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10.1. One Dimensional Arrays

10.1.1. Basic Syntax

A string is an immutable sequence of characters. Arrays provide more general sequences, with the same indexing notation, but with free choice of the type of the items in the sequence, and the ability to change the elements in the sequence.

For example, if we want the type for an array with int elements, it is int[]. In general for any element type, the type for an array of the element type is

type[]

so

int[] a;

declares a to refer to an array containing int elements. You do not know how many elements will be allowed in this array from this declaration. We must give further information to create the corresponding array object. All object can be created using the new syntax. An array must get a definite length, which can be a literal integer of any integer expression. For example

int[] a;
a = new int[4];

or combined with the declaration,

int[] a = new int[4];

creates an array that holds 4 integers. The elements of the array must get initial values. Numerical arrays get initialized to all 0’s with this syntax.

For a variety of reasons, including bookkeeping by the compiler, the actual data for an array is not stored directly in the memory location allocated by the declaration. The array could have any number of items, and hence the memory requirements are not known at compile time. Like all other object (as opposed to primitive) types, what is actually stored at the memory location declared for a is a reference to the actual place where the data for the array is stored. In actual compiler implementation this reference is an address in memory. In diagrams we will illustrate object references with an arrow pointing to the actual location for the object’s data. For example after a is initialized:

new array

The small box beside a is meant to indicate the memory space allocated when a is declared. As you can see that space does not actually contain the array, but only a reference to the array, pointing to the actual sequence of data for the array. To make it easy to refer to the elements in the diagram, we also label the indices associated with each element, though they are not actual a part of what is stored in memory.

The general syntax to create a new array is

new type[ length ]

After the type, there are square brackets enclosing an expression for the length of the array - this length is unchangeable after creation.

The elements inside an array can to referenced with the same index notation used earlier for strings.

a[2]

refers to the element at index 2 (third element because of 0 based indexing).

Unlike with strings, this element can not only be read, but also be assigned to:

a[0] = 7;
a[1] = 5;
a[2] = 9;
a[3] = 6;

These four assignment statements would replace the original 0 values for each element in the array.

This is a verbose way to specify all array values. An array with the same final data could be created with the single declaration:

int[] b = {7, 5, 9, 6};
new array initialized with braces

The list in braces ONLY is allowed as an initialization of a variable in a declaration, not in a later assignment statement. Technically it is an initializer, not an array literal.

Individual array elements can both be used in expressions, and be assigned to. Continuing with the earlier example code:

a[2] = 4*a[1] - a[3];

a[2] now equals 4*5 - 6 = 14.

Arrays, like strings, have a Length property:

Console.WriteLine(b.Length); // prints 4

Just as we saw that using a variable for an index was useful with strings, array elements are almost always referred to with an index variable in practice. A very common pattern is to deal with each element in sequence, and the syntax is the same as for a string. Print all elements of array b:

for (int i= 0; i < b.Length, i++) {
   Console.WriteLine(b[i]);
}

You could also use while syntax. The foreach syntax would be:

foreach( int x : b) {
   Console.WriteLine(x);
}

The int type for x matches the element type of the array b.

The shorter foreach syntax is not as general as the for syntax. For example, to print only the first 3 elements of b:

for(int i= 0; i < 3; i++) {
   Console.WriteLine(b[i]);
}

but the foreach syntax would not work, since it must process all elements.

Also use the for syntax to assign new values to the array elements, rather than just use the values in expressions:

for(int i= 0; i < b.Length; i++) {
   b[i] = 5*i;
}

Now the array b of our earlier examples (of length 4) would contain 0, 5, 10, and 15.

10.1.2. Parameters to Main

The Main function may take an array of strings as parameter, as in example PrintParam.cs:

   /** Demonstrate the use of command line parameters. */
   static void Main(string[] args)
   {
      Console.WriteLine("There are {0} command line parameters.", args.Length);
      foreach(string s in args) {
         Console.WriteLine(s);
      }

   }

By convention, the formal parameter for Main is called args, short for arguments.

Compile and run the program from the command line. Run it again with some things at the end of the line like:

mono PrintParam.exe hi there 123

This should print for you:

There are 3 command line parameters.
hi
there
123

See what quoted strings do. Run the command:

mono PrintParam.exe "hi there" 123

This should print for you:

There are 2 command line parameters.
hi there
123

The quotes are important in many places. For instance the message in the hg commit -m message command must be one parameter. That generally requires quotes, unless you are given to one-word descriptions.

10.1.3. String Method Split

A string method producing an array:

string[] Split(char separator )

Returns an array of substrings from this string. They are the pieces left after chopping out the separator character from the string. Example:

csharp> var fruitString = "apple pear banana";
csharp> string[] fruit = fruitString.Split(' ');
csharp> fruit;
{ "apple", "pear", "banana" }
csharp> fruit[1];
"pear"

Split is useful for parsing a line with several parts:

string line = InputLine("Enter integers on a line");
string[] tokens = line.Split(' ');
int[] nums = new int[tokens.Length];
for (int i = 0; i < nums.Length; i++) {
   nums[i] = int.Parse(tokens[i]);
}

Here if the user enters “2 5 22”, then tokens is an array containing strings “2”, “5”, and “22”. If we want them all converted to integers and place in a new array, we need to create an array of the same length, and loop through, parsing each string in tokens into an integer in the corresponding location in nums.

10.1.4. References and Aliases

Object variables like arrays being references has important implications for assignment.

With a primitive type like an int, an assignment copies the data:

copying an int

In the diagram, the contents of the memory box labeled a is copied to the memory box labeled d. The value of d starts off equal to the value of a, but can later be changed independently.

Contrast an assignment with arrays. The value that is copied is the reference, not the array data itself, so both end up pointing at the same actual array:

copying an array reference

Hereafter, array assignments like:

a[2] = -10;
d[1] = 55;

would both change the same array. Now a and d are essentially names for the same thing (the actual array). The technical term matches English: The names are aliases.

This may seem like a pretty silly discussion. Why bother to give two different names to the same object? Isn’t one enough? In fact it is very important in function/method calls. An array reference can be passed as an actual value, and it is the array reference that is copied to the formal parameter, so the formal parameter name is an alias for the actual parameter name.

Note

If an array passed as a parameter to a method has elements changed in the method, then the change affects the actual parameter array. The change remains in the actual parameter array after the method has terminated.

For example, consider the following function:

/** Modify a by multiplying all elements by multiplier. */
static void Scale(int[] a, int multiplier)
{
   for (int i = 0; i < a.Length; i++) {
      a[i] *= multiplier;  // or:  a[i] = a[i] * multiplier
   }
}

The fragment:

int[] nums = {2, 4, 1};
Scale(nums, 5);

would change nums, so it ends up containing elements 10, 20, and 5.

10.1.5. Anonymous Array Initialization

Sometimes you want to use an array with specific values only as a parameter to a function. You could write something like

int[] temp = {3, 1, 7};
SomeFunc(temp);

but if temp is never going to be referenced again, you can do this without using a name:

SomeFunc(new int[] {3, 1, 7});

It is essential to include the new int[], not just the {3, 1, 7}.

Such an approach could also be used if you want to return a fixed length array, where you have values for each parts, as in:

return new int[] {minVal, maxVal};

10.1.5.1. Command Line Adder Exercise

Write a program Adder.cs that calculates and prints the sum of command line parameters, so

mono Adder.exe 2 5 22

would print 29.

10.1.5.2. Trim All Exercise

Write a program Trimmer.cs that includes and tests a function with heading:

/** Trim all elements of s and replace them in the array.
    Example: If a contains {" is  ", " it", "trimmed?   "}
    then after the function call the array contains
    {"is", "it", "trimmed?"}.   */
static void TrimAll(string[] a)

10.1.5.3. Count Duplicates Exercise

Write a program CountDups.cs that includes and tests a function with heading:

/** Return the number of duplicate pairs in an array a.
 * Example: for elements 2, 5, 1, 5, 2, 5
 * the return value would be 4 (one pair of 2's three pairs of 5's. */
public static int dups(int[] a)

10.1.5.4. Mirror Array Exercise

Write a program MakeMirror.cs that includes and tests a function with heading:

/** Create a new array with the elements of a in the opposite order.
 * {"aA", "bB", "cC"} produces a new array {"cC", "bB", "aA"}
 */
public static string[] Mirror(string[] a)

10.1.5.5. Reverse Array Exercise

Write a program ReverseArray.cs that includes and tests a function with heading:

/** Reverse the order of array elements.
 * {"aA", "bB", "cC"} -> {"cC", "bB", "aA"}
 */
public static void Reverse(string[] a)

10.1.5.6. Histogram Exercise

Write a program MakeHistogram.cs that includes and tests a function with heading:

/** Return a histogram array counting repetitions of values
 *  start through end in array a.  The count for value start+i
 *  is in index i of the returned array.  For example:
 *  Histogram(new int[]{2, 0, 3, 5, 3, 5}, 0, 5) returns
 *  a new array containing {1, 0, 1, 2, 0, 2}.  */
public static int[] Histogram(int[] a, int start, int end)

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