Slicing

If you assign a = 123, what happens if you try to get the second digit of a via a[1]?

Show answer

Numbers are not stored in the written representation, so they can’t be treated like strings.

a = 123
print(a[1])

TypeError: 'int' object is not subscriptable

More slicing

What does the following program print?

atom_name = 'carbon'
print('atom_name[1:3] is:', atom_name[1:3])
Show answer 
atom_name[1:3] is: ar

Even more slicing

Open a Notebook to help answer the following questions:

  1. What does thing[low:] (without a value after the colon) do?
  2. What does thing[:high] (without a value before the colon) do?
  3. What does thing[:] (just a colon) do?
  4. What does thing[-2] do?
Show answer
  1. Slices from low to the end (inclusive)
  2. Slices from the start to high (exclusive)
  3. Slices from the start to the end
  4. Selects the second last element

Decimals

What type of value is 3.4? How can you find out?

Show answer

It is a floating-point number (often abbreviated “float”).

print(type(3.4))

<class 'float'>

Choose a Type

What type of value (integer, floating point number, or character string) would you use to represent each of the following?

  1. Number of days since the start of the year.
  2. Time elapsed from the start of the year until now in days.
  3. Serial number of a piece of lab equipment.
  4. A lab specimen’s age
  5. Current population of a city.
  6. Average population of a city over time.
Show answer

The answers to the questions are:

  1. Integer, since the number of days would lie between 1 and 365.
  2. Floating point, since fractional days are required
  3. Character string if serial number contains letters and numbers, otherwise integer if the serial number consists only of numerals
  4. This will vary! How do you define a specimen’s age? whole days since collection (integer)? date and time (string)?
  5. Choose floating point to represent population as large aggreates (eg millions), or integer to represent population in units of individuals.
  6. Floating point number, since an average is likely to have a fractional part.

Division Types

In Python 3, the // operator performs integer (whole-number) floor division, the / operator performs floating-point division, and the ‘%’ (or modulo) operator calculates and returns the remainder from integer division:

print('5 // 3:', 5//3)
print('5 / 3:', 5/3)
print('5 % 3:', 5%3)

5 // 3: 1
5 / 3: 1.6666666666666667
5 % 3: 2

However in Python 2 (and other languages), the / operator between two integer types perform a floor (//) division. To perform a float division, we have to convert one of the integers to float.

print('5 // 3:', 1)
print('5 / 3:', 1 )
print('5 / float(3):', 1.6666667 )
print('float(5) / 3:', 1.6666667 )
print('float(5 / 3):', 1.0 )
print('5 % 3:', 2)

If num_subjects is the number of subjects taking part in a study, and num_per_survey is the number that can take part in a single survey, write an expression that calculates the number of surveys needed to reach everyone once.

Show answer

We want the minimum number of surveys that reaches everyone once, which is the rounded up value of num_subjects / num_per_survey. This is equivalent to performing an integer division with // and adding 1.

num_subjects = 600
num_per_survey = 42
num_surveys = num_subjects // num_per_survey + 1

print(num_subjects, 'subjects,', num_per_survey, 'per survey:', num_surveys)

600 subjects, 42 per survey: 15

Arithmetic with Different Types

Which of the following will print 2.0? Note: there may be more than one right answer.

first = 1.0
second = "1"
third = "1.1"
  1. first + float(second)
  2. float(second) + float(third)
  3. first + int(third)
  4. first + int(float(third))
  5. int(first) + int(float(third))
  6. 2.0 * second
Show answer

Answer: 1 and 4

Complex Numbers

Python provides complex numbers, which are written as 1.0+2.0j. If val is an imaginary number, its real and imaginary parts can be accessed using dot notation as val.real and val.imag.

  1. Why do you think Python uses j instead of i for the imaginary part?
  2. What do you expect 1+2j + 3 to produce?
  3. What do you expect ‘4j’ to be? What about 4 j or `4 + j’? >
Show answer
  1. Standard mathematics treatments typically use i to denote an imaginary number. However, from media reports it was an early convention established from electrical engineering that now presents a technically expensive area to change. Stack Overflow provides additional explanation and discussion
  2. 4+2j
  3. 4j, syntax error, depends on the value of j

See the notebook.

Back to Python basics - part one.