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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 currently have the deck of cards coded, but it is unshuffled. This is for programming the card game of War if it helps. I need to shuffle the deck, but whenever I do, it will only shuffle together the card numbers and the suits, not the full card. For example, I have A identified as an ace and the suits come after each number. A normal card would be "AH" (an ace of hearts) or "6D" (a six of diamonds). Instead, it will output "5A" as one of the cards, as in a 5 of aces. I don't know how to fix this, but the code that I currently have is this:
card_nums = ('A23456789TJQK')';
card_suits = ('HDSC')';
unshuffled_deck = [repmat(card_nums,4,1),repmat(card_suits,13,1)];
disp(unshuffled_deck)
shuffled_deck = unshuffled_deck(randperm(numel(unshuffled_deck)));
disp(shuffled_deck)
I would appreciate any help with this, and thank you very much for your time!
You're creating a random permutation of all of the elements from both columns of unshuffled_deck combined. Instead you need to create a random permutation of the rows of unshuffled_deck:
shuffled_deck = unshuffled_deck(randperm(size(unshuffled_deck,1)),:);
The call to size gives you the number of rows in the deck array, then we get a random permutation of the row indices, and copy the row (value, suit) as a single entity.
Here's a version using a structure array in response to #Carl Witthoft's comment. I was afraid it would add too much complexity to the solution, but it really isn't bad:
card_nums = ('A23456789TJQK')';
card_suits = ('HDSC')';
deck_nums = repmat(card_nums,4,1);
deck_suits = repmat(card_suits,13,1);
cell_nums = cellstr(deck_nums).'; %// Change strings to cell arrays...
cell_suits = cellstr(deck_suits).'; %// so we can use them in struct
%// Construct a struct array with fields 'value' and 'suit'
unshuffled_deck = struct('value',cell_nums,'suit',cell_suits);
disp('unshuffled deck:');
disp([unshuffled_deck.value;unshuffled_deck.suit]);
%// Shuffle the deck using the number of elements in the structure array
shuffled_deck = unshuffled_deck(randperm(numel(unshuffled_deck)));
disp('shuffled deck:');
disp([shuffled_deck.value; shuffled_deck.suit]);
Here's a test run:
unshuffled deck:
A23456789TJQKA23456789TJQKA23456789TJQKA23456789TJQK
HDSCHDSCHDSCHDSCHDSCHDSCHDSCHDSCHDSCHDSCHDSCHDSCHDSC
shuffled deck:
4976TT93KTJQJATK953A75QA82Q6226K5J784J4A3372486K859Q
CHSSSHCDSCSSHDDCDSHHCDHSDDCDHCCHHCHHHDDCSCDSSCHDSCSD
To access an individual card, you can do:
>> shuffled_deck(2)
ans =
scalar structure containing the fields:
value = 9
suit = H
Or you can access the individual fields:
>> shuffled_deck(2).value
ans = 9
>> shuffled_deck(2).suit
ans = H
Unfortunately, I don't know of any way to simply index the struct array and get, for instance, 9H as you would in a regular array using disp(shuffled_deck(2,:)). In this case, the only option I know of is to explicitly concatenate each field:
disp([shuffled_deck(2).value,shuffled_deck(2).suit]);
I would like to be able to initialise a big table in matlab easily.
Say I have the bounds x, y, z = 5, 4, 3. I want to be able to make a 5x4x3 table where each element is a struct that stores count and sum. Count and sum in this struct should be 0 when initialised.
I thought it would be enough to do this:
table = []
table(5,4,3) = struct('sum', 0, 'count', 0)
And this would work for a double but not with a structure evidently.
Any ideas?
EDIT:
As another question, (bonus if you will) is there a way to force matlab to store the struct, but when you access the element (i.e., table(1, 2, 3)) get it to return the average (i.e., table(1,2,3).sum/table(1,2,3).count).
Its not vital to the question but it would certainly be cool.
You'll need just to replace the line table = [] to avoid the error, that is
clear table;
table(5,4,3) = struct('sum', 0, 'count', 0)
works fine. Note, however, that this command only initializes one field of your array, i.e., the memory allocation is incomplete. To initialize all fields of your array, you can use
table2(1:5,1:4,1:3) = struct('sum', 0, 'count', 0)
to visualize the difference, use whos, which returns
>> whos
Name Size Bytes Class Attributes
table 5x4x3 736 struct
table2 5x4x3 8288 struct
Your second question can be solved, for instance, by using anonymous functions
myMean = #(a) a.sum./a.count; %define the function
myMean(table2(2,2,2)) % access the mean in the field (2,2,2)
I am trying to initialize a structure in MATLAB similar to how C code does
typedef struct{
float x;
float y;
} Data
Data datapts[100];
From matlab, I know this is how to create a structure:
Data = structure('x',0,'y',0)
but how do you create 100 instances of it?
Or is this not usually done in MATLAB? Does MATLAB prefer dynamic allocation whenever there is new data to add?
Thanks for all your help..
I don't know C, so I don't know how your code initializes the structure. However, consider these two possibilities:
1. A struct array data with 100 elements, each of which has two fields x and y
You can initialize an empty struct with
data = struct('x', cell(100,1), 'y', cell(100,1));
and you access each element of the struct array as data(1) and each of these is a struct. Typically, these are used when you have several equivalent "things" with the same set of properties, but different values for each.
Example:
elements = struct(...
'name', {'Hydrogen', 'Helium', 'Lithium'},...
'atomicWeight', {1, 4, 7}, ...
'symbol', {'H', 'He', 'Li'});
elements(1)
ans =
name: 'Hydrogen'
atomicWeight: 1
symbol: 'H'
So you can access each individual struct to get to its properties. Now if you wanted to append a struct array with the next 10 elements to this list, you can use cat, just like you would for matrices.
2. A struct data with two fields x and y, each with 100 elements
You can initialize this as
data = struct('x',zeros(100,1),'y',zeros(100,1));
and you access each element of the field as data.x(1). This is typically used when you have one "thing" with several properties that can possibly hold different values.
Example:
weather=struct('time',{{'6:00','12:00','18:00','24:00'}},...
'temperature',[23,28,25,21]);
Once you understand structs and struct arrays and how they're used and indexed, you can use them in more complicated ways than in the simple illustration above.
repmat(Data,100,1);
You can assign data to it with:
Data(1).x = 10;
Data(1).y = 20;
In addition to the other methods described by #yoda and #Jacob, you can use cell2struct.
HI! I am looking for a document that will define what the word "rows[0]" means. this is for BIRT in the Eclipse framework. Perhaps this is a Javascript word? I dunno... been searching like mad and have found nothing yet. Any ideas?
rows is a shortcut to dataSet.rows. Returns the current data rows (of type DataRow[]) for the data set associated with this report item instance. If this report element has no data set, this property is undefined.
Source: http://www.eclipse.org/birt/phoenix/ref/ROM_Scripting_SPEC.pdf
Typically code like rows[x] is accessing an element inside an array. Any intro to programming book should be able to define that for you.
rows[0] would be accessing the first element in the array.
That operation has several names depending on the language, but generally the same concept. In Java, it's an array access expression in C#, it's an indexer or array access operator. As with just about anything, C++ is more complicated, but basically the [] operator takes a collection of something or an array and pulls out (or assigns to) a specific numbered element in that collection or array (generally starting at 0). So in C# ...
// create a list of integers
List<int> lst = new List<int>() { 1, 2, 3, 4, 5 };
// access list
int x = lst[0]; // get the first element of the list, x = 1 afterwords
x = lst[2]; // get the third element of the list, x = 3 afterwords
x = lst[4]; // get the fifth element of the list, x = 5 afterwords
x = lst[5]; // IndexOutOfBounds Exception