Python loops - for, while

Loops in Python

while loop
Use while loop, when you don't know in advance how many times you will repeat the block of code.
Make sure the condition/expression will be false eventually, otherwise loop will be in infinite loop.
Indentation is very important in Python language.

while loop syntax in Python:-
"while" expression ":" block-of-code
  ["else" ":" block-of-code]

while i < 6:
  print (i)
  i = i + 1

while x < 3 :
print (x)
x = x + 1
else:
print ('the else')


for loop
Use for loop, in Python, when you know number of times the loop should iterate.

for loop syntax in Python:-
"for" target_list "in" expression_list ":" block-of-code
  ["else" ":" block-of-code]

for i in range(8):
print (i)

for x in [1,7,13,2] : print (x)

for i in range(0, len(data), 2):

print (data[i])

for ch in "Hello":
print (ch)

for n,a in enumerate("Hello World"):
print ('alphabet', a, 'index number', n)

a = ['cat', 'window', 'defend']
for x in a:
print (x, len(x))

for key, value in comp_arr.items():
print ('%s: %s' % (key, value))

data = [ ("C20H20O3", 308.371), ("C22H20O2", 316.393),
("C24H40N4O2", 416.6), ("C14H25N5O3", 311.38), ("C15H20O2", 232.3181) ]
for (formula, mw) in data:
print ("The molecular weight of %s is %s" % (formula, mw))

for line in open('log.txt', 'r'): # loops through each line in file
print(line)

for key in dictionary.keys(): # loops through each key in Python dictionary
  print(key)

for key, value in dictionary.iteritems(): print ('A %s has %d' % (key, value))

for key, value in comp_arr.items():
  print ('%s: %s' % (key, value))

break - exit the loop
continue - go to next iteration
pass - nothing

for looop examples with break and continue
>>> for value in [3, 1, 4, 1, 5, 9, 2]:
print ("Checking", value)
if value > 8:
print ("Exiting for loop")
break
elif value < 3:
print ("Ignoring")
continue
print ("The square is", value**2)

for x in range(5):
    print (x)
    break
else :
    print ('I am in else block')

async with EXPR as VAR:
    BLOCK

async for TARGET in ITER:
    BLOCK
else:
    BLOCK2


Related Python Articles:  Functions in Python   Comprehensions in Python