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Python 2 –Conditionals and
loops
Lecture 17 – COMPSCI111/111G
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Today’s lecture
Recap of yesterday’s lecture
if statements
while loops
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Recap
Introduced the IDLE IDE, variables
Basic arithmetic operators Modulus (%) operator
print() function can be used to display text, arithmetic operations, variables etc.
input() function allows you to capture the user’s input from the keyboard int() converts the string value from input() into an
integer float() converts the string value from input() into
a floating point value
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Recap int() and float() can also convert
integers/floating point numbers to other data types
Example:x = 20.56print(int(x)) #output is 20
Example:y = 10print(float(y)) #output is 10.0
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IF statement Conditional activity (ie. ‘if this then do that’) is
an important part of many programs
The if statement lets you introduce conditional activity into your program
Statements that are executed when if is true must be tabbed underneath the if statement
Syntax:if [logical condition]:
[lines of code here]else:
[lines of code here]
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Example
determine if a number is odd or even, and print out an appropriate message.
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Is it an even num?
The number is odd The number is even
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Logical conditions
A logical condition will either evaluate to true or false
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Meaning Operator ExampleLess than < a < b
Less than or equal to <= a <= b
Greater than > a > b
Greater than or equal to >= a >= b
Equal to == a == b
Not equal to != a != b
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Logical conditions
You can combine logical conditions using the Boolean operators
if a and b: If the test in a and b evaluate to true, then the logical
condition will be true e.g. if x > 1 and y < 2
if a or b: If either a or b evaluate to true, then the overall
logical condition will be true e.g. if x > 1 or y < 2
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Logical conditions if not(a):
Inverts the result of a e.g. if not(5>6)
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IF statement example Write a program that asks the user to enter a
number between 1 and 10 (inclusive). The program will print out "Correct" if the number is in the range and "Incorrect" if the number is outside the range.
Example output (bold text is the user’s input):
Please enter a number (1-10): 34Incorrect
Please enter a number (1-10): 6Correct
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L24Demo1.py
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Example:
Prompt the user for a number
Convert it to an integer
If between 1 and 10 (inclusive)• Print “Correct”
Else • Print “Incorrect”
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IF statement examplenumber = int(input("Please enter a number (1-10): "))
if number >= 1 and number <= 10: print("Correct")
else:print("Incorrect")
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L24Demo2.py
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Exercise 1
Write a program that asks the user to enter a number. The program should determine if the number is odd or even, and print out an appropriate message.
Example output (bold text is the user’s input) :
Please enter a number: 56You entered 56 which is even
Please enter a number: 33You entered 33 which is odd
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TRY IT OUT!L24Ex1.py
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Algorithm:
Prompt the user for a number
Convert it to an integer
If it is an even number• “You entered”, _______,“which is even”
Else • “You entered”, _______,“which is odd”
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Loops
Allows you to repeat certain statements for as long as the loop’s logical condition evaluates to true
Statements that are executed when the while’s condition is true must be tabbed underneath the while statement
Syntax:
while [logical condition]:[lines of code here while condition is true]
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Loops
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Execute eachstatement in
the block
Iscondition
true?
statements before the loop
statements after the loop
No
Yes
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Example
Write a program to print the numbers from 1 to 5current = 1while current <= 5:
print(current)current = current + 1
print("Finished!")
Output:12345Finished! 17
L24Demo3.py
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Loops
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Print the valueIncrement current by 1Current <=5?
Set current to 1
Print ("Finished")
No
Yes
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Exercise 2
Write a program that repeatedly asks the user to enter a number between 1 and 10 (inclusive). When they do so, the program should print “Well done” on the screen and end
Example output (bold text is the user’s input):
Enter a number between 1 and 10: 99Enter a number between 1 and 10: -1Enter a number between 1 and 10: 10Well done
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L24Ex2.py
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Flow chart
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Prompt the user for another number
number<1?Or
number>10?
Prompt the user for a number
Print (“Well done")
No
Yes
Enter a number between 1 and 10: 99Enter a number between 1 and 10: -1Enter a number between 1 and 10: 10Well done
TRY IT OUT!
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Exercise 3 Write a program that repeatedly asks the user to
enter a number. If the number is even, then “x is even” (where x is the number) should be printed on screen. If the number is odd, then “x is odd” should be printed on screen. The program must print “Thanks” and end when the user types ‘0’
Example output (bold text is the user’s input):
Please enter a number: 4545 is oddPlease enter a number: 1212 is evenPlease enter a number: 0Thanks
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L24Ex3.py
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Flow chart
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Prompt the user for another number
number != 0
prompt the user for a number
print (“Thanks")
No
Yes
Please enter a number: 4545 is oddPlease enter a number: 1212 is evenPlease enter a number: 0Thanks
TRY IT OUT!
___ is odd
even?
___ is even
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Summary if statements allow you to introduce
conditional activities into your program
while loops allow you to repeat certain statements for as long as the logical condition evaluates to true
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Exercises
What is the output of the following statements?
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temp = 90if temp >= 90:
print("hot")temp = temp - 10
if temp < 70:print("cold")
if temp == 80:print("just right")