Logic
Boolean Values
The boolean type has only two values: True
and False
. Let's assign a boolean value to a variable and verify the type using the builtin function type()
:
python_is_fun = True
print(python_is_fun)
True
type(python_is_fun)
bool
Let's assign the value False
to a variable and again verify the type:
math_is_scary = False
print(math_is_scary)
False
type(math_is_scary)
bool
Comparison Operators
Comparison operators produce Boolean values as output. For example, if we have variables x
and y
with numeric values, we can evaluate the expression x < y
and the result is a boolean value either True
or False
.
Comparison Operator  Description 

< 
strictly less than 
<= 
less than or equal 
> 
strictly greater than 
>= 
greater than or equal 
== 
equal 
!= 
not equal 
For example:
1 == 2
False
1 < 2
True
2 == 2
True
3 != 3.14159
True
20.00000001 >= 20
True
Boolean Operators
We combine logical expressions using boolean operators and
, or
and not
.
Boolean Operator  Description 

A and B 
returns True if both A and B are True 
A or B 
returns True if either A or B is True 
not A 
returns True if A is False 
For example:
(1 < 2) and (3 != 5)
True
(1 < 2) and (3 < 1)
False
(1 < 2) or (3 < 1)
True
not (1000 <= 999)
True
if statements
An if statement consists of one or more blocks of code such that only one block is executed depending on logical expressions. Let's do an example:
# Determine if roots of polynomial ax^2 + bx + c = 0
# are real, repeated or complex using the
# quadratic formula x = (b \pm \sqrt{b^2  4ac})/2a
a = 10
b = 234
c = 1984
discriminant = b**2  4*a*c
if discriminant > 0:
print("Discriminant =", discriminant)
print("Roots are real and distinct.")
elif discriminant < 0:
print("Discriminant =", discriminant)
print("Roots are complex.")
else:
print("Discriminant =", discriminant)
print("Roots are real and repeated.")
Discriminant = 24604
Roots are complex.
The main points to observe:
 Start with the
if
keyword.  Write a logical expression (returning
True
orFalse
).  End line with a colon
:
.  Indent block 4 spaces after
if
statement.  Include
elif
andelse
statements if needed.  Only one of the blocks
if
,elif
andelse
is executed.  The block following an
else
statement will execute only if all other logical expressions before it areFalse
.
Examples
Invertible Matrix
Represent a 2 by 2 square matrix as a list of lists. For example, represent the matrix
$$ \begin{bmatrix} 2 & 1 \\ 5 & 7 \end{bmatrix} $$
as the list of lists [[2,1],[5,7]]
.
Write a function called invertible
which takes an input parameter M
, a list of lists representing a 2 by 2 matrix, and returns True
if the matrix M
is invertible and False
if not.
def invertible(M):
'''Determine if M is invertible.
Parameters

M : list of lists
Representation of a 2 by 2 matrix M = [[a,b],[c,d]].
Returns

bool
True if M is invertible and False if not.
Examples

>>> invertible([[1,2],[3,4]])
True
'''
# A matrix M is invertible if and only if
# the determinant is not zero where
# det(M) = ad  bc for M = [[a,b],[c,d]]
determinant = M[0][0] * M[1][1]  M[0][1] * M[1][0]
if determinant != 0:
return True
else:
return False
Let's test our function:
invertible([[1,2],[3,4]])
True
invertible([[1,1],[3,3]])
False
Concavity of a Polynomial
Write a function called concave_up
which takes input parameters p
and a
where p
is a list representing a polynomial $p(x)$ and a
is a number, and returns True
if the function $p(x)$ is concave up at $x=a$ (ie. its second derivative is positive at $x=a$, $p''(a) > 0$).
We'll use the second derivative test for polynomials. In particular, if we have a polynomial of degree $n$
$$ p(x) = c_0 + c_1 x + c_2 x^2 + \cdots + c_n x^n $$
then the second derivative of $p(x)$ at $x=a$ is the sum
$$ p''(a) = 2(1) c_2 + 3(2)c_3 a + 4(3)c_4 a^2 + \cdots + n(n1)c_n a^{n2} $$
def concave_up(p,a):
'''Determine if the polynomial p(x) is concave up at x=a.
Parameters

p : list of numbers
List [a_0,a_1,a_2,...,a_n] represents the polynomial
p(x) = a_0 + a_1*x + a_2*x**2 + ... + a_n*x**n
Returns

bool
True if p(x) is concave up at x=a (ie. p''(a) > 0) and False otherwise.
Examples

>>> concave_up([1,0,2],0)
False
>>> concave_up([1,0,2],0)
True
'''
# Degree of the polynomial p(x)
degree = len(p)  1
# p''(a) == 0 if degree of p(x) is less than 2
if degree < 2:
return False
else:
# Compute the second derivative p''(a)
DDp_a = sum([k*(k1)*p[k]*a**(k2) for k in range(2,degree + 1)])
if DDp_a > 0:
return True
else:
return False
Let's test our function on $p(x) = 1 + x  x^3$ at $x=2$. Since $p''(x) = 6x$ and $p''(2) = 12 < 0$, the polynomial is concave down at $x=2$.
p = [1,1,0,1]
a = 2
concavity = concave_up(p,a)
print(concavity)
False
Exercises

The discriminant of a cubic polynomial $p(x) = ax^3 + bx^2 + cx + d$ is
$$ \Delta = b^2c^2  4ac^3  4b^3d  27a^2d^2 + 18abcd $$
The discriminant gives us information about the roots of the polynomial $p(x)$:
 if $\Delta > 0$, then $p(x)$ has 3 distinct real roots
 if $\Delta < 0$, then $p(x)$ has 2 distinct complex roots and 1 real root
 if $\Delta = 0$, then $p(x)$ has at least 2 (real or complex) roots which are the same
Represent a cubic polynomial $p(x) = ax^3 + bx^2 + cx + d$ as a list
[d,c,b,a]
of numbers. (Note the order of the coefficients is increasing degree.) For example, the polynomial $p(x) = x^3  x + 1$ is[1,1,0,1]
.Write a function called
cubic_roots
which takes an input parameterp
, a list of length 4 representing a cubic polynomial, and returnsTrue
if $p(x)$ has 3 real distinct roots andFalse
otherwise. 
Represent a 2 by 2 square matrix as a list of lists. For example, represent the matrix
$$ \begin{bmatrix} 2 & 1 \\ 5 & 7 \end{bmatrix} $$
as the list of lists
[[2,1],[5,7]]
. Write a function calledinverse_a
which takes an input parametera
and returns a list of lists representing the inverse of the matrix$$ \begin{bmatrix} 1 & a \\ a & 1 \end{bmatrix} $$

Write a function called
real_eigenvalues
which takes an input parameterM
, a list of lists representing a 2 by 2 matrix (as in the previous exercise), and returnsTrue
if the eigenvalues of the matrixM
are real numebrs andFalse
if not.