Loop introduction to basic python tutorials

Loops are used to execute blocks repeatedly. From the selection structure in the last lecture, we have seen how to indent a block's membership. We'll use a similar notation for loops.

The for loop

The for loop needs to pre-set the number of loops (n), and then execute the statement that belongs to for n times.

The basic structure is

for  The element  in  The sequence : 

    statement

For example, let's edit a file called fordemo.py

for a in [3,4.4,'life']:

    print a

This loop is to take one element at a time from the table [3,4.4,'life'] (remember: a table is a sequence), assign that element to a, and then perform an operation under for (print).

Introduces a new Python function range() to help you set up tables.

idx = range(5)

print idx

You can see that idx is [0,1,2,3,4]

This function creates a new table. The elements of this table are all integers, starting at 0, and the next element is 1 larger than the previous one, all the way to the upper bound (not including the upper bound itself) written in the function.

(there's a lot more about range(), if you're interested, there's a change in range() in Python 3.)

For example,

for a in range(10):

    print a**2

The while loop

The use of while is

while  conditions :

    statement

While is going to loop through it until it's False

For example,

while i < 10:

    print i

    i = i + 1

Interrupt cycle

continue   #  During one execution of the loop, if encountered continue,  Then skip this execution and proceed to the next 
break      #  Stop executing the entire loop 
for i in range(10):

    if i == 2: 

        continue

    print i

 

When the loop executes to I = 2, if the condition holds, trigger continue, skip this execution (do not execute print), and proceed to the next execution (I = 3).

for i in range(10):

    if i == 2:        

        break

    print i

When the loop reaches I = 2, the if condition is true, the break is triggered, and the entire loop is stopped.

conclusion

Range ()

For element in sequence:

While conditions:

The continue

break