Python Review

for beginners

1. print()

print(value, ..., sep=' ', end='\n', file=sys.stdout, flush=False) to output something

by default:

with the separation of

      >>> print(1,'23asd',[1,2,3],(1,2))
      1 23asd [1, 2, 3] (1, 2)

with the end of ‘\n’

      >>> for i in range(1,4):
      ...		print(i)
      ...
      1
      2
      3

ignore file=sys.stdout, flush=False

change the arguements

change the separation

      >>> print(1,'23asd',[1,2,3],(1,2), sep='\n')
      1
      23asd
      [1, 2, 3]
      (1, 2)	

change the end

      >>>	for i in range(1,4):
      ...		print(i,end=' ')
      ...
      1 2 3	

*debug: while facing with a great block of program without knowing what goes wrong, we add some print() to figure out which step produces problem help(print) to get more info

2. string(s)

something enclosed with " or '

operations:

len(s) to work out the length of s

      >>> s='123sd67fhjk'
      >>> len(s)
      11

index & slice

index: s[i] to enter the (i+1)th element of s

      >>> s[0]
      '1'
      >>> s[-1]
      'k'

slice: s[i:j:k] to take out a slice starting with index i , ending up with index (j-1), with the step of k

      >>> s[1:10:3]
      '2df'
      >>> s[2:5]
      '3sd'
      >>> s[3:]
      'sd67fhjk'
      >>> s[:-4]
      '123sd67'

does not support item assignment

      >>> s[2]=4
      Traceback (most recent call last):
            File "<stdin>", line 1, in <module>
      TypeError: 'str' object does not support item assignment

s1 + s2 to produce an addition of s1 and s2
str(a) where a is an int or float or list or tuple etc. to transform a into ‘a’ with type str
help(str) to get more info

3. list(l) & tuple(t)

list (mutable)
- elements enclosed with []
- elements can be int or float or str or list or tuple or dict etc.
- len(l) as string index and slice mostly as string
  - support item assignment
    >>> l=[1,2,3] >>> l[2]=5 >>> l [1,2,5]
- l.append(a) to push a into l (and return None) python >>> l=[1,2,3] >>> l.append(5) >>> l [1,2,3,5] >>> l1=l.append(6) >>> print(l1) None
- l.pop(i) to pop out the ith element (and return it)
```
      >>> l=[1,2,3,4,5,6,7]
      >>> l.pop()
      7
      >>> l.pop(3)
      4
      >>> l
      [1,2,3,5,6]
```
- l.reverse() to return a reverse order of l
- l.sort() to return l with an ascending order
- help(list) to get more info
tuple (immutable)
- elements enclosed with ()
- elements can be int or float or str or list or tuple or dict etc.
- len(l) as string
- index and slice as string
  - does not support item assignment python >>> t=(1,2,3) >>> t[2] 3 >>> t[2]=4 Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: 'tuple' object does not support item assignment
- help(tuple) to get more info

quick view

	str	list	tuple
len( )	Yes	Yes	Yes
index	Yes	Yes	Yes
silce	Yes	Yes	Yes
item assignment	Nope	Yes	Nope
append( )	Nope	Yes	Nope
pop( )	Nope	Nope	Nope

4. dictionary(dict)

click Dictionaries and refer to Chap5.5
help(dict) to get more info

5. Defining functions:

click defining functions and refer to Chap4.6

6. for & while & if

for

      >>> # Measure some strings:
      ... words = ['cat', 'window', 'defenestrate']
      >>> for w in words:
      ...     print(w, len(w))
      ...
      cat 3
      window 6
      defenestrate 12

while

      >>> a, b = 0, 1
      >>> while b < 1000:
      ...     print(b, end=',')
      ...     a, b = b, a+b
      ...
      1,1,2,3,5,8,13,21,34,55,89,144,233,377,610,987,

      >>> x = int(input("Please enter an integer: "))
      Please enter an integer: 42
      >>> if x < 0:
      ...     x = 0
      ...     print('Negative changed to zero')
      ... elif x == 0:
      ...     print('Zero')
      ... elif x == 1:
      ...     print('Single')
      ... else:
      ...     print('More')
      ...
      More

7. break & continue

break

      >>> for n in range(2, 10):
      ...     for x in range(2, n):
      ...         if n % x == 0:
      ...             print(n, 'equals', x, '*', n//x)
      ...             break
      ...     else:
      ...         # loop fell through without finding a factor
      ...         print(n, 'is a prime number')
      ...
      2 is a prime number
      3 is a prime number
      4 equals 2 * 2
      5 is a prime number
      6 equals 2 * 3
      7 is a prime number
      8 equals 2 * 4
      9 equals 3 * 3

continue

      >>> for num in range(2, 10):
      ...     if num % 2 == 0:
      ...         print("Found an even number", num)
      ...         continue
      ...     print("Found a number", num)
      Found an even number 2
      Found a number 3
      Found an even number 4
      Found a number 5
      Found an even number 6
      Found a number 7
      Found an even number 8
      Found a number 9

8. pass

The pass statement does nothing.

      >>> while True:
      ...     pass  # Busy-wait for keyboard interrupt (Ctrl+C)
      ...
      >>>
      >>> def f(x):
      ...     pass   # Remember to implement this!
      ...

9. Other useful functions

*range()

     >>> for i in range(1,10,3):
     ... 	print(i)
     ...
     1
     4
     7
     >>> for i in range(3,6):
     ... 	print(i)
     ...
     3
     4
     5
     >>> for i in range(3):
     ... 	print(i)
     ...
     0
     1
     2

click The range() Function and refer to Chap4.3

*eavl()

     >>> s='123'
     >>> s=eval(s)
     >>> s
     123
     >>>
     >>> l='[1,2,3,4]'
     >>> l=eval(l)
     >>> l
     [1,2,3,4]
     >>>
     >>> t='(1,2,3)'
     >>> t=eval(t)
     >>> t
     (1,2,3)
     >>>
     >>> d='{1: 'a', 2: 'b'}'
     >>> d=eval(d)
     >>> d
     {1: 'a', 2: 'b'}

''.join()

     >>> l=['2','3','4','qwe','asd']
     >>> r='-enhhh-'.join(l)
     >>> r
     '2-enhhh-3-enhhh-4-enhhh-qwe-enhhh-asd'

split()

     >>> s='123|242|sdfsd'
     >>> l=s.split('|')
     >>> l #return a list
     ['123', '242', 'sdfsd'] 

count()

     >>> l=['1','4','1']
     >>> l.count('1')
     2

more info: zhufm@shanghaitech.edu.cn

Python review

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