Arrays

1
Arrays
2
Arrays

• So far the variables we have made store one value
  at a time. If you need to store and use 10
  integers, then 10 int variables must be declared.
• These variables are called atomic or scalar,
  which means they cannot be further divided into
  smaller pieces.

3
Arrays

• Often it is the case that we have a group of
  values of the same data type that form a logical
  group, for example a list of grades for a class.
• Such a list is called a 1-dimensional array.

4
One-Dimensional Arrays

• To declare an array, we use the following syntax
• data-type array-namenumber-of-items
• For example
• char grades50
• This creates an array of 50 characters that could
  be used to hold letter grades.

5
Other array declarations

• double amount38 //holds 38 values of type
  double.
• int volts6 //holds 6 integer values.
• const int BUFFER_SIZE 100
• float bufferBUFFER_SIZE //100 floats.

6
Whats the Point?

• Arrays save time in that you dont have to
  manually declare a 100 floating point variables.
• Arrays allow for easy updates of a given value.
  To do this we make use of the array index. Each
  element in an array is accessed by supplying the
  appropriate index value. We see this in the next
  slide.

7
Array Indices

• The grades array from our first example holds 50
  characters.
• grades0 refers to the first element in the
  array.
• grades1 is the second element
• grades2 is the third element
• grades49 is the fiftieth element

8
Using Array Variables

• Just like atomic variables
• grades0 a / assigns a to the first
  element in grades /
• grades1 grades0
• grades2 grades1 3 //assigns d to
  grades2

9
More array indices

• The array index can be specified using variables
  or expressions, but you must be careful of two
  things
• The variable or expression evaluates to an
  integer
• The value is in the range of the array. In the
  grades example, the index range is something
  between 0 and 49.

10
Using a loop with arrays

• Arrays and for loops go hand-in-hand.
• Consider the following code
• int volts5
• //Note i 0 and i lt 5
• for (int i 0 i lt 5 i)
• voltsi 70
• //What does this do?

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Example Program

• / ask user for 5 temperature values, and then
  output their sum /
• include ltiostreamgt
• using namespace std
• int main()
• const int MAXTEMPS 5
• int i, tempMAXTEMPS, total 0
• for (i 0 i lt MAXTEMPS i)
• cout ltlt Enter a temperature
• cin gtgt tempi
• (cont)

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Example Program (cont)

• cout ltlt \nThe total of the temperatures
• for (i 0 i lt MAXTEMPS i)
• cout ltlt ltlt tempi
• total total tempi
• cout ltlt is ltlt total ltlt endl
• return 0

13
Array Initialization

• Arrays are initialized like scalar variables,
  except the list of values are placed in braces.
  For example
• int volts5 2, 9, 100, 34, 27
• char grades3 a, c, b
• Initialization of large arrays can be split into
  rows
• double states10 5.4, 3.8, 3.4, 10.6, 9.2,
• 1.8, 10.3, 14.0, 2.3, 1.4

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Array Initialization (cont)

• You do not have to initialize every element of an
  array
• / initializes hits0, hits1, and hits2 to
  the values indicated all the rest are set to 0
  /
• int hits6 133, 25, 10001
• The size of the array may be omitted if you
  initialized all of its values at declaration
• / grades has three elements /
• int grades 79, 90, 81
• / same as above /
• int grades3 79, 90, 81

15
Two-dimensional Arrays

• A two-dimensional array consists of both rows and
  columns of elements. It is essentially a matrix.
• To declare a two-dimensional array, we merely use
  two sets of square brackets.
• The first contains the number of rows
• The second contains the number of columns
• //Creates a 2D array with 3 rows and 4 columns
• int vals34

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Indices in 2D arrays

• Assume that the two dimensional array called val
  is declared and looks like the following

• To access the cell containing 6, we reference
  val13, that is, row 1, column 3.

17
Using 2D arrays

• Just like 1D arrays, once you have specified the
  index, you are just working with a single
  variable of the given data type.
• Assignments and usage is still the same
• sumRow0 val00 val01 val02
• val03
• //assigns 72 to cell at row 2, column 3
• val23 72

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Initializing 2D arrays

• You can use additional braces to indicate when
  rows start and end, but you dont have to do
  that.
• int val34 8,16,9,52,
• 3,15,27,6,
• 14,25,2,10
• Or
• int val34 8,16,9,52,
• 3,15,27,6,
• 14,25,2,10
• Or (correct, but not as clear as the first two)
• int val34 8,16,9,52,3,15,27,6,14,25,2,10
  

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More on 2D arrays

• Initialization of 2D arrays is done in row order.
• 2D arrays work well with (for) loops like 1D
  arrays. However, to access all elements,
  typically you will need nested loops for 2D
  arrays. Can you see why?

20
Example Program

• include ltiostreamgt
• include ltiomanipgt
• using namespace std
• int main()
• int vals34 8,16,9,52, 3,15,27,6,
  14,25,2,10
• for (int i 0 i lt 3 i)
• cout ltlt endl
• for (int j 0 j lt 4 j)
• cout ltlt valij ltlt
• return 0

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Multi-Dimensional Arrays

• Arrays can have higher dimensions.
• Everything works about the same as indicated
  here.
• Note multidimensional arrays can have quite a few
  elements so you must be mindful of your memory
  capacity
• int big100010001000 //a billion ints

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Common Programming Errors

• Addressing indices that are out of bounds of the
  array range. This will run-time crash or at the
  very least a logical error. Be careful especially
  when using expressions to compute the indices.
• Remember, indexing starts with 0, not 1!!!
• Forgetting to declare the array (either
  altogether or forgetting the )
• Forgetting to initialize an array. Some compilers
  set everything to zero, some do not.