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.