1 Python Chapter 4 Branching statements and loops © Samuel Marateck 2010

2 Python Chapter 4 Branching statements and loops © Samuel Marateck 2010

3 The type of a variable is determined by what is defined by the type of the literal assigned to it. Example: value = 12 So value is an integer identifier. If you then assign ‘asd’ to value, value = ‘asd’ What type a variable is value now?

4 value becomes a string variable.

5 We now introduce a new type of literal, the boolean literal. It can have two values, True and False. Note that these two literals must be capitalized.

6 In computer science, an expression consist of a variable, literal or both. So examples of expressions are: x, 3.3, x+2, and ‘asd’.

7 A boolean expression must have a true or false value. Can you give an example of a boolean expression ?

8 Can you give an example of a boolean expression ? True and False are the simplest boolean expressions.

9 Normally, boolean expressions have a comparison operator. The following slide is a table of some of these operators.

10 OperatorExplanation ==Equal to !=Not equal to >Greater than >=Greater than or equal to <Less than <=Less than or equal to

11 Note that the operators consisting of two characters cannot have an embedded blank. So > = would cause an error. It should be written >=.

12 Next we show how to use a boolean expression in an if statement.

13 if x == 3: print(x) print(‘nyu’) print(‘end’)

14 The boolean expression in if x == 3: is x==3. If it’s true, then the block of statements following the if is executed. Here the block is: print(x) print(‘nyu’) Note that the block must be indented.

15 The Python environment automatically indents statements in the block. To end the block you must manually unindent.

16 if x == 3: print(x) print(‘nyu’) print(‘end’) After the block is executed, the statements after the block, here print(‘end’) executed. If the boolean expression is false, the block is skipped and the statements after the block, here print(‘end’), are executed.

17 Now we explore the if-else construct:

18 if x == 3: print(x) print(‘nyu’) else: print(x**2) print(‘end’)

19 If x==3 is true the block print(x) print(‘nyu’) is executed, then print(‘end’) is executed. If x==3 is false, the block after the else:, print(x**2) is executed. Then print(‘end’) is executed.

20 In if True : print(‘doggy’) the literal True is the boolean expression and it is true, so print(‘doggy’) is executed.

21 The for loop In for j in range(10): print(j) the range of j is 0 ≤ j ≤ 9 so print(j) is executed ten times, once for each value of j in this range. Note that j is an integer and is called the loop index.

22 To add the integers from 1 to 10, set a variable sum to zero and add to it in the loop sum = 0 for j in range(11): sum = sum + j print(sum) Since the print is indented, it is executed with sum = sum + j For each value of j where now 0 ≤ j ≤ 10. The last two statements is the block comprising the scope of the loop.

23 A table showing how sum is incremented. jsumsum + j

24 A table showing how sum is incremented. jsumsum + j

25 A table showing how sum is incremented. jsumsum + j

26 A table showing how sum is incremented. jsumsum + j

27 A table showing how sum is incremented. jsumsum + j etc

28 A table showing how sum is incremented. jsumsum + j etc

29 Since when j = 0, sum is not incremented, we can rewrite the loop as: sum = 0 for j in range(1, 11): sum = sum + j print(sum) Now the range of j is 1 ≤ j ≤ 10 and the increment by default is 1.

30 If you want the increment to be 2, write: for j in range(1, 11, 2):

31 If you want j to decrease, starting at 10, and going to 1, write: for j in range(10, 0, -1): The -1 indicates a negative increment of 1. the range of j is 10>= j >= 1.

32 In general for a positive for loop increment, the second number in range must be greater than the first for the loop to be executed at least once. So in for j in range(2,2) the loop will be executed zero times. The second 2 means that the upper limit of the loop index is 1.

33 In general for a negative for loop increment, the second number in range must be less than the first for the loop to be executed at least once. So in for j in range(2,2, -1) the loop will be executed zero times. The second 2 means that the limit of the loop index is 3.

34 If you want the sum to be printed only at the end of the loop’s execution, unindent the print, for j in range(1, 11): sum = sum + j print(sum)

35 In general, in for j in range(n), the range of j is 0 ≤ j ≤ n-1.

36 If you want to build a string consisting of increasing numbers, e.g., , try s = ‘’ for j in range(0,20): s = s + j print(s) Because s is a string, the ‘+’ here means concatenation not addition.

37 While wont the following program compile? How do you correct it? s = ‘’ for j in range(0,20): s = s + j print(s)

38 Since s is a string, j must be converted to a string by writing str(j) s = ‘’ for j in range(0,20): s = s + str(j) print(s)

39 A table showing how the string s is incremented. jss + str(j) 0‘’‘0’

40 A table showing how the string s is incremented. jss + str(j) 0‘’‘0’ 1 ’01’

41 A table showing how the string s is incremented. jss + str(j) 0‘’‘0’ 1 ’01’ 2 ’012’

42 A table showing how the string s is incremented. jss + str(j) 0‘’‘0’ 1 ’01’ 2 ’012’ 3 ’0123’

43 A table showing how the string s is incremented. jss + str(j) 0‘’‘0’ 1 ’01’ 2 ’012’ 3 ’0123’ 4 ’01234’

44 A table showing how the string s is incremented. jss + str(j) 0‘’‘0’ 1 ’01’ 2 ’012’ 3 ’0123’ 4 ’01234’ 5 ’012345’

45 A table showing how the string s is incremented. jss + str(j) 0‘’‘0’ 1 ’01’ 2 ’012’ 3 ’0123’ 4 ’01234’ 5 ’012345’ 6 ’ ’ etc

46 How do you get the computer to print only digits, e.g.,

47 Let’s pause here and ask, what is 4//10?

48 Let’s pause here and ask, what is 4//10? The answer is 0. So the remainder, 4%10, is 4. What is 14//10?

49 What is 14//10? The answer is 1. So the remainder, 14%10, is 4. How should we rewrite the program to get ?

50 s = ‘’ for j in range(0,20): n = j%10 s = s + str(n) print(s)

51 Using characters in a for Given a string s = ‘asd1’, in for c in s: print(c) during execution, the Python virtual machine iterates though the string s and prints each character in the string.

52 You don’t have to use c and s. So given the string t = ‘asd1’, you could write for b in t: print(b) and get the same results. How do you write a program that counts the characters in a string? inu

53 s = ‘asd1’, n = 0 for c in s: n = n + 1 print(‘number of characters is ‘, n)

54 What happens if instead you write: s = ‘asd1’, for c in s: n = 0 n = n + 1 print(‘number of characters is ‘, n)

55 Since n= 0 is in the loop, the counter n is set to zero for each character.

56 Nested loops How do you produce the following pattern in which the number of columns and rows is dictated by the loop indices. XXXXXX

57 How do you write a program that produces: XXXXXX and then skip to a new line?

58 for j in range(6): print(x, end = ‘’) print() # skips to a new line

59 How do we repeat this pattern 3 times?

60 We nest for j in range(6): print(x, end = ‘’) print() # skips to a new line in another loop

61 for k in range(3): for j in range(6): print(x, end = ‘’) print() # skips to a new line Note that the print() is executed after the for j loop, so it’s executed three times not six times. The loop index of the outer loop k must be different than the loop index of the inner loop, j.