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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(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(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)
1

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)
<ipython-input-10-2ae56e567bff> in <module>
----> 1 '1' < 2

TypeError: '<' not supported between instances of 'str' and 'int'
In [11]:
'5' < 2
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-11-4b266c2a1d9b> in <module>
----> 1 '5' < 2

TypeError: '<' not supported between instances of 'str' and 'int'
In [12]:
'1' > 2
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-12-142f2d5d83a7> in <module>
----> 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 type(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("James")
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': 'James'})
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)
  File "<ipython-input-30-61c7132d9aa5>", line 2
    print(x)
        ^
IndentationError: expected an indented block

but:

In [31]:
if x > 0:
    print(x)
2

 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")
  File "<ipython-input-32-1045ed7694fb>", 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