CMU 15-112: Fundamentals of Programming and Computer Science
Class Notes: Conditionals

1. if statement
2. if-else statement
3. if-else expression
4. if-elif-else statement
5. Incorrect usage

1. if statement   video

   def f(x):
       print("A", end="")
       if (x == 0):
           print("B", end="")
           print("C", end="")
       print("D")
   
   f(0)
   f(1)

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   A more interesting example:

   # These examples define abs(n), which is a nice example here, but it is
   # also a builtin function, so you do not need to define it to use it.
   
   def abs1(n):
       if (n < 0):
           n = -n
       return n
   
   # again, with same-line indenting
   
   def abs2(n):
       if (n < 0): n = -n # only indent this way for very short lines (if at all)
       return n
   
   # again, with multiple return statements
   
   def abs3(n):
       if (n < 0):
           return -n
       return n
   
   # aside: you can do this with boolean arithmetic, but don't!
   
   def abs4(n):
       return (n < 0)*(-n) + (n>=0)*(n) # this is awful!
   
   # now show that they all work properly:
   
   print("abs1(5) =", abs1(5), "and abs1(-5) =", abs1(-5))
   print("abs2(5) =", abs2(5), "and abs2(-5) =", abs2(-5))
   print("abs3(5) =", abs3(5), "and abs3(-5) =", abs3(-5))
   print("abs4(5) =", abs4(5), "and abs4(-5) =", abs4(-5))

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2. if-else statement   video

   def f(x):
       print("A", end="")
       if (x == 0):
           print("B", end="")
           print("C", end="")
       else:
           print("D", end="")
           if (x == 1):
               print("E", end="")
           else:
               print("F", end="")
       print("G")
   
   f(0)
   f(1)
   f(2)

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   Revisiting abs(n):

   def abs5(n):
       if (n >= 0):
           return n
       else:
           return -n
   
   # or, if you prefer...
   
   def abs6(n):
       if (n >= 0):
           sign = +1
       else:
           sign = -1
       return sign * n
   
   print("abs5(5) =", abs5(5), "and abs5(-5) =", abs5(-5))
   print("abs6(5) =", abs6(5), "and abs6(-5) =", abs6(-5))

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3. if-else expression   video

   # if-else expression (not an if-else statement!)
   
   def abs7(n):
       return n if (n >= 0) else -n
   
   print("abs7(5) =", abs7(5), "and abs7(-5) =", abs7(-5))

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4. if-elif-else statement   video

   def f(x):
       print("A", end="")
       if (x == 0):
           print("B", end="")
           print("C", end="")
       elif (x == 1):
           print("D", end="")
       else:
           print("E", end="")
           if (x == 2):
               print("F", end="")
           else:
               print("G", end="")
       print("H")
   
   f(0)
   f(1)
   f(2)
   f(3)

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   A more interesting example:

   def numberOfRoots(a, b, c):
       # Returns number of roots (zeros) of y = a*x**2 + b*x + c
       d = b**2 - 4*a*c
       if (d > 0):
           return 2
       elif (d == 0):
           return 1
       else:
           return 0
   
   print("y = 4*x**2 + 5*x + 1 has", numberOfRoots(4,5,1), "root(s).")
   print("y = 4*x**2 + 4*x + 1 has", numberOfRoots(4,4,1), "root(s).")
   print("y = 4*x**2 + 3*x + 1 has", numberOfRoots(4,3,1), "root(s).")

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   Another example:

   def getGrade(score):
       if (score >= 90):
           grade = "A"
       elif (score >= 80):
           grade = "B"
       elif (score >= 70):
           grade = "C"
       elif (score >= 60):
           grade = "D"
       else:
           grade = "F"
       return grade
   
   print("103 -->", getGrade(103))
   print(" 88 -->", getGrade(88))
   print(" 70 -->", getGrade(70))
   print(" 61 -->", getGrade(61))
   print(" 22 -->", getGrade(22))

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5. Incorrect usage   video
   See here.