exception types, and that is:
Often there is complex program state that maybe difficult to have control over at a particular point in code e.g a DB connection goes down so data for an input cannot be retrieved etc... This kind of issue often results in an Exception of some kind that has to bubble up to a higher level because where it occurs has no way of dealing with it at that point.
IndexOutOfRangeException is generally different in that it in most cases it is pretty trivial to check for at the point where the exception is being raised. Generally this kind of exception get thrown by some code that could very easily deal with the issue at the place it is occurring - just by checking the actual length of the array. You don't want to 'fix' this by handling this exception higher up - but instead by ensuring its not thrown in the first instance - which in most cases is easy to do by checking the array length.
Another way of putting this is that other exceptions can arise due to genuine lack of control over input or program state BUT IndexOutOfRangeException more often than not is simply just pilot (programmer) error.
These two exceptions are common in various programming languages and as others said it's when you access an element with an index greater than the size of the array. For example:
var array = [1,2,3];
/* var lastElement = array[3] this will throw an exception, because indices
start from zero, length of the array is 3, but its last index is 2. */
The main reason behind this is compilers usually don't check this stuff, hence they will only express themselves at runtime.
Similar to this:
Why don't modern compilers catch attempts to make out-of-bounds access to arrays?
I am doing some of CodeWars challenges recently and I've got a problem with this one.
"You are given an array (which will have a length of at least 3, but could be very large) containing integers. The array is either entirely comprised of odd integers or entirely comprised of even integers except for a single integer N. Write a method that takes the array as an argument and returns this "outlier" N."
I've looked at some solutions, that are already on our website, but I want to solve the problem using my own approach.
The main problem in my code, seems to be that it ignores negative numbers even though I've implemented Math.abs() method in scala.
If you have an idea how to get around it, that is more than welcome.
Thanks a lot
object Parity {
var even = 0
var odd = 0
var result = 0
def findOutlier(integers: List[Int]): Int = {
for (y <- 0 until integers.length) {
if (Math.abs(integers(y)) % 2 == 0)
even += 1
else
odd += 1
}
if (even == 1) {
for (y <- 0 until integers.length) {
if (Math.abs(integers(y)) % 2 == 0)
result = integers(y)
}
} else {
for (y <- 0 until integers.length) {
if (Math.abs(integers(y)) % 2 != 0)
result = integers(y)
}
}
result
}
Your code handles negative numbers just fine. The problem is that you rely on mutable sate, which leaks between runs of your code. Your code behaves as follows:
val l = List(1,3,5,6,7)
println(Parity.findOutlier(l)) //6
println(Parity.findOutlier(l)) //7
println(Parity.findOutlier(l)) //7
The first run is correct. However, when you run it the second time, even, odd, and result all have the values from your previous run still in them. If you define them inside of your findOutlier method instead of in the Parity object, then your code gives correct results.
Additionally, I highly recommend reading over the methods available to a Scala List. You should almost never need to loop through a List like that, and there are a number of much more concise solutions to the problem. Mutable var's are also a pretty big red flag in Scala code, as are excessive if statements.
How to increment for-loop variable dynamically as per some condition.
For example.
var col = 10
for (i <- col until 10) {
if (Some condition)
i = i+2; // Reassignment to val, compile error
println(i)
}
How it is possible in scala.
Lots of low level languages allow you to do that via the C like for loop but that's not what a for loop is really meant for. In most languages, a for loop is used when you know in advance (when the loop starts) how many iterations you will need. Otherwise, a while loop is used.
You should use a while loop for that in scala.
var i = 0
while(i<10) {
if (Some condition)
i = i+2
println(i)
i+=1
}
If you dont want to use mutable variables you can try functional way for this
def loop(start: Int) {
if (some condition) {
loop(start + 2)
} else {
loop(start - 1) // whatever you want to do.
}
}
And as normal recursion function you'll need some condition to break the flow, I just wanted to give an idea of what can be done.
Hope this helps!!!
Ideally, you wouldn't use var for this. fold works pretty well with immutable values, whether it is an int, list, map...
It lets you set a default value (e.g. 0) to the variable you want to return and also iterate through the values(e.g i) changing that value (e.g accumulator) on every iteration.
val value = (1 to 10).fold(0)((loopVariable,i) => {
if(i == condition)
loopVariable+1
else
loopVariable
})
println(value)
Example
I'm making an app where I have to put a lot of if/else statements. I know you can do as in the title in some other coding language, but I'm not sure if you can do it in Swift.
How do you shorten this:
if x == y || x == z {
//do something
}
To something like this:
if x == y || z {
//do something
}
Perhaps you could consider using an array and checking to see if x is in the array, like in the following example:
let (x, y, z) = (3, 8, 3)
if [y, z].contains(x) {
//True
}
If you're comparing objects (like UIImage), use containsObject instead of contains:
if [x, y, z].containsObject(y) {
//True
}
I conject that there is no sensible language (swift included), that distributes comparison == across logical or ||.
The way you've written it - x == y || x == z - is the most compact form.
Jack Greenhill's answer does indeed go into the right direction. However with more and more values, his method will get very inefficient, since it has to check every element of the array against equality, therefore complexity O(n).
A very underrated data structure, which can do this kind of operation in O(1) should be used instead: The Set. It uses hash values to check quickly whether a value is present or not. You can use it like this:
let x = 3
let values : Set = [1, 3, 6, 1, 7] // {6, 7, 3, 1}
if values.contains(x) {
// ...
}
This takes the same amount of time, whether values contains just one or 1000 elements. An array would be 1000 times slower.
Oftentimes the decision to use an array is made before even considering a set. If your elements don't have any order and can only occur once (which is actually more often the case than you'd think), you probably want a set. A set gives you useful methods, such as union, isSubset, interception and more, for free by just putting your elements in it. The only additional requirement for the element type is to conform to Hashable.
Perhaps this:
switch x {
case y, z : // do then
default : // do else
}
I want to write a binary search program in Scala without using any if-else statements at all. Basic strategy which i have devised uses the return value of comparison between array[mid] and search_key. In short :
1) Generate return value based on comparison between array[mid] and search_key
2) Create a unique string using that return value.
3) Call function using 'reflection' with help of that string
So my question is ...is there any computational logic which returns different values in this case ? How can i achieve this ? For example :
if array[mid] == search_key , return 0
if array[mid] > search_key , return 1
if array[mid] < search_key , return 2
If anyone have any better solution for this problem, please also suggest that.
The easy way that does something similar is
array(mid) compareTo search_key
This line is equivalent to
if (array(mid) == search_key) 0
else if (array(mid) < search_key) -1
else 1 // if (array(mid) > search_key)
As for the best way to do it, you could make a sequence of actions to take, and compute an index into that sequence. If you are interested you can see the full code in https://gist.github.com/kolmar/bcfc94ee4051ee7eb3a1