Arrays and Strings

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1 Arrays and Strings

2 Arrays

Recall: Fibonacci() and Arrays 1 1 2 3 5 8 13 21 34 55 a Fibonacci(n) a ß

3 Recall: Fibonacci() and Arrays a Fibonacci(n) a ß array of length n a[1] ß 1 a[2] ß 1 for i ß 3 to n a[i] ß a[i-1] + a[i-2] return a

4 Declaring Arrays var a [10]int var b [100]string var c [20]float64 Declare arrays of the given type and length. array elements:! index of array:!

5 Accessing/Changing Array Elements in Go Array elements can be accessed by putting their index between [ ] following the array name: fmt.println(a[7],a[8]) a[0] = 10 b[30] = hi there i := c[i] = 3.1 c[2*i] = c[i] x[i] can appear on left side of assignment to set a value.

6 Accessing/Changing Array Elements in Go The length of an array can be found with len(x), where x is an array. Array indices start at 0. The first element is x[0]; last element is x[len(x) - 1]. It s an error to try to access elements past the end (or before the beginning) of the array: var d [100]int d[len(d)-1] = 3 d[len(d)] = 3 d[-60] = 7 // OK // ERROR! // ERROR!

7 Command Line Arguments with os.args HW2: enter command line information that is input as parameters into a function directly. Package os provides access to these parameters: go run myprogram.go a 3 77 another param os.args is an array of strings. os.args[1] is a os.args[2] is 3 os.args[3] is 77 os.args[4] is another param os.args[0] is myprogram

8 Exiting a Program with os.exit() if terminating because of an error, use os.exit(1). if terminating normally, use os.exit(0). func RandomWalk(n, s int) { if n <= 0 { fmt.println( n must be positive! ) os.exit(1) } /* Rest of program here */ } Think: How can we make sure that we have the right number of input parameters to a program?

9 Multidimensional Arrays as 2-D Tables var a [7][4]int a[1][2] = 19 y := a[3][0] // gives 0 x := len(a) // gives 7 Think of as an array of length 7, where each element contains an array of length 4 7 rows columns

10 Multidimensional Arrays as 2-D Tables var a [7][4]int a[1][2] = 19 y := a[3][0] // gives 0 x := len(a) // gives 7 Think of as an array of length 7, where each element contains an array of length 4 7 rows columns Think: How can we determine the number of columns?

11 Multidimensional Arrays as 2-D Tables var a [7][4]int a[1][2] = 19 y := a[3][0] // gives 0 x := len(a) // gives 7 Think of as an array of length 7, where each element contains an array of length 4 7 rows columns z := len(a[0]) // gives 4

12 Strings

13 String Conversion Pain Exercise: What should be the value of MJordan? MJordan := string(45)

14 String Conversion Pain Exercise: What should be the value of MJordan? MJordan := string(45) Answer: It seems like it should be 45. But the answer is -.

15 How Strings are Represented Internally Logical interpretation Internal representation H e l l o, W o r l d! Each item in a string is called a character. A character in Go is represented using a byte variable type that is a number.

16 ASCII Chart (You don t need to know these numbers) Binary Dec Glyph ` a b c d e f g h i j k l m n o p q r s t u v w x y z { } ~ Binary Dec Glyph A B C D E F G H I J K L M N O P Q R S T U V W X Y Z [ \ ] ^ _ Binary Dec Glyph (space) ! " # $ % & ' ( ) * , / : ; < = > ?

17 Restoring Sanity: strconv Package

18 Restoring Sanity: strconv Package MJordan := strconv.itoa(45) // gives 45 x, err := strconv.atoi( 107") //gives 107 if err!= nil { os.exit(1) } y, err := strconv.parsefloat( 3.1, 64) if err!= nil { os.exit(1) } Check out FormatFloat() for float to string conversion.

19 strings Package Contains many built-in string algorithms. Example: testing whether a string contains another: var a = hi, there var b = the if strings.contains(a, b) { fmt.println( String a contains string b! ) }

20 Recall: String + Operator s1 + s2 concatenate strings s1 and s2! hi + there == hithere what s + up + doc? == what s up doc?

21 Indexing Strings Strings work like arrays in some ways: Strings have fixed length. You can find the length of string s with len(s). You can access elements of string s with s[i]. s := Hi There! fmt.println(s[0]) // prints H fmt.println(s[len(s)-1]) // prints! if s[0] == H // single quotes for single symbols H i T h e r e!

22 Substrings in Go s := Hi There! fmt.println(s[3:5]) // prints Th fmt.println(s[1:]) // prints i There! fmt.println(s[:4]) // prints Hi T var str string = s[3:6] fmt.println(str) fmt.println(str[0]) // prints The // prints T H i T h e r e!

23 Recall: Counting Pattern in Text 1. Start from the first position of Text and check whether Pattern appears in Text starting at its first position. 2. If yes, draw a dot on a piece of paper. 3. Move to the second position of Text and check whether Pattern appears in Text starting at its second position. 4. If yes, draw another dot on the same piece of paper. 5. Continue until you reach the end of Text. 6. Count the number of dots on the piece of paper.

24 Example: Implement PatternCount() Pattern Counting Problem: Count the number of times that a pattern appears in a longer string. Input: Strings Text and Pattern. Output: The number of times that Pattern appears in Text. fmt.println(strings.count( ATATA, ATA )) // prints 1, not 2. Exercise: Write a PatternCount() function handling overlapping instances of Pattern solving this problem.

25 Example: Implement PatternCount() func PatternCount(Pattern, Text string) int { //fill in details here }

26 Example: Reverse Complementing a String Reverse Complement Problem: Input: A DNA string Text. Output: The reverse complement of Text. 5 3 A G T C G C A T A G T T C A G C G T A T C A 3 5 Think: How could we solve this problem top-down?

27 Example: Reverse Complementing a String ReverseComplement() Complement() Reverse()

28 (Unlike Arrays) Strings are Not Editable! func Complement(s string) string { for i:=0; i<len(s); i++ { switch s[i] { case 'A': s[i] = 'T //ERROR! case 'T': s[i] = 'A //ERROR! case 'C': s[i] = 'G //ERROR! case 'G': s[i] = 'C //ERROR! } } return s }

29 Reverse Complementing a String func Complement(s string) string { s2 := "" for i:=0; i<len(s); i++ { switch s[i] { case 'A': s2 += 'T // type! case 'T': s2 += 'A // type! case 'C': s2 += 'G // type! case 'G': s2 += 'C // type! } } return s2 }

30 Reverse Complementing a String func Complement(s string) string { s2 := "" for i:=0; i<len(s); i++ { switch s[i] { case 'A': s2 += T case 'T': s2 += A case 'C': s2 += G case 'G': s2 += C } } return s2 } Exercise: Write a Reverse() function reversing a string.

31 Reverse Complementing a String func Reverse(s string) string { s2 := n := len(s) for i:=0; i<n; i++ { s2 += string(s[n-i-1]) } return s2 }

32 Slices

33 Last Time: Random Walk Input: Integers n and s. Output: Print each step of a random walk with s steps in an n x n chessboard.

Output: Print each step of a random self-avoiding walk with s steps in an n x n

34 Self-Avoiding Random Walks Self-avoiding walk: that never visits the same square of the checkerboard twice. Input: Integers n and s. Output: Print each step of a random self-avoiding walk with s steps in an n x n chessboard (start in the middle). Exercise: Write pseudocode for a function that prints a random self-avoiding walk.

35 Pseudocode for Random Self-Avoiding Walk SelfAvoidingRandomWalk(n, s) field ß n x n boolean array holding all false (x, y) ß (n/2, n/2) field[x][y] ß true Print(x, y) for i ß 0 to (steps 1) (xnext, ynext) = (x, y) while field[xnext][ynext] (xnext, ynext) = RandomStep(x, y, 10) (x, y) = (xnext, ynext) field[x][y] = true Print(x,y) Note the code reuse of RandomStep()...

36 Just Two Little Problems 1. Corner case: If the path works itself into a corner with nowhere to go, the algorithm will have an infinite loop! 2. In Go, arrays cannot have length equal to a variable. func SelfAvoidingRandomWalk(n, s int) { var field [n][n]bool //throws error! //... blah }

37 Who First Computed Earth s Circumference?

Eratosthenes of Cyrene (276 195 BC) Also:

38 Eratosthenes of Cyrene ( BC) Also: first algorithm for identifying prime numbers

39 Finding Primes Prime Number Problem: Input: An integer n. Output: All integers p < n such that p is prime.

40 Sieve of Eratosthenes

41 Sieve of Eratosthenes Prime Number Problem: Input: An integer n. Output: All integers p < n such that p is prime. Exercise: Write a program in pseudocode that takes n as input and returns an array iscomposite such that: iscomposite[i] = true if i is composite iscomposite[i] = false if i is prime

len(iscomposite) { for i := 2*biggestPrime; i < len(iscomposite); i += biggestprime { iscomposite[i] =

42 Fill in Details Later PrimeSieve(n) iscomposite ß array of n false boolean values biggestprime ß 2 while biggestprime < n func primesieve(iscomposite []bool) { var biggestprime = 2 for biggestprime < len(iscomposite) { for i := 2*biggestPrime; i < len(iscomposite); i += biggestprime { iscomposite[i] = true } biggestprime++ for biggestprime < len(iscomposite) && iscomposite[biggestprime] { biggestprime++ } } }

43 Strings

44 Need to Implement: Substring() and Count() FrequentWords(Text, k) FrequentPatterns ß an empty list c ß empty array of length Text - k for i ß 0 to Text - k Pattern ß Substring(Text, i, k) c[i] ß Count(Text, Pattern) maxcount ß Max(a) for i ß 0 to Text - k if a[i] = maxcount add Substring(Text, i, k) to FrequentPatterns FrequentPatterns ß RemoveDuplicates(FrequentPatterns) return FrequentPatterns

45 String + Operator s1 + s2 concatenate strings s1 and s2! hi + there == hithere what s + up + doc? == what s up doc?

Indexing Strings Strings work like arrays in some ways: Strings have fixed length. You can find the length of string s with len(s). You can access elements of string s with s[i].

46 Indexing Strings Strings work like arrays in some ways: Strings have fixed length. You can find the length of string s with len(s). You can access elements of string s with s[i]. You cannot modify a string once it has been created. s := Hi There! fmt.println(s[0]) // prints H fmt.println(s[len(s)-1]) // prints! s[3] = t // ERROR! Can t assign to strings if s[0] == H //note the single quotes for individual symbols.

47 Substrings in Go s := Hi There! fmt.println(s[3:5]) // prints Th fmt.println(s[1:]) // prints i There! fmt.println(s[:4]) // prints Hi T var str string = s[3:6] fmt.println(str) fmt.println(str[0]) // prints The // prints T H i T h e r e!

Recall: Counting Pattern in Text 1. Start from the first position of Text and check whether Pattern appears in Text starting at its first position. 2. If yes, draw a dot on a piece of paper. 3.

48 Recall: Counting Pattern in Text 1. Start from the first position of Text and check whether Pattern appears in Text starting at its first position. 2. If yes, draw a dot on a piece of paper. 3. Move to the second position of Text and check whether Pattern appears in Text starting at its second position. 4. If yes, draw another dot on the same piece of paper. 5. Continue until you reach the end of Text. 6. Count the number of dots on the piece of paper.

49 Example: Implement Count() Pattern Counting Problem: Count the number of times that a pattern appears in a longer string. Input: Strings Text and Pattern. Output: The number of times that Pattern appears in Text. Note: our Count() function should handle overlaps (unlike strings.count()). Think: do you see why strings is before Count()? Exercise: Write a Count() function in Go solving this problem.

50 Example: Reverse Complementing a String Reverse Complement Problem: Input: A DNA string Text. Output: The reverse complement of Text. 5 3 A G T C G C A T A G T T C A G C G T A T C A 3 5 Exercise: How could we solve this? What are the barriers?

51 Strings are Not Editable! func Complement(s string) string { for i:=0; i<len(s); i++ { switch s[i] { case 'A': s[i] = 'T //ERROR! case 'T': s[i] = 'A //ERROR! case 'C': s[i] = 'G //ERROR! case 'G': s[i] = 'C //ERROR! } } return s }

52 Self-Avoiding Random Walks Self-avoiding walk: a walk that never visits the same square of the checkerboard twice. Input: Integers n and s. Output: Print each step of a random self-avoiding walk with s steps in an n x n chessboard (start in the middle).

Pseudocode for Random Self-Avoiding Walk SelfAvoidingRandomWalk(n, s) field ß n x n boolean array holding all false (x, y) ß (n/2, n/2) field[x][y] ß true Print(x,y) for i ß 0 to

53 Pseudocode for Random Self-Avoiding Walk SelfAvoidingRandomWalk(n, s) field ß n x n boolean array holding all false (x, y) ß (n/2, n/2) field[x][y] ß true Print(x,y) for i ß 0 to (steps 1) (xnext, ynext) = (x, y) while field[xnext][ynext] (xnext, ynext) = RandomStep(x, y, 10) (x, y) = (xnext, ynext) field[x][y] = true Print(x,y) Note the code reuse of RandomStep()...

54 Just Two Little Problems 1. Corner case: If the path works itself into a corner with nowhere to go, the algorithm will have an infinite loop! 2. In Go, arrays cannot have length equal to a variable. func SelfAvoidingRandomWalk(n, s int) { var field [n][n]bool //throws error! //... blah }

55 Let s Next Look at Max() FrequentWords(Text, k) FrequentPatterns ß an empty list c ß empty array of length Text - k for i ß 0 to Text - k Pattern ß Substring(Text, i, k) c[i] ß Count(Text, Pattern) maxcount ß Max(a) for i ß 0 to Text - k if a[i] = maxcount add Substring(Text, i, k) to FrequentPatterns FrequentPatterns ß RemoveDuplicates(FrequentPatterns) return FrequentPatterns

Issue #2: Arrays Copied in Function Calls func Max(A [10000000]int) int { m := 0 for i := range A { if A[i] > m { m = A[i] } } return m } func main() { var numbers [10000000]int } // fill

56 Issue #2: Arrays Copied in Function Calls func Max(A [ ]int) int { m := 0 for i := range A { if A[i] > m { m = A[i] } } return m } func main() { var numbers [ ]int } // fill numbers with random integers for i := range numbers { numbers[i] = rand.int() } fmt.println(max(numbers)) A new array A is created and the contents from numbers is copied over (wasteful of memory).

57 (See Burrows-Wheeler Transform Slides)

58 Slices

Slices: Everything Arrays in Go Should Be A slice variable is declared by not specifying a size in [] var a []int // at this point a has the special value nil // and can t be used as an array a =

59 Slices: Everything Arrays in Go Should Be A slice variable is declared by not specifying a size in [] var a []int // at this point a has the special value nil // and can t be used as an array a = make([]int, 10, 20) Creates an array of size 20 with a slice of size 10 inside it. 0! 0! 0! 0! 0! 0! 0! 0! 0! 0! 0! 0! 0! 0! 0! 0! 0! 0! 0! 0! 0! 1! 2! 3! 4! 5! 6! 7! 8! 9! 10! 11! 12! 13! 14! 15! 16! 17! 18! 19! Length of this slice is 10! Underlying array has size 20!

60 Omitting the Third Parameter var a []int a = make([]int, 10) // this is the same as make([]int, 10, 10)

61 We Can Still Change Items var a []int a = make([]int, 10) a[3] = -18

62 Passing a Variable as Slice Length var n int = 5050 var a []string a = make([]string, n) // this is OK!

63 Defining a Slice in One Line var n int = 100 b := make([]string, n) // this is OK!

64 Recall: Literals... The Other Side of = Integer literals: a sequence of digits String literals: a sequence of characters between Hi there! 1+ = Unicode strings are supported. bool (Boolean) literals: either true or false true false

65 Array and Slice Literals Can also write slice literals (useful if you have a short list of data that may change):! var a = []float64{3.2, -30, 84, 62} var prime = []int{2,3,5,7,11}

66 Array and Slice Literals Can also write slice literals (useful if you have a short list of data that may change):! var a = []float64{3.2, -30, 84, 62} var prime = []int{2,3,5,7,11} We can also use array literals (useful for a short list of data that is fixed):! var a = [3]float64{3.2, -30.0, } var prime = [5]int{2,3,5,7,11}

67 The append() Operation If we want to make a slice bigger by adding something to the end of it... s := make([]int, 10) 0! 0! 0! 0! 0 0! 0! 0! 0 0! 0! 1! 2! 3! 4! 5! 6! 7! 8! 9!

68 The append() Operation If we want to make a slice bigger by adding something to the end of it... s := make([]int, 10) s = append(s, 5) 0! 0! 0! 0! 0 0! 0! 0! 0 0! 5 0! 1! 2! 3! 4! 5! 6! 7! 8! 9! 10

More on Strings & Arrays

More on Strings & Arrays 02-201 Indexing Strings Strings work like arrays in some ways: Strings have fixed length. You can find the length of string s with len(s). You can access elements of string s with