# Big O: Space Complexity

The space means **additional space**. This space doesn’t care how much size the input is.

### Memory

**Heap**: store variables we assigned

**Stack**: keep track of the function calls

### What takes memory space?

* **Variables**
    
* **Data Structures**
    
* **Function Call**
    
* **Allocations**
    

**Example 1:**

In the function `sayBooooooo`, it takes one more space for `i := 0`. Ignore the size of the input slice `num`. The space complexity is `O(1)`.

In the function `sliceOfHiNTimes`, it creates a new slice `hiSlice`. Then, fill this slice with the string “Hi“ as an item, and each “Hi“ item occupies memory. Follow the Big O rule 2, removing the constants (`O(1 + n)` —> `O(n)`). The space complexity is `O(n)`

```go
package main

import "fmt"

func main() {
	num := []int{1, 2, 3, 4, 5, 6, 7}

	// space complexity: O(1)
	// one more space for i
	sayBooooooo(num)

	// space complexity: O(n)
	sliceOfHiNTimes(7)
}

func sayBooooooo(input []int) {
	for i := 0; i < len(input); i++ {
		fmt.Println("Say booooooo!")
	}
}

func sliceOfHiNTimes(times int) []string {
	// create a new slice
	hiSlice := make([]string, times)

	// fill this slice with loop
	// each item in the slice occupies a memory space
	for i := 0; i < times; i++ {
		hiSlice[i] = "Hi"
	}
	fmt.Println(hiSlice)
	return hiSlice
}
```

**Example 2:**

Find a user’s first and last tweet content. The space complexity is `O(1)`.

```go
package main

import "fmt"

func main() {
	// find first tweet and last tweet
	tweets := []string{"hi, this is my first tweet", "great", "holiday"}
	findFirstAndLastTweet(tweets)

}

func findFirstAndLastTweet(tweets []string) {
	firstTweet := tweets[0]            // O(1)
	lastTweet := tweets[len(tweets)-1] // O(1)

	fmt.Println("First tweet:", firstTweet, ",Last tweet:", lastTweet)
}
```