Prelude: Syntax and Semantics

Syntax: grammar

This is about forming grammatically correct sentences. {{ .Site.Title }}

Prelude: Syntax and Semantics

Syntax: grammar

This is about forming grammatically correct sentences. {{ .Site.Title }}

Harry Sejal love. ← This is not even a well-formed sentence.

Prelude: Syntax and Semantics

Syntax: grammar

This is about forming grammatically correct sentences. {{ .Site.Title }}

Harry Sejal love. ← This is not even a well-formed sentence.

Static Semantics: meaning

This is about creating meaningful sentences.

Prelude: Syntax and Semantics

Syntax: grammar

This is about forming grammatically correct sentences. {{ .Site.Title }}

Harry Sejal love. ← This is not even a well-formed sentence.

Static Semantics: meaning

This is about creating meaningful sentences.

Harry subtracts Sejal. ← This is a well-formed sentence that is not meaningful.

Prelude: Syntax and Semantics

Syntax: grammar

This is about forming grammatically correct sentences. {{ .Site.Title }}

Harry Sejal love. ← This is not even a well-formed sentence.

Static Semantics: meaning

This is about creating meaningful sentences.

Harry subtracts Sejal. ← This is a well-formed sentence that is not meaningful.

Semantics: intent

This is about making sure that the meaning matches with your intent.

Prelude: Syntax and Semantics

Syntax: grammar

This is about forming grammatically correct sentences. {{ .Site.Title }}

Harry Sejal love. ← This is not even a well-formed sentence.

Static Semantics: meaning

This is about creating meaningful sentences.

Harry subtracts Sejal. ← This is a well-formed sentence that is not meaningful.

Semantics: intent

This is about making sure that the meaning matches with your intent.

Harry killed Sejal. ← This is meaningful but wrong!

A typical Python program

statement_1
def function1(...,...):
  ...
statement_2
statement_3
def function2(...,...):
  ...
def function3(...,...):
  ...
statement_4
.
.
.

A collection of statements and functions.

A typical Python program

statement_1
def function1(...,...):
  ...
statement_2
statement_3
def function2(...,...):
  ...
def function3(...,...):
  ...
statement_4
.
.
.

A collection of statements and functions.

Are not required to appear in any particular order, except that a function must be defined before it is called.

Assignment Statements

Assign a value to a name.

The left hand side is a name (aka variable).

The right hand side is an expression.

i = 5
j = 2*i
j = 5*j
course_name = "Computing"
course_code = "ES112"

Operations in the expression depend on the types of values being combined.

Assignment Statements

Assign a value to a name.

The left hand side is a name (aka variable).

The right hand side is an expression.

i = 5
j = 2*i
j = 5*j
course_name = "Computing"
course_code = "ES112"

Operations in the expression depend on the types of values being combined.

Eg., Arithmetic operations can be performed on numeric values. Strings can be concatenated.

Numeric values

Flavors of numbers

• Integers (type: int)

  • Eg, -3, 5, 450, -9034, 234, 3214

• Floating Point (type: float)

  • Eg, -3.34, 5.5, 23.4, -63.4, 456.434

Numeric values

Flavors of numbers

• Integers (type: int)

  • Eg, -3, 5, 450, -9034, 234, 3214

• Floating Point (type: float)

  • Eg, -3.34, 5.5, 23.4, -63.4, 456.434

int v/s float

• Floating point numbers break up into the mantissa and the exponent.

  • Eg. .00314159265359 * 10^3

• Internally, a value is stored as a finite sequence of zeroes and ones.

Operations on Numbers

• Normal arithmetic operations:

  • addition (+), subtraction (-), multiplication (*), division (/)

Operations on Numbers

• Normal arithmetic operations:

  • addition (+), subtraction (-), multiplication (*), division (/)

  • modulus or remainder (%), integer division or quotient (//)

Operations on Numbers

• Normal arithmetic operations:

  • addition (+), subtraction (-), multiplication (*), division (/)

  • modulus or remainder (%), integer division or quotient (//)

  • Exponentiation (**)

Operations on Numbers

• Normal arithmetic operations:

  • addition (+), subtraction (-), multiplication (*), division (/)

  • modulus or remainder (%), integer division or quotient (//)

  • Exponentiation (**)

  • Python knows that floats generalize ints.

  • In particular, Python allows you to mix ints and floats.

Operations on Numbers

• Normal arithmetic operations:

  • addition (+), subtraction (-), multiplication (*), division (/)

  • modulus or remainder (%), integer division or quotient (//)

  • Exponentiation (**)

  • Python knows that floats generalize ints.

  • In particular, Python allows you to mix ints and floats.

  • Division always produces floats, even if you are dividing two ints.

Operations on Numbers

• Normal arithmetic operations:

  • addition (+), subtraction (-), multiplication (*), division (/)

  • modulus or remainder (%), integer division or quotient (//)

  • Exponentiation (**)

  • Python knows that floats generalize ints.

  • In particular, Python allows you to mix ints and floats.

  • Division always produces floats, even if you are dividing two ints.

  • Addition and multiplication works on numbers and sequences.

Operations on Numbers

• Normal arithmetic operations:

  • addition (+), subtraction (-), multiplication (*), division (/)

  • modulus or remainder (%), integer division or quotient (//)

  • Exponentiation (**)

  • Python knows that floats generalize ints.

  • In particular, Python allows you to mix ints and floats.

  • Division always produces floats, even if you are dividing two ints.

  • Addition and multiplication works on numbers and sequences.

  • Integer division floors the result, instead of rounding towards zero.*

* Even when negative numbers are divided. Here's why.

Operations on Numbers

• Normal arithmetic operations:

  • addition (+), subtraction (-), multiplication (*), division (/)

  • modulus or remainder (%), integer division or quotient (//)

  • Exponentiation (**)

>>> y = 7//3
>>> print(y)
2
>>> type(y)
<class 'int'>

>>> x = 7/3
>>> print(x)
2.3333333333333335
>>> type(x)
<class 'float'>

Operations on Numbers

• Fancy arithmetic operations:

  • log(), sqrt(), sin(), ...

  • Built in, but not available by default.

  • import math

>>> help(math.log)
Help on built-in function log in module math:
log(...)
    log(x[, base])
    Return the logarithm of x to the given base.
    If the base not specified, returns the natural logarithm (base e) of x.

Operations on Numbers

• Fancy arithmetic operations:

  • log(), sqrt(), sin(), ...

  • Built in, but not available by default.

  • import math

>>> help(math.log)
Help on built-in function log in module math:
log(...)
    log(x[, base])
    Return the logarithm of x to the given base.
    If the base not specified, returns the natural logarithm (base e) of x.

>>> math.log(2**5,2)
5.0
>>> int(math.log(2**5,2))
5

Names, Values and Types

Values have types.

Types determine what operations make sense.

Names, Values and Types

Values have types.

Types determine what operations make sense.

Names inherit types from their values.

Names, Values and Types

Values have types.

Types determine what operations make sense.

Names inherit types from their values.

• In many languages, names are declared along with their type.

Names, Values and Types

Values have types.

Types determine what operations make sense.

Names inherit types from their values.

• In many languages, names are declared along with their type.

• Python takes the responsibility of making a best guess.

Names, Values and Types

Values have types.

Types determine what operations make sense.

Names inherit types from their values.

• In many languages, names are declared along with their type.

• Python takes the responsibility of making a best guess.

• The type of a name is not fixed explicitly in advance.

Names, Values and Types

Values have types.

Types determine what operations make sense.

Names inherit types from their values.

• In many languages, names are declared along with their type.

• Python takes the responsibility of making a best guess.

• The type of a name is not fixed explicitly in advance.

• Even worse,...

🤕 Changing Types

It is possible for names to be assigned values of different types as the program evolves.

🤕 Changing Types

It is possible for names to be assigned values of different types as the program evolves.

i = 5  #i is an int

🤕 Changing Types

It is possible for names to be assigned values of different types as the program evolves.

i = 5  #i is an int

i = 234**2 #still an int...

🤕 Changing Types

It is possible for names to be assigned values of different types as the program evolves.

i = 5  #i is an int

i = 234**2 #still an int...

j = i/7 #j is float, creats a float

🤕 Changing Types

It is possible for names to be assigned values of different types as the program evolves.

i = 5  #i is an int

i = 234**2 #still an int...

j = i/7 #j is float, creats a float

.
.
.

🤕 Changing Types

It is possible for names to be assigned values of different types as the program evolves.

i = 5  #i is an int

i = 234**2 #still an int...

j = i/7 #j is float, creats a float

.
.
.

i = 2*j #i is now a float!  ☠

🤕 Changing Types

It is possible for names to be assigned values of different types as the program evolves.

i = 5  #i is an int

i = 234**2 #still an int...

j = i/7 #j is float, creats a float

.
.
.

i = 2*j #i is now a float!  ☠

This is not recommended: it is confusing and leads to debugging nightmares.

🤕 Changing Types

It is possible for names to be assigned values of different types as the program evolves.

i = 5  #i is an int

i = 234**2 #still an int...

j = i/7 #j is float, creats a float

.
.
.

i = 2*j #i is now a float!  ☠

This is not recommended: it is confusing and leads to debugging nightmares.

Use consistent naming conventions.

Boolean Types

>>> type(True)
<class 'bool'>

>>> type(False)
<class 'bool'>

Boolean Types

>>> type(True)
<class 'bool'>

>>> type(False)
<class 'bool'>

>>> not True
False
>>> not False
True

Boolean Types

>>> type(True)
<class 'bool'>

>>> type(False)
<class 'bool'>

>>> True and True
True
>>> True and False
False
>>> False and True
False
>>> False and False
False

Boolean Types

>>> type(True)
<class 'bool'>

>>> type(False)
<class 'bool'>

>>> True or True
True
>>> True or False
True
>>> False or True
True
>>> False or False
False

Comparison Operators

• Equality (==), Not Equals (!=)

• Greater than or equal to (>=), Less than or equal to (<=)

• Greater Than (>), Less than (<)

>>> x = 3
>>> y = 3.0

Comparison Operators

• Equality (==), Not Equals (!=)

• Greater than or equal to (>=), Less than or equal to (<=)

• Greater Than (>), Less than (<)

>>> x = 3
>>> y = 3.0

>>> x == y

Comparison Operators

• Equality (==), Not Equals (!=)

• Greater than or equal to (>=), Less than or equal to (<=)

• Greater Than (>), Less than (<)

>>> x = 3
>>> y = 3.0

>>> x == y

True

Comparison Operators

• Equality (==), Not Equals (!=)

• Greater than or equal to (>=), Less than or equal to (<=)

• Greater Than (>), Less than (<)

>>> x = 3
>>> y = 3.0

>>> x == y

True

>>> type(x) == type(y)

Comparison Operators

• Equality (==), Not Equals (!=)

• Greater than or equal to (>=), Less than or equal to (<=)

• Greater Than (>), Less than (<)

>>> x = 3
>>> y = 3.0

>>> x == y

True

>>> type(x) == type(y)

False

Comparison Operators

• Equality (==), Not Equals (!=)

• Greater than or equal to (>=), Less than or equal to (<=)

• Greater Than (>), Less than (<)

>>> x = 3
>>> y = 3.0

>>> x == y

True

>>> type(x) == type(y)

False

>>> True < False
False

Exponentiation

The expression:

has two possible interpretations:

Similarly,

has two possible interpretations:

From the Manual...

help('**')

The power operator binds more tightly than unary operators on its left; it binds less tightly than unary operators on its right.

From the Manual...

help('**')

The power operator binds more tightly than unary operators on its left; it binds less tightly than unary operators on its right.

For int operands, the result has the same type as the operands unless the second argument is negative. In that case, all arguments are converted to float and a float result is delivered.

From the Manual...

help('**')

The power operator binds more tightly than unary operators on its left; it binds less tightly than unary operators on its right.

For int operands, the result has the same type as the operands unless the second argument is negative. In that case, all arguments are converted to float and a float result is delivered.

For example, "10**2" returns "100", but "10**-2" returns "0.01".

From the Manual...

help('**')

The power operator binds more tightly than unary operators on its left; it binds less tightly than unary operators on its right.

For int operands, the result has the same type as the operands unless the second argument is negative. In that case, all arguments are converted to float and a float result is delivered.

For example, "10**2" returns "100", but "10**-2" returns "0.01".

Raising "0.0" to a negative power results in a "ZeroDivisionError".

From the Manual...

help('**')

The power operator binds more tightly than unary operators on its left; it binds less tightly than unary operators on its right.

For int operands, the result has the same type as the operands unless the second argument is negative. In that case, all arguments are converted to float and a float result is delivered.

For example, "10**2" returns "100", but "10**-2" returns "0.01".

Raising "0.0" to a negative power results in a "ZeroDivisionError".

Raising a negative number to a fractional power results in a "complex" number. (In earlier versions it raised a "ValueError".)

Examples

>>> (4**12)/2

Examples

>>> (4**12)/2

8388608.0

Examples

>>> (4**12)/2

8388608.0

>>> 4**(12/2)

Examples

>>> (4**12)/2

8388608.0

>>> 4**(12/2)

4096.0

Examples

>>> (4**12)/2

8388608.0

>>> 4**(12/2)

4096.0

>>> 4**12/2

Examples

>>> (4**12)/2

8388608.0

>>> 4**(12/2)

4096.0

>>> 4**12/2

8388608.0

Examples

>>> (4**12)/2

8388608.0

>>> 4**(12/2)

4096.0

>>> 4**12/2

8388608.0

>>> 10**-2

Examples

>>> (4**12)/2

8388608.0

>>> 4**(12/2)

4096.0

>>> 4**12/2

8388608.0

>>> 10**-2

0.01

Examples

>>> (4**12)/2

8388608.0

>>> 4**(12/2)

4096.0

>>> 4**12/2

8388608.0

>>> 10**-2

0.01

>>> -10**2

Examples

>>> (4**12)/2

8388608.0

>>> 4**(12/2)

4096.0

>>> 4**12/2

8388608.0

>>> 10**-2

0.01

>>> -10**2

-100

Comparisons

if a == 3 or b == 3 or c == 3:

V/S.

if a or b or c == 3:

Comparisons

if a == 3 or b == 3 or c == 3:

V/S.

if a or b or c == 3:

Since OR has a lower precedence than ==, the second statement will be evaluated as:

(X) if a or b or c == 3:

Comparisons

if a == 3 or b == 3 or c == 3:

V/S.

if a or b or c == 3:

Since OR has a lower precedence than ==, the second statement will be evaluated as:

(X) if a or b or c == 3:

which is very different from:

(Y) if a == 3 or b == 3 or c == 3:

Comparisons

if a == 3 or b == 3 or c == 3:

V/S.

if a or b or c == 3:

Since OR has a lower precedence than ==, the second statement will be evaluated as:

(X) if a or b or c == 3:

which is very different from:

(Y) if a == 3 or b == 3 or c == 3:

💀 Note that there are values of a,b,c for which X will evaluate to True while Y will evaluate to False.

The Precedence Table 😴

Manipulating Text

• Computation is a lot more than number crunching.

Manipulating Text

• Computation is a lot more than number crunching.

• Text processing is increasingly important.

Manipulating Text

• Computation is a lot more than number crunching.

• Text processing is increasingly important.

  • Document preparation

  • Importing/exporting spreadsheet data.

  • Maching queries to content.

Manipulating Text

• Computation is a lot more than number crunching.

• Text processing is increasingly important.

  • Document preparation

  • Importing/exporting spreadsheet data.

  • Maching queries to content.

String Types

Manipulating Text

• Computation is a lot more than number crunching.

• Text processing is increasingly important.

  • Document preparation

  • Importing/exporting spreadsheet data.

  • Maching queries to content.

String Types

• Type string str, a sequence of characters.

Manipulating Text

• Computation is a lot more than number crunching.

• Text processing is increasingly important.

  • Document preparation

  • Importing/exporting spreadsheet data.

  • Maching queries to content.

String Types

• Type string str, a sequence of characters.

  • A single character is a string of length one.

  • No separate char type.

String Types (Contd.)

• Enclose in quotes: single, double or triple.

String Types (Contd.)

• Enclose in quotes: single, double or triple.

s = "ES112 Computing"

String Types (Contd.)

• Enclose in quotes: single, double or triple.

s = "ES112 Computing"

>>> w = 'ES112 'Computing''
  File "<stdin>", line 1
    w = 'ES112 'Computing''
                        ^
SyntaxError: invalid syntax

String Types (Contd.)

• Enclose in quotes: single, double or triple.

s = "ES112 Computing"

>>> w = 'ES112 'Computing''
  File "<stdin>", line 1
    w = 'ES112 'Computing''
                        ^
SyntaxError: invalid syntax

w = 'ES112 \'Computing\''
x = "ES112 'Computing'"

String Types (Contd.)

• Enclose in quotes: single, double or triple.

s = "ES112 Computing"

>>> w = 'ES112 'Computing''
  File "<stdin>", line 1
    w = 'ES112 'Computing''
                        ^
SyntaxError: invalid syntax

w = 'ES112 \'Computing\''
x = "ES112 'Computing'"

>>> y = "Students say that "ES112 Computing" is their favorite course."

SyntaxError: invalid syntax

String Types (Contd.)

• Enclose in quotes: single, double or triple.

s = "ES112 Computing"

>>> w = 'ES112 'Computing''
  File "<stdin>", line 1
    w = 'ES112 'Computing''
                        ^
SyntaxError: invalid syntax

w = 'ES112 \'Computing\''
x = "ES112 'Computing'"

>>> y = "Students say that "ES112 Computing" is their favorite course."

SyntaxError: invalid syntax

z = '''Students say that "ES112 Computing" is their favorite course.'''

Strings as Sequences

Strings as Sequences

• Strings in Python are a sequence or list of characters.

Strings as Sequences

• Strings in Python are a sequence or list of characters.

• Positions 0, 1, ..., n-1 for a string of length n

Strings as Sequences

• Strings in Python are a sequence or list of characters.

• Positions 0, 1, ..., n-1 for a string of length n

• Positions -1, -2, ... count backwards from the end.

>>> s = "hello world"

>>> s = "hello world"

>>> s[0]

>>> s = "hello world"

>>> s[0]

'h'

>>> s = "hello world"

>>> s[0]

'h'

>>> s[-1]

>>> s = "hello world"

>>> s[0]

'h'

>>> s[-1]

'd'

>>> s = "hello world"

>>> s[0]

'h'

>>> s[-1]

'd'

>>> len(s)

>>> s = "hello world"

>>> s[0]

'h'

>>> s[-1]

'd'

>>> len(s)

11

>>> s = "hello world"

>>> s[0]

'h'

>>> s[-1]

'd'

>>> len(s)

11

>>> s[0:len(s)]

>>> s = "hello world"

>>> s[0]

'h'

>>> s[-1]

'd'

>>> len(s)

11

>>> s[0:len(s)]

'hello world'

>>> s = "hello world"

>>> s[0]

'h'

>>> s[-1]

'd'

>>> len(s)

11

>>> s[0:len(s)]

'hello world'

>>> s[3:7]

>>> s = "hello world"

>>> s[0]

'h'

>>> s[-1]

'd'

>>> len(s)

11

>>> s[0:len(s)]

'hello world'

>>> s[3:7]

'lo w'

String Operators

Concatenation via + and Slicing via :

String Operators

Concatenation via + and Slicing via :

For strings s and w, s + w is sw.

String Operators

Concatenation via + and Slicing via :

For strings s and w, s + w is sw.

A slice is a segment of a string.

String Operators

Concatenation via + and Slicing via :

For strings s and w, s + w is sw.

A slice is a segment of a string.

s[i:j] starts at s[i] and ends at s[j-1].

String Operators

Concatenation via + and Slicing via :

For strings s and w, s + w is sw.

A slice is a segment of a string.

s[i:j] starts at s[i] and ends at s[j-1].

>>> s = "hello"
>>> w = "world"
>>> s + w

String Operators

Concatenation via + and Slicing via :

For strings s and w, s + w is sw.

A slice is a segment of a string.

s[i:j] starts at s[i] and ends at s[j-1].

>>> s = "hello"
>>> w = "world"
>>> s + w

'helloworld'

String Operators

Concatenation via + and Slicing via :

For strings s and w, s + w is sw.

A slice is a segment of a string.

s[i:j] starts at s[i] and ends at s[j-1].

>>> s = "hello"
>>> w = "world"
>>> s + w

'helloworld'

>>> s + ' ' + w

String Operators

Concatenation via + and Slicing via :

For strings s and w, s + w is sw.

A slice is a segment of a string.

s[i:j] starts at s[i] and ends at s[j-1].

>>> s = "hello"
>>> w = "world"
>>> s + w

'helloworld'

>>> s + ' ' + w

'hello world'

String Operators

Concatenation via + and Slicing via :

String Operators

Concatenation via + and Slicing via :

>>> x = "hello world"
>>> x[0:5] + x[5:]

String Operators

Concatenation via + and Slicing via :

>>> x = "hello world"
>>> x[0:5] + x[5:]

'hello world'

>>> x = "hello world"
>>> x[0:5] + x[5:0]

String Operators

Concatenation via + and Slicing via :

>>> x = "hello world"
>>> x[0:5] + x[5:]

'hello world'

>>> x = "hello world"
>>> x[0:5] + x[5:0]

'hello worl'

String Operators

Concatenation via + and Slicing via :

>>> x = "hello world"
>>> x[0:5] + x[5:]

'hello world'

>>> x = "hello world"
>>> x[0:5] + x[5:0]

'hello worl'

>>> x = "hello world"
>>> x[5]

String Operators

Concatenation via + and Slicing via :

>>> x = "hello world"
>>> x[0:5] + x[5:]

'hello world'

>>> x = "hello world"
>>> x[0:5] + x[5:0]

'hello worl'

>>> x = "hello world"
>>> x[5]

' '