Variables and Assignment

Overview

Teaching: 10 min
Exercises: 10 min

Questions

• How can I store data in programs?

Objectives

• Write programs that assign scalar values to variables and perform calculations with those values.

• Correctly trace value changes in programs that use scalar assignment.

Use variables to store values.

• Variables are names for values.
• In Python the = symbol assigns the value on the right to the name on the left.
• The variable is created when a value is assigned to it.
• Here, Python assigns a volume size to a variable roi_vol and a name in quotation marks to a variable roi_label.

roi_vol = 130
roi_label = 'hippocampus'

• Variable names:
  • cannot start with a digit
  • cannot contain spaces, quotation marks, or other punctuation
  • may contain an underscore (typically used to separate words in long variable names)
• Underscores at the start like __roi_vol have a special meaning so we won’t do that until we understand the convention.

Variables can be displayed by simply executing them as an expression

roi_vol

130

roi_label

'hippocampus'

Variables must be created before they are used.

• If a variable doesn’t exist yet, or if the name has been mis-spelled, Python reports an error.
  • Unlike some languages, which “guess” a default value.

brain_volume

----------------------------------------------------------
NameError                Traceback (most recent call last)
<ipython-input-12-dec7c88e2504> in <module>()
----> 1 brain_volume

NameError: name 'brain_volume' is not defined

• The last line of an error message is usually the most informative.

Use comments to add documentation to programs.

# This sentence isn't executed by Python.
# print(brain_volume)
brain_volume = 1351   # Neither is this - anything after '#' is ignored.

Variables can be used in calculations.

• We can use variables in calculations just as if they were values.
  • Remember, we assigned 130 to roi_vol a few lines ago.

corrected_roi_vol = roi_vol + 3
# Volume with a correction for a systematic bias: 
corrected_roi_vol

133

Use an index to get a single character from a string.

• Sometimes called a “subscript”.
• Each character in a string is in a particular location.
• Use the location’s index in square brackets to get the character.
• Locations are numbered from 0 rather than 1.

roi_label = 'hippocampus'
roi_label[0]

h

Use a slice to get a substring.

• A slice extracts elements, based on a start and stop value
• A slice consists of [start:stop].
• From the start value (inclusive) up to but not including the stop value (exclusive).
• So the difference between stop and start is the slice’s length.

roi_label[0:5]

hippo

A slice with a different step

• We don’t have to take every element in the range we specify. We can specify the step by which we take elements from the string.

roi_label[0:5:2]

hpo

Use the built-in function len to find the length of a string.

len(roi_label)

11

• Nested functions are evaluated from the inside out, just like in mathematics.

Python is case-sensitive.

• Python thinks that upper- and lower-case letters are different, so Name and name are different variables.
• There are conventions for using upper-case letters at the start of variable names so we will use lower-case letters for now.

Use print to display values.

• Python has a built-in function called print that prints things as text.
• Call the function (i.e., tell Python to run it) by using its name.
• Provide values to the function (i.e., the things to print) in parentheses.
• The values passed to the function are called ‘arguments’

print(roi_label)

hippocampus

print('The', roi_label, 'is', roi_vol, 'cubic mm')

The hippocampus is 130 cubic mm

• print automatically puts a single space between items to separate them.
• And wraps around to a new line at the end.

Use meaningful variable names.

• Python doesn’t care what you call variables as long as they obey the rules (alphanumeric characters and the underscore).

flabadab = 42
ewr_422_yY = 'Ahmed'
print(ewr_422_yY, 'is', flabadab, 'years old')
flabadab = 700
ewr_422_yY = 'frontal cortex'
print('The',ewr_422_yY, 'is', flabadab, 'cubic mm')

Ahmed is 42 years old
The frontal cortex is 700 cubic mm

• Use meaningful variable names to help other people understand what the program does.
• The most important “other person” is your future self.

Variables exist in the IPython environment

We can list the variables we have defined, along with some details about them, by executing the IPython magic %whos:

%whos

• These variables do not exist on the hard disk of the computer. They will not continue to exist if we stop our current IPython kernel (by executing exit). Ideally, in any time in our analysis we should be able to remove these variables and regenerate them again by running a script file containing all of the commands we used to get to our current state.

• We will learn the tools that IPython provides to do this.

• For now we will simply learn how to delete all of the variables in the current environment. This is done using the %reset IPython magic and typing y and enter to confirm:

%reset

Once deleted, variables cannot be recovered. Proceed (y/[n])? 

y

Swapping Values

Draw a table showing the values of the variables in this program after each statement is executed. In simple terms, what do the last three lines of this program do?

lowest = 1.0
highest = 3.0
volume = lowest
lowest = highest
highest = volume

Predicting Values

What is the final value of important_label in the program below? (Try to predict the value without running the program, then check your prediction.)

label = "hippocampus"
important_label = label
label = "visual"

Challenge

If you assign roi_vol = 130, what happens if you try to get the second digit of roi_vol?

Solution

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

roi_vol = 130
print(roi_vol[1])

TypeError: 'int' object is not subscriptable

Choosing a Name

Which is a better variable name, m, min, or minutes? Why? Hint: think about which code you would rather inherit from someone who is leaving the lab:

1. ts = m * 60 + s
2. tot_sec = min * 60 + sec
3. total_seconds = minutes * 60 + seconds

Solution

minutes is better because min might mean something like “minimum” (and actually does in Python, but we haven’t seen that yet).

Slicing

What does the following program print?

sequence_type = 'MP_RAGE'
print('sequence_type[1:3] is:', sequence_type[1:3])

Solution

sequence_type[1:3] is: P_

Key Points

• Use variables to store values.

• Use print to display values.

• Variables must be created before they are used.

• Variables can be used in calculations.

• Use an index to get a single character from a string.

• Use a slice to get a substring.

• Use the built-in function len to find the length of a string.

• Python is case-sensitive.

• Use meaningful variable names.

• Variables in the current environment can be displayed or deleted using the %whos and %reset IPython magics.