Lecture 2

Today: variables, types, defining your own functions, if-else

Variables

How to use variables in Python.

In [1]:
# this is how you assign a variables
x = 2
y = 3
In [2]:
x+y
Out[2]:
5
In [3]:
x**2 + x + 1
Out[3]:
7
In [4]:
# you can re-assign and lose the old value
x = 3
In [5]:
x+y
Out[5]:
6
In [6]:
# variables can hold strings as well
strng = "will print"


In [7]:
# by the way, print is a handy function, if you want to print more than one value
print("hellp")
print(x)
print("print " + strng, 1, 2, x, x+1)

hellp
3
print will print 1 2 3 4

Types

Every variable (and every expression has a type)

In [8]:
# take a look at this
x = 2/2
print(x)
y = 1
print(y)

1.0
1

what's going on? It's because 2/2 has a different type than 1.

In [9]:
# division by integer has float type
type(2/2)
Out[9]:
float
In [10]:
type(1.0)
Out[10]:
float
In [11]:
type(1)
Out[11]:
int
In [12]:
# let's look at some other types
type("hellooo")
Out[12]:
str
In [13]:
# even functions have a type
type(abs)
Out[13]:
builtin_function_or_method
In [14]:
import math
type(math.cos)
Out[14]:
builtin_function_or_method
In [15]:
# even modules have type
type(math)
Out[15]:
module

types are important in programming. you must always think about the types of the objects you are working with.



Defining functions

In [16]:
def f(x):
    return x*x
In [17]:
f(3)
Out[17]:
9
In [18]:
# what is this going to do?
def g(x,y):
    print("One day, I will pass the Turing test.")
    z = x + y
    return z
    z = 1/0        # we never get to this line
    return z+1     # we never get to this line
In [19]:
g(1,2)

One day, I will pass the Turing test.
Out[19]:
3

Every line inside the function is executed until return is called


Local vs global variables

Some things to be careful about when working with functions

In [20]:
# What's going to happen?
def f(x,y):
    zzz = 0   # local variable
    return x+y
In [21]:
f(2,1)
Out[21]:
3
In [22]:
print(zzz)    # error because zzz is a local variable, not known outside the function

---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-22-dbb4ee89e69d> in <module>()
----> 1 print(zzz)    # error because zzz is a local variable, not known outside the function

NameError: name 'zzz' is not defined


We didn't do the following during the lecture, but it's good to put here

In [23]:
ggg = 1
def f(x,y):
    ggg = 1000
    return x+y

print(f(1,2),ggg)

3 1

So ggg's value didn't change. Why? It was because ggg inside the function is a new, local variable. Not the global one.


Fix: use the global keyword

In [24]:
ggg = 1
def f(x,y):
    global ggg
    ggg = 1000
    return x+y

print(f(1,2),ggg)

3 1000
In [ ]: