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The “if”statement in Python

The “if”statement in Python
Nick Szirbik
Sept. 2020
Week 2 of the DAPOM course (4)

The general Python if-else syntax
if :

[else:
]

The statement blocks can contain one or more statements, or pass

The else “branch” is optional

The condition is a Boolean constant (True, False) , a Boolean variable, or a Boolean expression

Boolean expressions
2 < 5 # obviously True 3 > 7 # patently False
x == 11 # True only if x got the value 11 before this check
x > 10 # depends on the value of variable x
2 * x < x # cannot be True, no? # hereby, some of operators for Boolean expressions Meaning Math Symbol Python Symbols Less than < < Greater than > >
Less than or equal ≤ <= Greater than or equal ≥ >=
Equals = ==
Not equal ≠ !=

More Boolean operators: Logical and Membership operators
Logical operators are used to combine conditional statements:

Membership operators are used to test if an item or sequence of items is presented in an collection (list, set, dictionary, tuple):

Nested ifs and the use of elif (to make it nicer looking)
# nasty looking code, without elif

def letterGrade(score):
if score >= 90:
letter = ‘A’
else: # grade must be B, C, D or F
if score >= 80:
letter = ‘B’
else: # grade must be C, D or F
if score >= 70:
letter = ‘C’
else: # grade must D or F
if score >= 60:
letter = ‘D’
else:
letter = ‘F’
return letter
# nicer looking code with elif

def letterGrade(score):
if score >= 90:
letter = ‘A’
elif score >= 80:
letter = ‘B’
elif score >= 70:
letter = ‘C’
elif score >= 60:
letter = ‘D’
else:
letter = ‘F’
return letter

Finally: Importing, or getting access to existing code in Python
We can import (basically is an automatic copy/paste from another chunk of code) various other programs that we have access to

These are called in our jargon as: modules, packages, or libraries (in the old jargon: “subroutines”)

In the next practical, you will need some imports, for example:

import csv # importing a whole library, or
from csv import reader # importing only a single function

If you have questions
Please prepare these on paper, and ask them during the tutorial

You can ask in the break-out room of your team

Or you can ask during the plenary openings

If urgent, send an email to me (n.b.szirbik@rug.nl), but keep it short. If I have time, I try to respond immediately…

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