Flow Controls¶
Python uses the usual flow control statements known from other languages, with some twists.
There are three control flow statements in Python - if, for and while.
if Statement¶
The most well-known statement type is the if statement. For example:
if True:
print('Yup, it\'s true')
Yup, it's true
x = int(input("Please enter an integer: "))
if x < 0:
x = 0
print('Negative changed to zero')
elif x == 0: # keyword ‘elif’ is short for ‘else if’
print('Zero')
elif x == 1:
print('Single')
else:
print('More')
---------------------------------------------------------------------------
StdinNotImplementedError Traceback (most recent call last)
<ipython-input-2-5712790f94ca> in <module>
----> 1 x = int(input("Please enter an integer: "))
2 if x < 0:
3 x = 0
4 print('Negative changed to zero')
5 elif x == 0: # keyword ‘elif’ is short for ‘else if’
/opt/hostedtoolcache/Python/3.8.6/x64/lib/python3.8/site-packages/ipykernel/kernelbase.py in raw_input(self, prompt)
852 """
853 if not self._allow_stdin:
--> 854 raise StdinNotImplementedError(
855 "raw_input was called, but this frontend does not support input requests."
856 )
StdinNotImplementedError: raw_input was called, but this frontend does not support input requests.
for Statement¶
The for statement in Python differs a bit from what you may be used to in C or Pascal. Python’s for statement iterates over the items of any sequence (a list or a string), in the order that they appear in the sequence. For example (no pun intended):
words = ['Ram', 'Atul', 'Prashant']
for w in words:
print(w, len(w))
Ram 3
Atul 4
Prashant 8
The range() function¶
If you do need to iterate over a sequence of numbers, the built-in function range() comes in handy.
for i in range(5):
print(i)
0
1
2
3
4
for i in range(5, 10):
print(i)
5
6
7
8
9
for i in range(-10, -100, -30):
print(i)
-10
-40
-70
We can get a list from a range
list(range(4))
[0, 1, 2, 3]
break, continue and else¶
The break statement, like in C, breaks out of the innermost enclosing for or while loop.
Loop statements may have an else clause; it is executed when the loop terminates through exhaustion of the iterable (with for) or when the condition becomes false (with while), but not when the loop is terminated by a break statement.
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
When used with a loop, the else clause has more in common with the else clause of a try statement than it does with that of if statements: a try statement’s else clause runs when no exception occurs, and a loop’s else clause runs when no break occurs.
The continue statement, also borrowed from C, continues with the next iteration of the loop:
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
pass statement¶
The pass statement does nothing. It can be used when a statement is required syntactically but the program requires no action. For example:
class MyEmptyClass:
pass
pass can also be used as a place-holder for a function or conditional body when you are working on something new, to allow you to keep thinking at a more abstract level.
def initlog(*args):
pass # TODO: Implement this!
Exercise:¶
Print 1 - 10.
Print 1 - 10 except 5 and 8
Print if a number is even or odd
Print all divisor of number
# Please igone
for n in range(0, len([1,2,3])+1):
print(n)
0
1
2
3