Strings and Variables¶

Strings¶

As we have seen in the introduction a string is a string of letters or numbers – enclosed in quotes – that is not interpreted mathematically.

Operations such as + and * work differently on strings to on numerical data types.

print("Hello" + "World!")

Here, we did not enter a space when adding (joining) the two strings.

There are various ways we can include a space, the last one including a space character " ":

print("Hello " + "World!")
print("Hello" + " World!")
print("Hello" + " " + "World!")

The following two examples are equivalent:

print("Hello" + "Hello " + "Hello")
print("Hello " * 3)

However, strings cannot be multiplied together so this will not work:

print("Hello" * "World!")

Other things that do not work include: string + int and string * float (try them to see what happens).

Strings can be useful for displaying numerical in a more clear format. If you want to use a numerical value in a text string it first needs converting to string format using the built-in conversion function str:

print("The answer is " + str(6*7))

The conversion function takes a numerical argument (in brackets) and converts it into a string of characters. In the previous case the number 42 was converted into the plain text (ascii) characters 4 and 2.

String Methods and Functions¶

Strings have certain inbuilt functions called “methods” that can be used to change them.
Study the ones below to see what they do.

#Here we set a variable to store the string then use it instead of typing the whole thing
quote = "the QUICK BROWN Fox jumps over the LAZY Dog"

print( quote )
print( quote.upper() )
print( quote.lower() )
print( quote.title() )
print( quote.capitalize() )

Some functions take arguments, like the STR.replace(FIND, REPLACE) function, which replaces every instance of the string variable stored in FIND with that in REPLACE.

print( quote.replace("the", "every") )

Also functions can be chained in sequence:

print( quote.replace("BROWN", "RED").title().replace("Lazy", "Sleeping") )

Variables¶

One of the things that makes a programming language like Python more powerful than a calculator is the fact that we can give a name to an intermediate result.

The assignment statement: =¶

We give a name to a value using an equals sign (=) called the “assignment statement”.

The name appears on the left hand side of the equals sign and the value (or calculation) appears on the right hand side.

After a variable is assigned a value, the value will replace the name anywhere it is used in the script below it.

distance = 45.6  # assignment statement
time = 0.5  # another assignment statement
print(distance / time)  # the two names will be replace by their values

Names are called variables.

Variables can be useful to make a complicated expression more clear.

The calculation:

((4*9.81*0.05)/(3*0.47) * (2650.0 - 1000.0)/1000.0)**0.5

can be made a lot clearer by giving names to some of the parameters and intermediate results and by adding comments.

# Terminal velocity of spherical object

g = 9.81  # acceleration due to gravity (m/s^2^)
d = 0.05  # object diameter (m)
C_d = 0.47  # drag coefficient (1)
rho = 1000.0  # fluid density (kg/m^3^)
rho_s = 2650.0  # object density (kg/m^3^)
geometry_factor = (4*g*d)/(3*C_d)
buoyancy_factor = (rho_s - rho)/rho
V_t = (geometry_factor * buoyancy_factor)**0.5

print(V_t)

This makes it also much easier to make changes or corrections.

Exercise:¶

Let $a = 5$, $b = -1$ and $c=10$.

# YOUR CODE HERE

Evaluate $w$ where:¶

$w = 3 a - 5 b + 2 c$

#set w equal to the mathematical expression

# YOUR CODE HERE

print(w)

Expected Result: 40

Click here for Solution

Evaluate $w$ where:¶

$\displaystyle w = \frac{5 a - 2 b}{4 c + 2 a}$

Result: 0.54

# YOUR CODE HERE

print(w)

Expected Result: 0.54

Click here for Solution

Evaluate $w$ where:¶

$\displaystyle w = \sqrt{2 a - b}$

# YOUR CODE HERE

print(w)

Expected Result: 3.3166247903554

Click here for Solution

The word variable refers to the fact that we can change what value a name refers to.

The same name can be reused for different values.

a = 0.618
print(a)
a = 1.618
print(a)

IMPORTANT: Assignment is not equals!¶

Note that the use of the equals sign can be confusing at first.
The equals sign denotes assignment and not an equation in the mathematical sense.
A = B takes the rigt hand side (B) and assigns it to the variable (A) on the left.

Notes:¶

The following example will generate an error message since this not legal Python code.

2*3 = a  

If one variable is assigned to another, it takes its current value but they do not remain equal forever:

a = 1  # set a to 1
b = a  # b takes the current value of a
a = 2  # change the value of a
print(a, b)  # note that b does not change

The right hand side (RHS) is evaluated before assignment to the varible on the left (LHS), so the following examples are possible in Python:

a = 1
print(a)

# take the value of a and add 1 to it, assign this back to `a`
a = a + 1
print(a)

# multiply the current value of `a` by 2 and overwrite `a`
a = 2*a
print(a)

# replace the value of the variable `a` with its old value squared
a = a**2
print(a)

Note: in each case the old value of a is taken and used for the calculation, before replacing it using the assignment statement (=).

Example: Swapping two values¶

Suppose we have assigned the names varA and varB to two different values.
To swap the values, we have to use a temporary name to refer to one of the values.

varA = 7
varB = -99

print(varA, varB)

tmp = varA
varA = varB
varB = tmp

print(varA, varB)

Allowed Variable Names¶

A valid variable name consists of letters (A,a,B,b,…), digits (1,2,3,…) and underscores ( _ )
but ** cannot start with a digit**.
Spaces are not allowed in variable names.
You should not use an underscore at the start of a variable name, since this has a special meaning in Python.
Python is case-sensitive, which means that upper and lower case letters are distinct.

a = 5
b2 = a + a
quite_a_long_variable_name = 0.1
A = 50.0
outerRadius = 5.7
print(a, b2, quite_a_long_variable_name, A, outerRadius)

Example: Quadratic Formula¶

The following program finds the solutions of the quadratic equation $a x^2 + b x + c$ using the quadratic formula: $$x = \frac{-b\pm\sqrt{b^2-4ac}}{2a},$$ where $b^2-4ac$ is known as the discriminant.
We assume in this example that there are two solutions.

# coefficients of the quadratic a*x**2 + b*x + c

a = 1.0
b = -5.0
c = 6.0  # quadratic formula for the equation a*x**2 + b*x + c = 0

disc = b**2 - 4*a*c  # assumed to be positive
print(disc)

x1 = (-b + disc**0.5)/(2*a)
x2 = (-b - disc**0.5)/(2*a)

print(x1, x2)

Exercise run the quadratic formula program with different coefficients¶

Use an equations for which you know what the results should be:
- e.g.: $x^2 - 4 = 0$.

# YOUR CODE HERE

print(x1, x2)

Possible Solution

Find out what happens when the discriminant is negative.

# YOUR CODE HERE

print(x1, x2)

Possible Solution

Using Built-in Constants¶

Important constants like $\pi$ and $e$ can be accessed by loading the math library using import.
The imaginary number $i=\sqrt{-1}$ (called $j$ by electrical engineers) is accessed using 1j.

from math import pi, e

print(pi, e, e**(1j*pi) + 1)

Note that the last result is $1.2\times10^{-16}i = 0.00000000000000012i\approx 0$, due to floating-point precision (see appendix of Section 1).

Strings as Variables¶

Strings can also be assigned to variable names, and it important to not confuse the two. For example:

word1 = "Hello World!"
print("word1")
print(word1)

Notice that in the first instance the string word1 that was enclosed in quotes was print ed and in the second the string value hello assigned to the variable named word1 was printed.

Formatting Numbers in Strings¶

Integer or floating point numbers can also also be combined with strings in other ways. They can be directly interspersed with text strings by separating them with commas without conversion:

print("The answer is", 40+2, "but not", "40"+"2")

Notice that spaces were automatically added in this case.
There are other ways of formatting objects including numbers in text output (we will see more later).
One is the including special syntax such as %d for integers and %f for floating point numbers (there are others).
The value to replace it with follows the string, preceded by a percent symbol.

import math
pi = math.pi
print("Pi is %d as an integer" % pi)
print("Pi is %f, approximately" % pi)
print("Pi is %.3f to 3 decimal places" % pi)

In the third case %.3f is used to display the value to three decimal places. This can be very useful for displaying the results of calculations neatly. We will see another method for including numbers in strings later in the unit.

Another method is the format() function (study how each of the arguments works and make notes on your observations):

mysentence = "This is how to put {} and {} in the sentence {}.".format("this", "that", "here")

print(mysentence)

mysentence2 = "This is how to place {2} then {0} and finish with {1}.".format("zero", "one", "two")

print(mysentence2)

this = "An integer: {:.0f}, two dp: {:.2f}, full precision: {}, ".format(pi, pi, pi)
print(this)

final = "Base of natural logs: {1:.3f}, pi = {0:.5f}".format(pi, e)

print(final)

Formatted “f-strings”¶

From Python 3.6 onwards a new method: “f-strings” were introduced to Python:

from math import pi, e
a=42

#the first argument is a plain variable name, .3f gives it to 3 decimal (floating point) places, and the third to 3 digits
formatted_string = f"The answer is {a}, the number pi is {pi:.3f} and the base of natural logarithms is {e:.3}."

print(formatted_string)

This makes formatting numbers in text much easier than before. Strings can also be formatted into strings, as the following multi-line string (using three quote marks on either side) shows:

Multiline Strings¶

Finally, strings can also be formatted as multi-line strings using three quote marks on either side:

a=42
palindrome = "do geese see god"

multiline = f"""Hello I am a multiline string,
as you can see my lines can be broken!

I can also include the contants of variables like {a}.

Not only that, other strings!:

This: {palindrome}

That: {palindrome.upper()}

Something else: {(palindrome+" ")*3}

"""

print(multiline)

Summary¶

# a comment is ignored

import math  # importing a module

-999  # an integer
0.0999  # a floating point number
1.987e-6  # scientific/engineering notation
print(88)  # printing a value
print(1, 4, 9)  # printing multiple values
print(10**(2*3) - 1)  # integer arithmetic
print(7/2)   # integer arithmetic does what you would expect in real life
print(7.0/2.0)  # floating point arithmetic
print(math.pi*100)  # built-in constant
num1 = 44  # assignment statement
num2 = 2*33
print(num1 + num2)

Operations: +, -, *, /, **, %
Functions: type, int, float, round
Constants: math.pi, math.e

Pitfalls¶

The operation * denotes multiplication and cannot be omitted. It can be helpful to read out a formula.
E.g., $5 (4 + 2)$ reads as five times four plus two.
In other programming languages or in in previous Python versions (e.g. Python 2), the symbol / denotes integer division. In them, we have 7/2 => 3, but 7.0/2 => 3.5 and 7/2.0 => 3.5. But this is not the case in Python 3.
Variable names are case-sensitive.
Do not confuse assignment statements and mathematical equations.

AR10366 (Computer Applications): Programming in Python

Strings and Variables¶

Strings¶

String Methods and Functions¶

Variables¶

The assignment statement: =¶

Exercise:¶

Evaluate \(w\) where:¶

Evaluate \(w\) where:¶

Evaluate \(w\) where:¶

IMPORTANT: Assignment is not equals!¶

Notes:¶

Example: Swapping two values¶

Allowed Variable Names¶

Example: Quadratic Formula¶

Exercise run the quadratic formula program with different coefficients¶

Using Built-in Constants¶

Strings as Variables¶

Formatting Numbers in Strings¶

Formatted “f-strings”¶

Multiline Strings¶

Summary¶

Pitfalls¶

Task 2 (2%)¶

Some partially filled out code is below to get you started.¶

Use the checking cell below after running your code above to see if it will pass the autograder…¶

Extra:¶

Extra Practice Example:¶

Projectile Motion¶