1. Strings:  Strings?  concatenate (join) +  make multiple copies using *  compare: > = <= !=  What else can we do?

2. A lot! Len, in def f(x): if "e" in x: return("e is in your message.") else: return("e is not in your message.") strvar = “puppies are cute” print(f(strvar)) z = len("cat")

3. Strings Positional Get a character at a particular index within a string stringvar1 = “ binary” Index 012345 stringvar2 = “computer” Index 012 3 4 5 6 7 So to use a particular item in the list: x=stringvar2[3] #”p” y=stringvar1[1] #”i” z=stringvar2[0] #”c” w=stringvar1[6] #??? Example: def f(par): randnum = randrange(0,6) return(“you get “ + par[randnum]) print(f((stringvar1))

4. String: stringvar1 = “ binary” Index 012345 stringvar2 = “computer” Index 01234567 get the length of a string len(stringvar1) # will give you 6, not 5! len(stringvar2) #will give you 8, not 7 Example: def f(par): y = len(par) randnum = randrange(0,y) #Why does this work? return(“you get “ + par[randnum]) print(f(stringvar2))

5. We can now do: def f(x,y): if (x == len(y)): return else: print(y[x]) return(f(x+1,y)) f(0,’kluge’)

6. Slicing (Different from Indexing)  Copying parts of strings: 0 1 2 3 4 5 | p | i | z | z | a | -5 -4 -3 -2 -1 def f(word): return(word[2:5] ) print(f(“pizza”)) def g(): word = “pizza” return(word[1:3]) def h(): word = “pizza” return(word[-4:-2]) def i(): word = “pizza” return(word[-4:3])

7. Shortcuts 0 1 2 3 4 5 | p | i | z | z | a | -5 -4 -3 -2 -1 word=“pizza” word[0:4] pizz word[:4] pizz word[2:5] zza word[2:] zza

8. # display a slice def g(s,f,wd): return("wd["+str(s)+":"+str(f)+"] is "+wd[s:f]) print(g(3,7,"sesquipedalian"))

9. Strings are Immutable word of the day  Can:  x = "catamaran"  print(x.count("a"))  print(x.index('a'))  Can’t (if it changes the string, we can’t do it)  x[3] = “y” #can’t do!

10. What does this do? def f(str1): str2 = “WuGmUmP” if str1.upper() == str2.upper(): return(“You’re one of us!”) else: return(“You’re not one of us!”) newstr = “wuGMuMp” print(f(newstr))

11. def rec(a,x,y): if (x == len(a)): return 0 elif (a[x] in y): return 1 + rec(a,x+1,y) else: return rec(a, x+1, y) a = "catamaran" y = "aeiou" print(str(rec(a,0,y)))

12. def rec(a,x,y): if (x == len(a)): return "" elif (a[x] in y): return rec(a,x+1,y) else: return (a[x] + rec(a, x+1, y)) a = "catamaran" y = "aeiou" print(str(rec(a,0,y))) 

13. def f(x,y,z): if y == len(x): return(z) else: if (x[y] == "p"): return(f(x,y+1,z+"m")) else: return(f(x,y+1,z+x[y])) print(f("puppy",0,""))

14. def f(x,a): if (x == len(a)//2): return a[x]==a[len(a)//2] else: if a[x] != a[len(a)-(x+1)]: return a[x] == a[len(a)-(x+1)] else: return(f(x+1,a)) print(f(0,"mom")) print(f(0,"mommy"))

15. def g(x,y,z,q): if (x == len(y)): return z else: if (y[x] not in ",!.:;?"): if q: return(g(x+1,y,z+1,False)) else: return(g(x+1,y,z,False)) else: return(g(x+1,y,z,True)) print(g(0,"Hello there, how are you today?",0,True))

16. def h(x,y,z,q): if x == len(y): return 0 else: if y[x] in "\"'": if (q == y[x]): return(z) elif q != y[x]: return h(x+1,y,0,y[x]) else: return h(x+1,y,z+1,q) print(h(0,"He said, 'Houston, we have a problem' in the movie",0,'+'))

18. While Loop def ThreeYearOld(): x = "" while (x != "Because."): x = input("But why?") return("Oh. Okay.") print(ThreeYearOld())  Look at this code:  What is our starting condition?  What must be true in order for the loop to start?  What will make the loop end?  What must become false?  Does something INSIDE the loop that changes so that eventually the loop will end?  Does this all remind you of something?

19. Loops:  While Loops:  Continue while condition is true  Must initialize to make condition true  In recursion we stop when the stopping contion is true – in while loops we stop when the while condition is false.  Something inside the loop must change so that the condition becomes false eventually.  What does this code print out? def f(x, counter): while counter < x: print(counter) counter = counter + 1 return(counter) print(f(5, 0)) def f2(x,counter): if (counter >= x): return (counter) else: print(counter) return(f2(x,counter+1)) print(f2(5,0))

20. While loops def f(total,count): while count >= 1: #This is the part that loops total += count count = count -1 return(total) print(f(0, int(input("Enter a number")))) Starting condition (What must be true to enter loop)? What makes the loop stop? What inside the loop changes so that the loop will stop?

21. While loops def f(total): while count >= 1: total += count count = count -1 return(total) print(f(0)) Starting condition? What makes the loop stop? What inside the loop changes so that the loop will stop?

22. While loops def f(total, count): while count >= 1: total += count count = count + 1 return(total) print(f(0, 4)) Starting condition? What makes the loop stop? What inside the loop changes so that the loop will stop?

23. Loops:  Anything that can be done recursively can be done with a while loop (and vice versa)  Certain problems work better with certain loops  loop must have a True/False condition while count >= 1:  While loops starts when the condition is True  It continues while the condition is True  It stops when the condition is False  Something must change inside the loop that will eventually make the True/False condition False while count >= 1: total += count count = count -1

24. Differences (syntactic):  We can initialize variables used by the while loop inside the function (BUT BEFORE THE WHILE LOOP!)  Why can’t we do this with recursion? def f(): x = 5 counter = 0 while counter < x: print(counter) counter = counter + 1 return(counter) print(f()) def f(x, counter): while counter < x: print(counter) counter = counter + 1 return(counter) print(f(5, 0))  In general, unless you know what you are doing, while loops should not call the function from within the function  In general, unless you know what you are doing, recursive function should not use while loops inside the function.

25.  Write a function that takes as an input parameter an integer and uses a while loop to print out "ha" that number of times.  Starting condition?  What makes the loop stop? (your True/False condition?)  What inside the loop changes so that the loop will stop? def func(x): while (x > 0): print("ha ",end= " ") x -= 1 return func(5) def func(x): stringvar = "" while (x > 0): stringvar += "ha ") x -= 1 return(stringvar) print(func(5)) Recursive: def func(x): if x <= 0: return else: print("ha ") return(func(x-1)) func(5) Recursive: def func(x,stringvar): if x <= 0: return(stringvar) else: return(func(x-1,stringvar+"ha ")) print(func(5, ""))

26. What does this do? def w2(x,y): tot = 0 while (y > 0): tot += x y -= 1 return(tot) print(w2(3,5))

27. What about this? def g(y,z): x = len(y)-1 while x >= 0: z += y[x] x-=1 return z print(g("nilbog",""))

28. What does this do? def f(message): newmessage = "" x = 0 while x < len(message): if message[x] == "g": newmessage += "l" else: newmessage += message[x] x += 1 return(newmessage) print(f("pogysyggabicaggy“))

29. This one? def c2(x): y = 0 k = 0 while (x > 0): y += x%2 * 10**k k += 1 x = x//2 return(y) print(c2(10))

30. What does this one do? def w4(x, z): y = randrange(0,x) print(y) g = -1 #Why did I do this? while (g != y): g = randrange(0,x) print(g) z+= 1 print(z) return("It took " + str(z)) print(w4(10,0))

31. Try: exponent:  Write a function that takes as input parameters 2 integers and uses a while loop to calculate x to the power of y (don’t use **) def c(x,y): z = 1; while (y > 0): z *= x y -= 1 return z

32. Write a while loop that converts a binary number to an integer def c3(x): y = 0 k = 0 while (x > 0): y += x%2 * 2 **k k+=1 x = x//10 return(y)

33. def z(ct): x = 0 while (ct >= 0): x += ct ct = ct - 1 return(x) print(z(5)) Versus: def z(ct): x = 0 while (ct >= 0): ct = ct - 1 x += ct return(x) print(z(5))