Loops are one essential part of programming.
In Python we have 2 kinds of loops: while loops and for loops.
while loops are defined using the
while keyword, and they repeat their block until the condition is evaluated as
condition = True while condition == True: print("The condition is True")
This is an infinite loop. It never ends.
Let’s halt the loop right after the first iteration:
condition = True while condition == True: print("The condition is True") condition = False print("After the loop")
In this case, the first iteration is ran, as the condition test is evaluated to
True, and at the second iteration the condition test evaluates to
False, so the control goes to the next instruction, after the loop.
It’s common to have a counter to stop the iteration after some number of cycles:
count = 0 while count < 10: print("The condition is True") count = count + 1 print("After the loop")
for loops we can tell Python to execute a block for a pre-determined amount of times, up front, and without the need of a separate variable and conditional to check its value.
For example we can iterate the items in a list:
items = [1, 2, 3, 4] for item in items: print(item)
Or, you can iterate a specific amount of times using the
for item in range(04): print(item)
range(4) creates a sequence that starts from 0 and contains 4 items:
[0, 1, 2, 3].
To get the index, you should wrap the sequence into the
items = [1, 2, 3, 4] for index, item in enumerate(items): print(index, item)
Break and continue
for loops can be interrupted inside the block, using two special keywords:
continue stops the current iteration and tells Python to execute the next one.
break stops the loop altogether, and goes on with the next instruction after the loop end.
The first example here prints
1, 3, 4. The second example prints
items = [1, 2, 3, 4] for item in items: if item == 2: continue print(item)
items = [1, 2, 3, 4] for item in items: if item == 2: break print(item)