Arrays in C++

Arrays in C++
An Introduction

Prof. David Bernstein
James Madison University

Computer Science Department

bernstdh@jmu.edu

Definitions

Array:
- A set of consecutive/contiguous memory locations used to store homogeneous data
Element:
- An item in an array
- (You refer to a particular element using its index)
Dimension:
- The number of elements in an array
- (An array of size/dimension n has indexes ranging from 0 to n-1 because the index is actually an offset from the first element)

Basics

A Typical Array Declaration:
- int scores[10];
A Typical Use of an Array:
- x = scores[3];

Strings as Arrays in C

The Issue:
- C does not have a string data type
- Instead, one uses an array of characters
An Observation:
- There is no easy way to know the dimension of an array
An Implication:
- All strings are terminated with the NUL character (i.e., '\0')

Strings as Arrays in C (cont.)

An Example

cppexamples/basics/strings.c

(Fragment: 0)

            char course[6];

    course[0] = 'C';
    course[1] = 'S';
    course[2] = '2';
    course[3] = '4';
    course[4] = '0';
    course[5] = '\0';

Strings as Arrays in C (cont.)

Assignment to an Array?

  char course[6];

  course = "CS240\0"; // Invalid

Strings as Arrays in C(cont.)

Using string.h

cppexamples/basics/strings.c

(Fragment: 1)

            char course[6];

    strcpy(course, "CS240"); // strcpy() will add the \0

Strings as Arrays in C

Potential Buffer Overflows:
- strcpy() can't know the size of the array so just copys whatever you tell it to, even if there isn't enough memory
Better Functions (that Still Require Care):
- strncpy() which doesn't null-terminate if there isn't space
- strncat() which does

Multidimensional Arrays

Arrays of Arrays:
- Are allowed
A Typical Declaration:
- int scores[30][5];
A Typical Use:
- scores[0][3] = 100;

Arrays of C Strings

An Example

cppexamples/basics/strings.c

(Fragment: 2)

            char        courses[3][6];
    
    strcpy(courses[0], "CS149");
    strcpy(courses[1], "CS159");
    strcpy(courses[2], "CS240");

Arrays of C Strings (cont.)

What happens if the second call to strcpy() in the above example is strcpy(courses[1], "GISAT200");?
Why?

An Aside: Strings in C++

The Idea:
- Use a class to encapsulate a string
The Advantages:
- Size can be known so no need for null-termination
- Size needn't be fixed

An Aside: Strings in C++

An Example

cppexamples/basics/strings.cpp

        #include <string>
using namespace std;

int main(void) {

  string course = "CS240";
  int n = course.length();

  string courses[3];
  courses[0] = "CS149";
  courses[1] = "GISAT200";
  courses[2] = "CS240";
}