Chapter 6: Arrays¶
CSE 2010 Week 9¶
Background¶
● So far in this course, we have learned how to declare variables of different data types (int, double,
string, char, bool), we have even learned how to create our own data types (classes).
● When we declare different variables and objects, even if they represent the same kind of value,
they are stored and accessed separately.
○ Example:
double salary1 = 65000.00;
double salary2 = 45000.00;
double salary3 = 38000.00;
● The above 3 variables are all the same data type and represent salaries, but they are stored in separate places in
memory and have no relation to one another.
● Using arrays, we can conveniently store and manage collections of values in easy-to-use containers.
Arrays¶
● Definition:¶
○ Static arrays are data structures (aka containers) that store an indexed sequence of
elements of the same data type.
○ The size of an array is fixed and determined at compilation time (hence the term static
arrays).
■ We will learn how to dynamically allocate arrays with pointers later.
● “Indexed sequence” means that each element in an array has an index¶
associated with it (like strings).¶
○ The first element of an array always has index 0, next element is index 1, next is index
2..and so on.
○ The last element of an array will always have index (size of array -1).
● Let’s look at different ways of defining arrays and what that looks like in¶
memory...¶
Array Definition: Example 1¶
● Syntax:¶
datatype arrayName[ ] = {element
1
, element
2
, ..., element
n
};
○ Will create an array of n elements with a size of n.
● Example:¶
int x[] = {6,10,17};
● How you can imagine it is stored in memory:¶
Array Definition: Example 2¶
● Syntax:
datatype arrayName[n];
○ Will create an “empty” array of size n
● Example:
double y[7];
● How you can imagine it is stored in memory:
● Note: Space for 6 elements will be allocated, but that does not mean that those spaces are empty or
initialized to zero. There may be old values stored in memory.
Array Definition: Example 3¶
● Syntax:
datatype arrayName[n]{0};
○ Will create an array of size n with all values set to 0
● Example:
double z[7]{0};
● How you can imagine it is stored in memory:
Array Definition: Example 4¶
● Syntax:
datatype arrayName[k] = {element 1 , element 2 , ..., elementn};
○ Will create an array of size k , initialized with n elements.
● Example:
double salaries[10]= {65000.00,45000.00,38000.00};
● How you can imagine it is stored in memory:
Array Definition: Example 5¶
● Strings are special data types, so they act a little different. Any value not
initialized will be set to an empty string.
● Example:
string names[6] = {“Bob”,”Sally”,”Jack”};
● How you can imagine it is stored in memory:
Accessing Array Elements¶
● Once an array has been defined, how do we access its elements?
● Remember that each element has an index # associated with it. In an array with a size
of n , the first element will always have index #0, and the last element will always have
index # n-.
● Using the [ ] operator (index operator), we can access the elements.
Syntax:
arrayName[i]
● Will access array element w/index i. You can then use the value as you normally would
use any value in an expression.
○ A cout statement
○ A function call
○ An arithmetic expression
○ Another assignment statement
Displaying Array Elements with a for loop¶
● When we have an initialized array, we know the size of the array, and know the range
of the indices.
● For example, if we have an array of size n, we know the indices will range from 0 -> n-
● Using this range, we can easily print everything inside the array with a for loop.
Displaying Array Elements with a range based for loop¶
● We can also use a range based for loop.¶
Changing Array Elements¶
● Using the [ ] operator, we can also have an array element on the left side¶
of an assignment statement, updating its value.¶
Example:¶
int x[] = {6,10,17}; //Elements in x = 6, 10, 17¶
x[2] = 15; // Assign element at index 2 the value of 15¶
//Elements in x = 6, 15, 17¶
Example¶
Storing user input into an array¶
● We can store a user’s input into an array using a loop.¶
● The size of an array is determined at compilation time, meaning that its¶
size cannot change as the program is running, so when you write your¶
program that is filling arrays with values you have to keep track of two¶
things:¶
- The max size of the array
- The actual number of elements in the array
Let’s look at an example of this (userArray.cpp on Canvas)¶
Arrays as Function Parameters¶
● Just like other types, we can have an array as a function parameter and pass them as arguments, but they
are special.
● Arrays are always passed by reference, even without the & operator in the parameter name.
● This means that any modification to an array parameter within a function WILL affect the array’s value in
memory.
● Example:
void fillArray(int a[], int size);
A function whose first parameter is an array of integers, and second parameter is the size of that
array. Changes made to the array in the function will affect the array in memory.
● If you pass an array into a function and don’t want to make changes to it, use the const keyword.
void printArray(const int a[], int size);
A function whose first parameter is a constant array of integers, and second parameter is the size
of that array. We cannot make changes to the array since it is a constant.