15 110 Principles of Computing Loop Structures Part
- Slides: 22
15 -110: Principles of Computing Loop Structures- Part III Lecture 9, September 30, 2018 Mohammad Hammoud Carnegie Mellon University in Qatar
Today… • Last Session: • Loop Structures- Part II • Today’s Session: • Loop Structures- Part III: • While Statement • Nested Loops • Various Examples • Announcements: • HA 3 will be out by tonight; it is due on Saturday Oct 6 by 11: 00 AM • Quiz I is on Sunday Oct 7
Recap: Fibonacci Sequence • Suppose we want to write a program that computes and outputs the nth Fibonacci number, where n is a value entered by a user • The Fibonacci sequence starts with 0 and 1 • After these first two numbers, each number in the sequence is computed as simply the sum of the previous two numbers • E. g. , 0, 1, 1, 2, 3, 5, 8, 13, 21, 34, …
Recap: Fibonacci Sequence def fibonacci(n): f_i = 0 f_j = 1 print(f_i, f_j, end = " ") for k in range(2, n+1): f_new = f_i + f_j print(f_new, end = " ") f_i = f_j = f_new
Recap: Fibonacci Sequence n = eval(input("Enter a number that is larger than 1 >> ")) if n < 2: print("You can only enter a number that is larger than 1!") else: fibonacci(n)
Definite Loops vs. Indefinite Loops • So far, we have considered only the case where the number of iterations is determined before the loop starts • This kind of loops is called definite loops and for is used in Python to write definite loops • But, what if we want to write loops, wherein we do not know the number of iterations beforehand? • This kind of loops is denoted as indefinite loops
The While Statement • In Python, an indefinite loop is implemented using a while statement while <condition>: <body> • <condition> is a Boolean expression, just like in if statements • <body> is, as usual, a sequence of one or more statements
The Flowchart of a While Loop Is <condition> True? Yes <body> No
Revisiting Average of a Series of Numbers • Here is how we have done it before: def main(): n = eval(input("How many numbers do you have? ")) sum = 0. 0 for i in range(n): x = eval(input("Enter a number >> ")) sum = sum + x print("n. The average of the numbers is", sum/n) main()
Revisiting Average of a Series of Numbers • Here is how we can do it now using a while statement: sum = 0. 0 n = eval(input("How many numbers do you have? ")) count = 0 while count < n: x = eval(input("Enter a number >> ")) sum = sum + x count = count + 1 print("The average of the " + str(n) + " numbers you entered is ", sum/n)
Revisiting Average of a Series of Numbers • Here is also another version that assumes no prior knowledge about the quantity of numbers the user will input sum = 0. 0 count = 0 more. Data = "yes" while more. Data == "yes": x = eval(input("Enter a number >> ")) sum = sum + x count = count + 1 more. Data = input("Do you have more numbers (yes or no)? ") print("The average of the " + str(count) + " numbers you entered is ", sum/count)
Printing Odd Numbers With Input Validation • Suppose we want to print the odd numbers between two user-input numbers (inclusive), say, start and end • The program assumes some conditions, whereby the start and end numbers shall be positive and end should be always greater than start • Hence, we should continue prompting the user for the correct input before proceeding with printing the odd numbers • This process is typically called input validation • Well-engineered programs should validate inputs whenever possible!
Printing Odd Numbers With Input Validation 1. while True: 2. start = eval(input("Enter start number: ")) 3. end = eval(input("Enter end number: ")) 4. if start >=0 and end >= 0 and end > start: 5. break It breaks the loop; execution continues at line 8. 6. else: 7. print("Please enter positive numbers, with end being greater than start") 8. 9. for i in range(start, end + 1): 10. if i % 2 == 0: It skips one iteration in the loop; execution 11. continue 12. print(i, end = " ") continues back at line 9.
Nested Loops • Like the if statement, loops can also be nested to produce sophisticated algorithms • Example: Write a program that prints the following rhombus shape * * * *
The Rhombus Example • One way (not necessarily the best way!) to think about this problem is to assume that the stars are within a matrix with equal rows and columns 1 2 3 4 1 * 3 7 8 9 * * 4 6 6 * 2 5 5 * * * Can you figure out the different relationships between rows and columns?
The Rhombus Example • One way (not necessarily the best way!) to think about this problem is to assume that the stars are within a matrix with equal rows and columns 1 2 3 4 1 * 3 7 8 9 * * 4 6 6 * 2 5 5 * * * Print a star when: 1) Row + Column == 6 2) Row + Column == 14 3) Row – Column == 4 4) Column – Row == 4
The Rhombus Example • Here is one way of writing the program in Python for i in range(1, 10): for j in range(1, 10): if ((i+j== 6) or (j-i==4) or (i+j == 14) or (i-j==4)): print("*", end = "") else: print(" ", end = "") print() Can you generalize this code?
The Rhombus Example • What are 6, 14, 4, and 4 below? 1 2 3 4 1 * 3 7 8 9 * * 4 6 6 * 2 5 5 * * * Print a star when: 1) Row + Column == 6 2) Row + Column == 14 3) Row – Column == 4 4) Column – Row == 4
The Rhombus Example • What are 6, 14, 4, and 4 below? 1 2 3 4 1 * 3 7 8 9 * * 4 6 6 * 2 5 5 * * *
The Rhombus Example: A More General Version while True: rows = eval(input("Enter number of rows: ")) columns = eval(input("Enter number of columns: ")) if rows != columns or rows % 2 != 1 or columns % 2 != 1: print("Please enter odd and equal rows and columns") else: break rows = abs(rows) columns = abs(columns)
The Rhombus Example: A More General Version for i in range(1, rows+1): for j in range(1, columns+1): if ((i+j== (columns//2 +2)) or (j-i==(columns//2)) or (i+j == (columns+ math. ceil(rows/2))) or (i-j==(columns//2))): print("*", end = "") else: print(" ", end = "") print()
Next Lecture… • Problem Solving