P4 - ArraysSlicesMaps

Arrays

var x [5]int

x := [5]float64{ 98, 93, 77, 82, 83 }

x := [5]float64{

98,

93,

77,

82,

83, //Note - even the last element ended by a comma (,)

}

x is an example of an array which is composed of 5 ints.

Length of array = len(<array_name>)

Index, Value = range <array_name>

package main

import "fmt"

func main() {

var x [5]int

x[4] = 100

fmt.Println(x)

}

You should see this:

[0 0 0 0 100]

Major issue with arrays

Their length is fixed. In order to add/remove an item, we actually had to change the size as well. Go's solution to this problem is to use a different type: slices

Slices

A slice is a segment of an array. Like arrays slices are indexable and have a length.
Unlike arrays this length is allowed to change.

Here's an example of a slice: var x []float64

The only difference between this and an array is the missing length between the brackets. In this case x has been created with a length of 0.
If you want to create a slice you should use the built-in make function:

x := make([]float64, 5)

This creates a slice that is associated with an underlying float64 array of length 5.
Slices are always associated with some array, and
although they can never be longer than the array, they can be smaller.
The make function also allows a 3rd parameter:

x := make([]float64, 5, 10)

10 represents the capacity of the underlying array which the slice points to.

Another way to create slices is to use the [low : high] expression:

arr := []float64{1,2,3,4,5}

x := arr[0:5]

low is the index of where to start the slice and high is the index where to end it (but not including the high index itself).

For example while

arr[0:5] returns [1,2,3,4,5], -->Index 5 is not included

arr[1:4] returns [2,3,4]. -->Index 4 is not included

For convenience we are also allowed to omit low, high or even both low and high.

arr[0:] is the same as arr[0:len(arr)],

arr[:5] is the same as arr[0:5] and

arr[:] is the same as arr[0:len(arr)].

Slice Functions

Go includes two built-in functions to assist with slices: append and copy. Here is an example of append:

func main() {

slice1 := []

int{1,2,3}

slice2 := append(slice1, 4, 5)

fmt.Println(slice1, slice2)

}

After running this program slice1 has [1,2,3] and slice2 has [1,2,3,4,5]. append creates a new slice by taking an existing slice (the first argument) and appending all the following arguments to it.

Here is an example of copy:

func main() {

slice1 := []

int{1,2,3}

slice2 := make([]int, 2)

copy(slice2, slice1)

fmt.Println(slice1, slice2)

}

After running this program slice1 has [1,2,3] and slice2 has [1,2]. The contents of slice1 are copied into slice2, but since slice2 has room for only two elements only the first two elements of slice1 are copied.

Maps

A map is an un-ordered collection of key-value pairs. Also known as an associative array, a hash table or a dictionary, maps are used to look up a value by its associated key. Here's an example of a map in Go:

var x map[ <Key_Type> ] <Value_Type>

var x map[string]int