Python - 4. Control Structures

From:http://interactivepython.org/courselib/static/pythonds/Introduction/ControlStructures.html

Control Structures

As we noted earlier, algorithms require two important control structures: iteration and selection.

• Iteration
  • while

>>> counter = 1
>>> while counter <= 5:
...     print("Hello, world")
...     counter = counter + 1


Hello, world
Hello, world
Hello, world
Hello, world
Hello, world

• • for

>>> for item in [1,3,6,2,5]:
...    print(item)
...
1
3
6
2
5

>>> for item in [1,3,6,2,5]:
...    print(item)
...
1
3
6
2
5

wordlist = ['cat','dog','rabbit']
letterlist = [ ]
for aword in wordlist:
for aletter in aword:
letterlist.append(aletter)
print(letterlist)

 

• Selection

Selection statements allow programmers to ask questions and then, based on the result, perform different actions.

• • ifelse

if n<0:
   print("Sorry, value is negative")
else:
   print(math.sqrt(n))

if score >= 90:
   print('A')
else:
   if score >=80:
      print('B')
   else:
      if score >= 70:
         print('C')
      else:
         if score >= 60:
            print('D')
         else:
            print('F')

if score >= 90:
   print('A')
elif score >=80:
   print('B')
elif score >= 70:
   print('C')
elif score >= 60:
   print('D')
else:
   print('F')

• if
  

  if n<0:
     n = abs(n)
  print(math.sqrt(n))

   
• list comprehension

>>> sqlist=[]
>>> for x in range(1,11):
         sqlist.append(x*x)

>>> sqlist
[1, 4, 9, 16, 25, 36, 49, 64, 81, 100]
>>>

>>> sqlist=[x*x for x in range(1,11)]
>>> sqlist
[1, 4, 9, 16, 25, 36, 49, 64, 81, 100]
>>>

>>> sqlist=[x*x for x in range(1,11) if x%2 != 0]
>>> sqlist
[1, 9, 25, 49, 81]
>>>

>>>[ch.upper() for ch in 'comprehension' if ch not in 'aeiou']
['C', 'M', 'P', 'R', 'H', 'N', 'S', 'N']
>>>