Loop Structures in Python Programming
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:
•
HA3 will be out by tonight; it is due on Saturday Oct 6 by 11:00AM
•
Quiz I is on Sunday Oct 7
•
Suppose we want to write a program that computes and outputs the
n
th 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
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_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)
•
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
Definite Loops vs. Indefinite Loops
•
In Python, an indefinite loop is implemented using a while statement
•
<condition>
is a Boolean expression, just like in if statements
•
<body>
is, as usual, a sequence of one or more statements
The While Statement
while <condition>:
<body>
The Flowchart of a While Loop
Is <condition>
True?
No
Yes
<body>
•
Here is how we have done it before:
Revisiting Average of a Series of Numbers
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("\nThe average of the numbers is", sum/n)main()
•
Here is how we can do it now using a while statement:
Revisiting Average of a Series of Numbers
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)•
Here is also another version that assumes no prior knowledge about
the quantity of numbers the user will input
Revisiting Average of a Series of Numbers
sum = 0.0
count = 0
moreData = "yes"
while moreData == "yes":
x = eval(input("Enter a number >> "))sum = sum + x
count = count + 1
moreData = input("Do you have more numbers (yes or no)? ")print("The average of the " + str(count) + " numbers you entered is ", sum/count)•
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
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
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:
11.
continue
12. print(i, end = " ")
It breaks the loop; execution continues at line 8.
It skips one iteration in the loop; execution
continues back at line 9.
•
Like the if statement, loops can also be nested to produce
sophisticated algorithms
•
Example
: Write a program that prints the following rhombus shape
Nested Loops
•
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
The Rhombus Example
Can you figure
out the different
relationships
between rows
and columns?
•
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
The Rhombus Example
Print a star when:
1)
Row + Column == 6
2)
Row + Column == 14
3)
Row – Column == 4
4)
Column – Row == 4
The Rhombus Example
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()
•
Here is one way of writing the program in Python
Can you generalize this code?
The Rhombus Example
•
What are 6, 14, 4, and 4 below?
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?
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
This lecture covers Loop Structures, specifically focusing on the while statement and nested loops in Python programming. It discusses the Fibonacci sequence and demonstrates how to write a program to compute the nth Fibonacci number. Additionally, it explains the difference between definite and indefinite loops, highlighting the implementation of indefinite loops using the while statement.
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Presentation Transcript
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: HA3 will be out by tonight; it is due on Saturday Oct 6 by 11:00AM 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_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 No Is <condition> True? Yes <body>
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("\nThe 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 moreData = "yes" while moreData == "yes": x = eval(input("Enter a number >> ")) sum = sum + x count = count + 1 moreData = 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 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: 11. continue 12. print(i, end = " ") continues back at line 9. It breaks the loop; execution continues at line 8. It skips one iteration in the loop; execution
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 5 6 7 8 9 1 * 2 * * Can you figure out the different relationships between rows and columns? 3 * * 4 * * 5 * * 6 * * 7 * * 8 * * 9 *
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 5 6 7 8 9 1 * 2 * * Print a star when: 1)Row + Column == 6 2)Row + Column == 14 3)Row Column == 4 4)Column Row == 4 3 * * 4 * * 5 * * 6 * * 7 * * 8 * * 9 *
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 5 6 7 8 9 1 * 2 * * Print a star when: 1)Row + Column == 6 2)Row + Column == 14 3)Row Column == 4 4)Column Row == 4 3 * * 4 * * 5 * * 6 * * 7 * * 8 * * 9 *
The Rhombus Example What are 6, 14, 4, and 4 below? 1 2 3 4 5 6 7 8 9 1 * 2 * * Print a star when: 1)Row + Column == 6 (i.e., Columns/2 +2) 2)Row + Column == 14 (i.e., Columns + ????/? ) 3)Row Column == 4 (i.e., Columns/2) 4)Column Row == 4 (i.e., Columns/2) 3 * * 4 * * 5 * * 6 * * 7 * * 8 * * 9 *
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
