Lesson 2: `if / else` and Functions

Conditionals I: The "if / else" statement
Built-in functions
Modules
Test your understanding: practice set 2

1. Conditionals I: The "`if / else`" statement

Programming is a lot like giving someone instructions or directions. For example, if I wanted to give you directions to my house, I might say...

Turn right onto Main Street

Turn left onto Maple Ave

If there is construction, continue straight on Maple Ave, turn right on Cat Lane, and left on Fake Street; else, cut through the empty lot to Fake Street

Go straight on Fake Street until house 123

The same directions, but in code:



In [ ]:

    
construction = False

print "Turn right onto Main Street"
print "Turn left onto Maple Ave"

if construction:
    print "Continue straight on Maple Ave" 
    print "Turn right onto Cat Lane"
    print "Turn left onto Fake Street"
else:
    print "Cut through the empty lot to Fake Street"
    print "Go straight on Fake Street until house 123"

This is called an "if / else" statement. It basically allows you to create a "fork" in the flow of your program based on a condition that you define. If the condition is True, the "if"-block of code is executed. If the condition is False, the else-block is executed.

Here, our condition is simply the value of the variable construction. Since we defined this variable to quite literally hold the value False (this is a special data type called a Boolean, more on that in a minute), this means that we skip over the if-block and only execute the else-block. If instead we had set construction to True, we would have executed only the if-block.

Let's define Booleans and if / else statements more formally now.

[ Definition ] Booleans

A Boolean ("bool") is a type of variable, like a string, int, or float.
However, a Boolean is much more restricted than these other data types because it is only allowed to take two values: True or False.
In Python, True and False are always capitalized and never in quotes.
Don't think of True and False as words! You can't treat them like you would strings. To Python, they're actually interpreted as the numbers 1 and 0, respectively.
Booleans are most often used to create the "conditional statements" used in if / else statements and loops.

[ Definition ] The `if / else` statement

Purpose: creates a fork in the flow of the program based on whether a conditional statement is True or False.

Syntax:

if (conditional statement):
    this code is executed
else:
    this code is executed

Notes:

Based on the Boolean (True / False) value of a conditional statement, either executes the if-block or the else-block
The "blocks" are indicated by indentation.
The else-block is optional.
Colons are required after the if condition and after the else.
All code that is part of the if or else blocks must be indented.

Example:



In [ ]:

    
x = 5
if (x > 0):
    print "x is positive"
else:
    print "x is negative"

So what types of conditionals are we allowed to use in an if / else statement? Anything that can be evaluated as True or False! For example, in natural language we might ask the following true/false questions:

is a True?

is a less than b?

is a equal to b?

is a equal to "ATGCTG"?

is (a greater than b) and (b greater than c)?

To ask these questions in our code, we need to use a special set of symbols/words. These are called the logical operators, because they allow us to form logical (true/false) statements. Below is a chart that lists the most common logical operators:

Most of these are pretty intuitive. The big one people tend to mess up on in the beginning is ==. Just remember: a single equals sign means assignment, and a double equals means is the same as/is equal to. You will NEVER use a single equals sign in a conditional statement because assignment is not allowed in a conditional! Only True / False questions are allowed!

`if / else` statements in action

Below are several examples of code using if / else statements. For each code block, first try to guess what the output will be, and then run the block to see the answer.



In [ ]:

    
a = True
if a:
    print "Hooray, a was true!"



In [ ]:

    
a = True
if a:
    print "Hooray, a was true!"
print "Goodbye now!"



In [ ]:

    
a = False
if a:
    print "Hooray, a was true!"
print "Goodbye now!"

Since the line print "Goodbye now!" is not indented, it is NOT considered part of the if-statement. Therefore, it is always printed regardless of whether the if-statement was True or False.



In [ ]:

    
a = True
b = False
if a and b:
    print "Apple"
else:
    print "Banana"

Since a and b are not both True, the conditional statement "a and b" as a whole is False. Therefore, we execute the else-block.



In [ ]:

    
a = True
b = False
if a and not b:
    print "Apple"
else:
    print "Banana"

By using "not" before b, we negate its current value (False), making b True. Thus the entire conditional as a whole becomes True, and we execute the if-block.



In [ ]:

    
a = True
b = False
if not a and b:
    print "Apple"
else:
    print "Banana"

"not" only applies to the variable directly in front of it (in this case, a). So here, a becomes False, so the conditional as a whole becomes False.



In [ ]:

    
a = True
b = False
if not (a and b):
    print "Apple"
else:
    print "Banana"

When we use parentheses in a conditional, whatever is within the parentheses is evaluated first. So here, the evaluation proceeds like this:

First Python decides how to evaluate (a and b). As we saw above, this must be False because a and b are not both True.

Then Python applies the "not", which flips that False into a True. So then the final answer is True!



In [ ]:

    
a = True
b = False
if a or b:
    print "Apple"
else:
    print "Banana"

As you would probably expect, when we use "or", we only need a or b to be True in order for the whole conditional to be True.



In [ ]:

    
cat = "Mittens"
if cat == "Mittens":
    print "Awwww"
else:
    print "Get lost, cat"



In [ ]:

    
a = 5
b = 10
if (a == 5) and (b > 0):
    print "Apple"
else:
    print "Banana"



In [ ]:

    
a = 5
b = 10
if ((a == 1) and (b > 0)) or (b == (2 * a)):
    print "Apple"
else:
    print "Banana"

Ok, this one is a little bit much! Try to avoid complex conditionals like this if possible, since it can be difficult to tell if they're actually testing what you think they're testing. If you do need to use a complex conditional, use parentheses to make it more obvious which terms will be evaluated first!

Note on indentation

Indentation is very important in Python; it’s how Python tells what code belongs to which control statements
Consecutive lines of code with the same indenting are sometimes called "blocks"
Indenting should only be done in specific circumstances (if statements are one example, and we'll see a few more soon). Indent anywhere else and you'll get an error.
You can indent by however much you want, but you must be consistent. Pick one indentation scheme (e.g. 1 tab per indent level, or 4 spaces) and stick to it.

[ Check yourself! ] `if/else` practice

Think you got it? In the code block below, write an if/else statement to print a different message depending on whether x is positive or negative.



In [ ]:

    
x = 6 * -5 - 4 * 2 + -7 * -8 + 3

# ******add your code here!*********

2. Built-in functions

Python provides some useful built-in functions that perform specific tasks. What makes them "built-in"? Simply that you don’t have to "import" anything in order to use them -- they're always available. This is in contrast the the non-built-in functions, which are packaged into modules of similar functions (e.g. "math") that you must import before using. More on this in a minute!

We've already seen some examples of built-in functions, such as print, int(), float(), and str(). Now we'll look at a few more that are particularly useful: raw_input(), len(), abs(), and round().

[ Definition ] `raw_input()`

Description: A built-in function that allows user input to be read from the terminal.

Syntax:

raw_input("Optional prompt: ")

Notes:

The execution of the code will pause when it reaches the raw_input() function and wait for the user to input something.
The input ends when the user hits "enter".
The user input that is read by raw_input() can then be stored in a variable and used in the code.
Important: This function always returns a string, even if the user entered a number! You must convert the input with int() or float() if you expect a number input.

Examples:



In [ ]:

    
name = raw_input("Your name: ")
print "Hi there", name, "!"



In [ ]:

    
age = int(raw_input("Your age: ")) #convert input to an int
print "Wow, I can't believe you're only", age

[ Definition ] `len()`

Description: Returns the length of a string (also works on certain data structures). Doesn’t work on numerical types.

Syntax:

len(string)

Examples:



In [ ]:

    
print len("cat")



In [ ]:

    
print len("hi there")



In [ ]:

    
seqLength = len("ATGGTCGCAT")
print seqLength

[ Definition ] `abs()`

Description: Returns the absolute value of a numerical value. Doesn't accept strings.

Syntax:

abs(number)

Examples:



In [ ]:

    
print abs(-10)



In [ ]:

    
print abs(int("-10"))



In [ ]:

    
positiveNum = abs(-23423)
print positiveNum

[ Definition ] `round()`

Description: Rounds a float to the indicated number of decimal places. If no number of decimal places is indicated, rounds to zero decimal places.

Synatx:

round(someNumber, numDecimalPlaces)

Examples:



In [ ]:

    
print round(10.12345)



In [ ]:

    
print round(10.12345, 2)



In [ ]:

    
print round(10.9999, 2)

If you want to learn more built in functions, go here: https://docs.python.org/2/library/functions.html

3. Modules

Modules are groups of additional functions that come with Python, but unlike the built-in functions we just saw, these functions aren't accessible until you import them. Why aren’t all functions just built-in? Basically, it improves speed and memory usage to only import what is needed (there are some other considerations, too, but we won't get into it here).

The functions in a module are usually all related to a certain kind of task or subject area. For example, there are modules for doing advanced math, generating random numbers, running code in parallel, accessing your computer's file system, and so on. We’ll go over just two modules today: math and random. See the full list here: https://docs.python.org/2.7/py-modindex.html

How to use a module

Using a module is very simple. First you import the module. Add this to the top of your script:

import <moduleName>

Then, to use a function of the module, you prefix the function name with the name of the module (using a period between them):

<moduleName>.<functionName>

(Replace <moduleName> with the name of the module you want, and <functionName> with the name of a function in the module.)

The <moduleName>.<functionName> synatx is needed so that Python knows where the function comes from. Sometimes, especially when using user created modules, there can be a function with the same name as a function that's already part of Python. Using this syntax prevents functions from overwriting each other or causing ambiguity.

[ Definition ] The `math` module

Description: Contains many advanced math-related functions.

See full list of functions here: https://docs.python.org/2/library/math.html

Examples:



In [ ]:

    
import math

print math.sqrt(4)
print math.log10(1000)
print math.sin(1)
print math.cos(0)

[ Definition ] The `random` module

Description: contains functions for generating random numbers.

See full list of functions here: https://docs.python.org/2/library/random.html

Examples:



In [ ]:

    
import random

print random.random()       # Return a random floating point number in the range [0.0, 1.0)
print random.randint(0, 10) # Return a random integer between the specified range (inclusive)
print random.gauss(5, 2)    # Draw from the normal distribution given a mean and standard deviation

# this code will output something different every time you run it!

4. Test your understanding: practice set 2

For the following blocks of code, first try to guess what the output will be, and then run the code yourself. These examples may introduce some ideas and common pitfalls that were not explicitly covered in the text above, so be sure to complete this section.

The first block below holds the variables that will be used in the problems. Since variables are shared across blocks in Jupyter notebooks, you just need to run this block once and then those variables can be used in any other code block.



In [ ]:

    
# RUN THIS BLOCK FIRST TO SET UP VARIABLES!
a = True
b = False
x = 2
y = -2
cat = "Mittens"



In [ ]:

    
print a



In [ ]:

    
print (not a)



In [ ]:

    
print (a == b)



In [ ]:

    
print (a != b)



In [ ]:

    
print (x == y)



In [ ]:

    
print (x > y)



In [ ]:

    
print (x = 2)



In [ ]:

    
print (a and b)



In [ ]:

    
print (a and not b)



In [ ]:

    
print (a or b)



In [ ]:

    
print (not b or a)



In [ ]:

    
print not (b or a)



In [ ]:

    
print (not b) or a



In [ ]:

    
print (not b and a)



In [ ]:

    
print not (b and a)



In [ ]:

    
print (not b) and a



In [ ]:

    
print (x == abs(y))



In [ ]:

    
print len(cat)



In [ ]:

    
print cat + x



In [ ]:

    
print cat + str(x)



In [ ]:

    
print float(x)



In [ ]:

    
print ("i" in cat)



In [ ]:

    
print ("g" in cat)



In [ ]:

    
print ("Mit" in cat)



In [ ]:

    
if (x % 2) == 0:
    print "x is even"
else:
    print "x is odd"



In [ ]:

    
if (x - 4*y) < 0:
    print "Invalid!"
else:
    print "Banana"



In [ ]:

    
if "Mit" in cat:
    print "Hey Mits!"
else:
    print "Where's Mits?"



In [ ]:

    
x = "C"
if x == "A" or "B":
    print "yes"
else:
    print "no"



In [ ]:

    
x = "C"
if (x == "A") or (x == "B"):
    print "yes"
else:
    print "no"

Surprised by the last two? It's important to note that when you want compare a variable against multiple things, you only compare it to one thing at a time. Although it makes sense in English to say, is x equal to A or B?, in Python you must write: ((x == "A") or (x == "B")) to accomplish this. The same goes for e.g. ((x > 5) and (x < 10)) and anything along those lines.

So why does the first version give the answer "yes"? Basically, anything that isn't False or the literal number 0 is considered to be True in Python. So when you say 'x == "A" or "B"', this evaluates to 'False or True', which is True!

Lesson 2: if / else and Functions

Table of Contents

1. Conditionals I: The "if / else" statement

[ Definition ] Booleans

[ Definition ] The if / else statement

if / else statements in action

Note on indentation

[ Check yourself! ] if/else practice

2. Built-in functions

[ Definition ] raw_input()

[ Definition ] len()

[ Definition ] abs()

[ Definition ] round()

3. Modules

How to use a module

[ Definition ] The math module

[ Definition ] The random module

4. Test your understanding: practice set 2

Lesson 2: `if / else` and Functions

1. Conditionals I: The "`if / else`" statement

[ Definition ] The `if / else` statement

`if / else` statements in action

[ Check yourself! ] `if/else` practice

[ Definition ] `raw_input()`

[ Definition ] `len()`

[ Definition ] `abs()`

[ Definition ] `round()`

[ Definition ] The `math` module

[ Definition ] The `random` module