Week 5

Arrays

Arrays are data structures consisting of related data items of the same type. More specifically, an array is a group of elements of the same type stored contiguously in memory.

Accessing Elements of an Array

To access an element we specify the array name followed by a position number in []. This position number is also known as a subscript or index. The index is 0-based so my_array[3] will be the fourth element of my_array.

Defining an Array

To define an array, we specify the type of the array followed by the number of elements that will be in the array in []. For example, int my_arr[3]; will reserve a space that is 3 ints wide in memory and assign it the label my_arr.

In C arrays are static entities meaning that they remain the same size throughout their lifetimes. That is, they can't be resized.

Note: in general we should use a #define statement for the size of an array so that we can avoid using magic numbers

Looping over an Array.

When looping over an array, it is recommended to use a size_t type for the counter control variable.

#include <stdio.h>

int main(void)
{
    for (size_t i = 0; i < 5; ++i)
    {
    n[i] = i;
    }
}

Initializer Lists

We can define an array with an initializer list to set all the elements to specific values. int n[5] = {32, 27, 64, 18, 95} will set n[0] to 32, n[1] to 27, etc.

The size of the initializer list does not need to be as large as the array. Any elements that are not specified will be set to 0. int n[5] = {0}; is equivalent to int n[5] = {0,0,0,0,0}.

Strings

A string in C is an array of char, terminated by a NUL character '\0' (ASCII 0). As a side note - an array of char does not necessarily need to be a string but a string is always and array of char.

There are a few ways to define a string:

// 1. String literal — auto-size, NUL added
char s1[] = "hello";

// 2. Fixed-size array with string literal — NUL added, extra space unused
char s2[10] = "hello";

// 3. Character array with initializer list — must manually add '\0'
char s3[] = { 'h', 'e', 'l', 'l', 'o', '\0' };

Note: for s2 style of defining a string, where we define the size we must always make sure that the size can accommodate the '\0' at the end. char s2[5] = "hello" would be invalid because all 5 spaces would be used up.

String Input

When we want to grab input from the user, we can define a fixed size buffer array to store the string in and then use a size format specifier to make sure we do not exceed the size of the buffer.

char buf[20];
scanf("%19s", buf); // note that we don't add the & here that we
        // normally do in scanf because buf is an array
        // and is already a pointer

Static vs Automatic Arrays

Recall that static, similar to Java, means that something is only ever initialized once during a program call. Every time its referred to by a function thereafter, it refers to that single instance.

This is in contrast to the default (automatic) array. Every time an automatic array initialized by a function it is a new instance of that array and is destroyed after we exit the function.

static int s_arr[3];  // keeps values between calls - initialized to 0 by default
int a_arr[3];         // new array each time — garbage values until initialized

Type	Lifetime	Initialized to
static array	Entire program	0 (by default)
automatic array	Each function call	Garbage (unless explicit)