Lecture Notes 08: More Conditionals

Recap: Conditions

Any of the IF blocks can ask about numeric relationships as seen with the operators:

• \(<\): less than   example: x < 2
• \(<=\): less or equal than   example: x <= 2
• \(>\): greater than   example: x > 2
• \(>=\): greater or equal than   example: x >= 2

• \(==\): equal to   example: x == 2
• \(!=\): different than   example: x != 2

Recap: Booleans

Boolean values are True or False.

These are the ones that let an IF body get executed or not.

You can directly print them, use them in expressions, or have them get printed as the result of Boolean expression:

print( True ) 
print( False )
print ( (0 < 3) == True )
print ( (-3 > 10) == True )
print( 1 > 2 )

Recap: Cascading IF statements

Remember, from last class:

Think of the following situation:

A professor issues two assignments X and Y.
And the rules for grading are:

1. If students flunk X (a passing grade is 60), they need a grade of 80 or above in Y to pass the class (C+)
.
2. If students pass X, and if they also get 80 or above in Y, students get a final grade of A (otherwise C+).

One answer:

 

Another answer:

Intervals

We said we could evaluate if a number belongs in the following types of intervals:

• \(<\): less than some number y; example: x < y; x could be in the interval [min integer, y)
• \(<=\): less or equal than   example: x <= y; x could be in the interval [-min integer, y]
• \(>\): greater than   example: x > y; x could be in the interval (y, max integer]
• \(>=\): greater or equal than   example: x >= 2; x could be in the interval [y, max integer]

Operator chaining

num = 0.5

if 0 < num < 1:
  print ("in between")
elif num==1:
  print ("Max")
else:
  print("Error")

Boolean Operators in Conditions

We can also use the following Boolean operators in Python:

• and: True only if both operands are True example: (x > 0) and (x < 1); x must be in the range (0,1)
• or: True if either operand is True example: (x < 0) or (x > 0); x could be any number except 0
• not: True if the operand is False example: not (x == 0); x could be any number except 0

num = 0.5

if num > 0 and num < 1:
  print ("in between")
elif num < 0 or num > 1:
  print ("Outside")
else:
  print("In a limit")

Multiple Complex Clauses

Each single section of a condition that can be evaluated to a True or a False is called a Clause.

Using the Boolean operators, we can combine complex clauses into very complex conditions.

There is a tradeof between nesting IF statements and using more complex conditions.
In general, clarity is preferable to compactness, but simple multi-clause conditions are acceptable.

Clauses with other variable types

How do we compare text?

The in Operator

In Python, the in operator in checks if a specified value is part of a sequence like a string. It also works with thing's we'll see soon, like arrays, lists, or tuples.

An example is the following:

hangman_word = "abracadabra"
guess = input("give me a letter: ")
if (guess in hangman_word):
  print("you guessed right!")
else:
  print("You guessed wrong!") 

Single-Round Battleship

Activity 5 [5 minutes]:

Homework