Control and Flow¶

Turing completeness¶

Now that we understand how we can use objects to store and model our data, we only need to be able to control the flow of our program in order to have a program that can, in principle, do anything!

Specifically we need to be able to:

Control whether a program statement should be executed or not, based on a variable. "Conditionality"
Jump back to an earlier point in the program, and run some statements again. "Branching"

Once we have these, we can write computer programs to process information in arbitrary ways: we are Turing Complete!

Conditionality¶

Conditionality is achieved through Python's if statement:

In [1]:

x = 5

if x < 0:
    print(f"{x} is negative")

The absence of output here means the if clause prevented the print statement from running.

In [2]:

x = -10

if x < 0:
    print(f"{x} is negative")

-10 is negative

The first time through, the print statement never happened.

The controlled statements are indented. Once we remove the indent, the statements will once again happen regardless.

Else and Elif¶

Python's if statement has optional elif (else-if) and else clauses:

In [3]:

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

x is positive

In [4]:

x = 5
if x < 0:
    print("x is negative")
elif x == 0:
    print("x is zero")
else:
    print("x is positive")

x is positive

Try editing the value of x here, and note that other sections are found.

In [5]:

choice = 'high'

if choice == 'high':
    print(1)
elif choice == 'medium':
    print(2)
else:
    print(3)

Comparison¶

True and False are used to represent boolean (true or false) values.

In [6]:

1 > 2

Out[6]:

False

Comparison on strings is alphabetical.

In [7]:

"UCL" > "KCL"

Out[7]:

True

But case sensitive:

In [8]:

"UCL" > "kcl"

Out[8]:

False

There's no automatic conversion of the string True to true:

In [9]:

True == "True"

Out[9]:

False

In python two there were subtle implied order comparisons between types, but it was bad style to rely on these. In python three, you cannot compare these.

In [10]:

'1' < 2

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Cell In[10], line 1
----> 1 '1' < 2

TypeError: '<' not supported between instances of 'str' and 'int'

In [11]:

'5' < 2

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Cell In[11], line 1
----> 1 '5' < 2

TypeError: '<' not supported between instances of 'str' and 'int'

In [12]:

'1' > 2

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Cell In[12], line 1
----> 1 '1' > 2

TypeError: '>' not supported between instances of 'str' and 'int'

Any statement that evaluates to True or False can be used to control an if Statement.

Automatic Falsehood¶

Various other things automatically count as true or false, which can make life easier when coding:

In [13]:

mytext = "Hello"

In [14]:

if mytext:
    print("Mytext is not empty")

Mytext is not empty

In [15]:

mytext2 = ""

In [16]:

if mytext2:
    print("Mytext2 is not empty")

We can use logical not and logical and to combine true and false:

In [17]:

x = 3.2
if not (x > 0 and isinstance(x, int)):
    print(x,"is not a positive integer")

3.2 is not a positive integer

not also understands magic conversion from false-like things to True or False.

In [18]:

not not "Who's there!" # Thanks to Mysterious Student

Out[18]:

True

In [19]:

bool("")

Out[19]:

False

In [20]:

bool("Graham")

Out[20]:

True

In [21]:

bool([])

Out[21]:

False

In [22]:

bool(['a'])

Out[22]:

True

In [23]:

bool({})

Out[23]:

False

In [24]:

bool({'name': 'Graham'})

Out[24]:

True

In [25]:

bool(0)

Out[25]:

False

In [26]:

bool(1)

Out[26]:

True

But subtly, although these quantities evaluate True or False in an if statement, they're not themselves actually True or False under ==:

In [27]:

[] == False

Out[27]:

False

In [28]:

bool([]) == False

Out[28]:

True

Indentation¶

In Python, indentation is semantically significant. You can choose how much indentation to use, so long as you are consistent, but four spaces is conventional. Please do not use tabs.

In the notebook, and most good editors, when you press <tab>, you get four spaces.

No indentation when it is expected, results in an error:

In [29]:

x = 2

In [30]:

if x > 0:
print(x)

  Cell In[30], line 2
    print(x)
    ^
IndentationError: expected an indented block

but:

In [31]:

if x > 0:
    print(x)

Pass¶

A statement expecting identation must have some indented code. This can be annoying when commenting things out. (With #)

In [32]:

if x > 0:
    # print x
    
print("Hello")

  Cell In[32], line 4
    print("Hello")
    ^
IndentationError: expected an indented block

So the pass statement is used to do nothing.

In [33]:

if x > 0:
    # print x
    pass

print("Hello")

Hello