Transcript 06.a While Loops - Chang.pptx

While Loops
CMSC 201
Overview
Today we will learn about:
• Looping Structures
• While loops
Looping
Loops let us do something until a boolean condition is
met.
The simplest loop is called a while loop.
The while loop
The syntax for a while loop is as follows:
line-1
while someCondition:
line-2
line-3
line-4
The while loop
line-1
while someCondition:
line-2
line-3
line-4
Condition is true
line-1
Condition is false
Condition is true
Condition is
false
line-2
line-3
line-4
Example
a = 5
while a > 0:
print(a)
a = a – 1
Prints
5
4
3
2
1
Notes
If the condition is false to begin with, the loop never
executes!
a = 4
b = 5
while a == b:
print("Hello")
print("Goodbye")
This will only print goodbye.
Notes
It is very important that the variable in the condition be
altered at some point during the loop.
a = 5
while a > 0:
print(a)
This is called an infinite loop, since a will always be
greater than zero.
Factorial Example
Computing factorial with a while loop.
n = int(input("Enter a number "))
counter = n
answer = 1
while counter > 0:
answer = answer * counter
counter = counter - 1
print("n factorial is ", answer)
Factorial Example
Loop control: counter counts down from n down to 1
n = int(input("Enter a number "))
counter = n
answer = 1
while counter > 0:
answer = answer * counter
counter = counter - 1
print("n factorial is ", answer)
Sanity check: what if n is 0 ? n is 1 ? n is -1?
Factorial Example
Calculating the answer.
n = int(input("Enter a number "))
counter = n
answer = 1
while counter > 0:
answer = answer * counter
counter = counter - 1
print("n factorial = ", answer)
Exercise
Use a while loop to print out every number between 0 and
100 that is divisible by three.
Loops + If Statements
Now that you know how to use loops, if statements, and
variables, you can create fairly complicated algorithms.
However, figuring out how to combine them can be
difficult!
In the next few homeworks and exercises, you will start
needing to figure out when to use each tool you’ve been
given so far. These are the basic elements of
programming!
Nested Loops
Nested loops are loops inside loops.
The first loop is the outer loop
The second one is the inner loop.
a = 0
while a < 5:
b = 0
while b < 4:
print(b)
b = b + 1
a = a + 1
Nested Loops
Loop control for outer loop:
a = 0
while a < 5:
b = 0
while b < 4:
print(b)
b = b + 1
a = a + 1
Nested Loops
Loop control for inner loop:
a = 0
while a < 5:
b = 0
while b < 4:
print(b)
b = b + 1
a = a + 1
Nested Loops
Loop control for outer and inner loops:
a = 0
while a < 5:
b = 0
while b < 4:
print(b)
b = b + 1
a = a + 1