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

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: