Introduction_to_Python

• reverse
Syntax: list.reverse() Reverse elements of list in place
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Example:
>>> list3 = [4, 12, 3, 9] >>> list3.reverse() >>> list3 [9, 3, 12, 4]
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Execute body until looping condition is met
Example:
>>> i = 0 >>> while i < 4: print i i += 1 0 1 2 3
Controlling Flow
• Loops - for
Syntax: for <var> in <sequence>: body
Controlling Flow
• Using range with for
Generate list used by for with range Example
>>> for i in range(4):
print i 0 1
2
3
Controlling Flow
• The continue statement
Generate a list of numbers from start to stop stepping every step start defaults to 0, step defaults to 1
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Example
>>> range(5) [0, 1, 2, 3, 4] >>> range(1, 9) [1, 2, 3, 4, 5, 6, 7, 8] >>> range(2, 20, 5) [2, 7, 12, 17]
Syntax: list.append(element) Add element to end of list
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Example:
>>> list1 = [3, '10', 2] >>> list1.append('new') >>> list1 [3, '10', 2, 'new']
Lists - Methods
[2, 3, 4, 5, 6]
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Missing Index implies end point
>>> sequence[:2] [0, 1] >>> sequence[3:] [3, 4, 5, 6, 7]
Tuples
• Immutable version of list
Syntax: (elem1, elem2, …) Items in tuple can not be altered
Example:
>>> [23, 'x'] * 4 [23, 'x', 23, 'x', 23, 'x', 23, 'x']
Indexing
• Indexing operator: [ ] • Positive indices count from the left • Negative indices count from the right
Built-in Function: len
• Syntax: len(object)
Return the length of object Example
>>> list1 = [1, 2, 3, 4, 5]
>>> len(list1) 5
>>> string1 = "length of a string" >>> len(string1) 18
0 a -7 1 b -6 2 c -5 3 d -4 4 e -3 5 f -2 6 g -1
sequence[0] == a sequence[6] == g sequence[2] == c
sequence[-7] == a sequence[-1] == g sequence[-5] == c
Slices
• Two indices separated by a colon
Available for both strings and lists Example
>>> sequence = [0, 1, 2, 3, 4, 5, 6, 7]
>>> sequence[1:4] [1, 2, 3] >>> sequence[2:-1]
Strings - Methods
• split
Syntax: string.split([sep]) Returns a list of strings
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Example:
>>> text = "1 2 4 5 1" >>> text.split() ['1', '2', '4', '5', '1']
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Execute body once for every element in sequence
Example:
>>> for i in [0, 1, 2, 3]: print i 0 1 2 3
Built-in Function: range
• Syntax: range([start,] stop[, step])
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Mutable
Example:
>>> list1 = [1, 'hello', 4+2j, 123.12] >>> list1 [1, 'hello', (4+2j), 123.12] >>> list1[0] = 'a' >>> list1
['a', 'hello', (4+2j), 123.12]
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Example:
>>> tuple1 = (1, 5, 10) >>> tuple1[2] = 2 Traceback (most recent call last): File "<pyshell#136>", line 1, in ? tuple1[2] = 2 TypeError: object doesn't support item assignment
Continue to next iteration of loop, skipping remainder of body Example:
>>> for x in range(8):
if x%2 == 0: continue print x 1 3 5 7
Controlling Flow
• The break statement
Байду номын сангаас
Lists - Methods
• sort
Syntax: list.sort([cmpfunc]) Sort list in place
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Example:
>>> list3 = [4, 12, 3, 9] >>> list3.sort() >>> list3 [3, 4, 9, 12]
Lists - Methods
Lists - Methods
• remove
Syntax: list.remove(element) Removes the first occurrence of element in list
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Example:
>>> list2 = [0, 1, 3, 4, 3, 5] >>> list2.remove(3) >>> list2 [0, 1, 4, 3, 5]
>>> test = "a, b, c, d, e" >>> test.split(',') ['a', ' b', ' c', ' d', ' e']
Strings - Methods
• strip
Syntax: string.strip() Remove leading and trailing white space (tabs, new lines, etc)
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Example:
>>> padded = " stuff " >>> padded.strip() 'stuff' >>> padded ' stuff '
>>> unpadded = padded.strip() >>> unpadded 'stuff'
Lists - Methods
• append
Introduction to Python
Data Structures