while¶

General Format¶

while test:          # Loop test
    statements       # Loop body
else:                # Optional else
    statements       # Run if didn't exit loop with break

break v.s. continue¶

break: jump out the closest enclosing loop
continue: jump to the top of the closest enclosing loop

pass¶

It's usually used to mean "to be filled in later".

In [1]:

def fun():
    pass

In [2]:

# ellipeses
# Supported in Python 3


def fun():
    ...

In [3]:

# Another usage of ellipeses

x = ...  # alternative to None
x

Out[3]:

Ellipsis

Loop else¶

Runs if and only if the loop is exited normally.

Example¶

In [4]:

# Without else

found = False
x = "spam"
while x and not found:
    if x[0] == "b":
        print("Found")
        found = True
    else:
        x = x[1:]
if not found:
    print("Not found")

Not found

In [5]:

# With else

x = "spam"
while x:
    if x[0] == "b":
        print("Found")
        break
    x = x[1:]
else:
    print("Not found")

Not found

for¶

General Format¶

for target in object:          # Assign object items to target
    statements                 # Repeated loop body: use target
else:                          # Optional else
    statements                 # Run if didn't exit loop with break

Tuple assignement in for loop¶

In [6]:

D = {1: "a", 2: "b", 3: "c"}
for key, value in D.items():
    print("Key: ", key, "  Value: ", value)

Key:  1   Value:  a
Key:  2   Value:  b
Key:  3   Value:  c

Python 3 extended sequence¶

In [7]:

for a, *b, c in [(1, 2), (3, 4, 5, 6)]:
    print(a, b, c)

1 [] 2
3 [4, 5] 6

File Scanner¶

read, readlines load a file all at once.

The following method might be *the best option for text files*.
It's usually the quickest way.
It uses iterator which doesn't load the entire file into memory all at once.

In [8]:

# Using iterator

for line in open("test.txt"):
    print(line)

readlines can still be useful when reversing a file

In [9]:

for line in reversed(open("test.txt").readlines()):
    print(line)

Loop Techniques¶

Use for instead of while whenever possible since it's usually quicker and easier.

while loop is a general looping statement
for is designed to iterate across items in a sequence or other iterable

range¶

In Python2 range returns a list while it returns iterator in Python3

range vs iterator¶

In most case, range should not be used in for and they can be replaced by using iterator.
This would make your code more efficient.

In [10]:

s = "spam"


# range
for i in range(len(s)):
    print(s[i])

# iterator
for i in s:
    print(i)

s
p
a
m
s
p
a
m

range vs comprehension¶

In [11]:

# range

L = [1, 2, 3, 4, 5]
for i in range(len(L)):
    L[i] += 1
L

Out[11]:

[2, 3, 4, 5, 6]

In [12]:

# comprehension

L = [1, 2, 3, 4, 5]
L = [x + 1 for x in L]
L

Out[12]:

[2, 3, 4, 5, 6]

Parallel Traversals: zip and map¶

zip allow multiple sequences work in the same loop (not overlapping in time)
zip returns a list in Python2 and returns an iterator in Python3

In [13]:

L1 = [1, 2, 3, 4]
L2 = [5, 6, 7, 8]

list(zip(L1, L2))

# In fact, N arguments can be used and we'll get N-ary tuple

Out[13]:

[(1, 5), (2, 6), (3, 7), (4, 8)]

zip truncates result tuples at the length of the shortest sequence

In [14]:

L1 = [1, 2]
L2 = [3, 4, 5, 6]

list(zip(L1, L2))

Out[14]:

[(1, 3), (2, 4)]

dictionay construction¶

In [15]:

key = [1, 2, 3]
value = "abc"

D = dict(zip(key, value))
D

Out[15]:

{1: 'a', 2: 'b', 3: 'c'}

enumerate¶

enumerate returns a gernerator object

In [16]:

s = "abcdefghijk"
for offset, item in enumerate(s, 1):
    print(offset, item)

1 a
2 b
3 c
4 d
5 e
6 f
7 g
8 h
9 i
10 j
11 k