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Loop - Python Cheat Sheet (DRAFT) by

Python - Computer Language

This is a draft cheat sheet. It is a work in progress and is not finished yet.

The range() Function

range(10)
-----
list(range(10))
-----
range(4,10)
-----
list(range(4,10))
-----
#range(begin,end, step)
list(range(4,50,5))
The built-in function range() is the right function to iterate over a sequence of numbers. It generates an iterator of arithmetic progre­ssions:

The "­for­" loop

primes = [2, 3, 5, 7]
for prime in primes:
    print(prime)
-------------------------------------
For loops can iterate over a sequence of numbers using the "­ran­ge" and "­xra­nge­" functions. The difference between range and xrange is that the range function returns a new list with numbers of that specified range, whereas xrange returns an iterator, which is more efficient. (Python 3 uses the range function, which acts like xrange). Note that the range function is zero based.

loop1.py

# Prints out the numbers 0,1,2,3,4
for x in range(5):
    print(x)

# Prints out 3,4,5
for x in range(3, 6):
    print(x)

# Prints out 3,5,7
for x in range(3, 8, 2):
    print(x)

"­whi­le" loops

# Prints out 0,1,2,3,4

count = 0
while count < 5:
    print(count)
    count += 1  # This is the same as count = count + 1
While loops repeat as long as a certain boolean condition is met.

"­bre­ak" and "­con­tin­ue" statements

# Prints out 0,1,2,3,4

count = 0
while True:
    print(count)
    count += 1
    if count >= 5:
        break

# Prints out only odd numbers - 1,3,5,7,9
for x in range(10):
    # Check if x is even
    if x % 2 == 0:
        continue
    print(x)
break is used to exit a for loop or a while loop, whereas continue is used to skip the current block, and return to the "­for­" or "­whi­le" statement.

can we use "­els­e" clause for loops

# Prints out 0,1,2,3,4 and then it prints "count value reached 5"

count=0
while(count<5):
    print(count)
    count +=1
else:
    print("count value reached %d" %(count))

# Prints out 1,2,3,4
for i in range(1, 10):
    if(i%5==0):
        break
    print(i)
else:
    print("this is not printed because for loop is terminated because of break but not due to fail in condition")
We can use else for loops. When the loop condition of "­for­" or "­whi­le" statement fails then code part in "­els­e" is executed. If break statement is executed inside for loop then the "­els­e" part is skipped. Note that "­els­e" part is executed even if there is a continue statement.