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

• Part 1 (Tue Lecture)
  1. if statement
  2. if-else statement
  3. if-elif-else statement
• Part 2 (Thu Lecture)
  4. if-else expression
  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-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))

4. 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))
   
   
5. Incorrect usage
   Note: this section concerns style and not correctness. All the examples here 'work', but some work more artfully than others. With that...
   • Negated Condition (with "else" clause)

     # Wrong: b = True if (not b): print('no') else: print('yes')

     # Right: b = True if (b): print('yes') else: print('no')

   • Empty "if" Clause

     # Wrong: b = False if (b): pass else: print('no')

     # Right: b = False if (not b): print('no')

   • Using "if" instead of "and"

     # Wrong (though not so wrong...): b1 = True b2 = True if (b1): if (b2): print('both!')

     # Right (well, maybe just "preferred"): b1 = True b2 = True if (b1 and b2): print('both!')

   • Using another "if" instead of "else"

     # Wrong: b = True if (b): print('yes') if (not b): print('no')

     # Right: b = True if (b): print('yes') else: print('no')

     Another example:

     # Wrong: x = 10 if (x < 5): print('small') if ((x >= 5) and (x < 10)): print('medium') if ((x >= 10) and (x < 15)): print('large') if (x >= 15): print('extra large')

     # Right: x = 10 if (x < 5): print('small') elif (x < 10): print('medium') elif (x < 15): print('large') else: print('extra large')

     Yet Another Example

     # Wrong: c = 'a' if ((c >= 'A') and (c <= 'Z')): print('Uppercase!') if ((c >= 'a') and (c <= 'z')): print('lowercase!') if ((c < 'A') or ((c > 'Z') and (c < 'a')) or (c > 'z')): print ('not a letter!')

     # Right: c = 'a' if ((c >= 'A') and (c <= 'Z')): print('Uppercase!') elif ((c >= 'a') and (c <= 'z')): print('lowercase!') else: print('not a letter!')

   • Using "arithmetic logic" instead of "boolean logic"

     # Wrong: x = 42 y = ((x > 0) and 99)

     # Right: x = 42 if (x > 0): y = 99