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

    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))

  2. if-else statement  
    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)

    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))

  3. if-else expression  
    # 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))

  4. if-elif-else statement  
    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)

    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).")

    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))

  5. Incorrect usage  
    See here.