Example use of "continue" statement in Python?

Question

The definition of the continue statement is:

The continue statement continues with the next iteration of the loop.

I can't find any good examples of code.

Could someone suggest some simple cases where continue is necessary?

Thallic

Answer 1

Here's a simple example:

for letter in 'Django':    
    if letter == 'D':
        continue
    print("Current Letter: " + letter)

Output will be:

Current Letter: j
Current Letter: a
Current Letter: n
Current Letter: g
Current Letter: o

It skips the rest of the current iteration (here: print) and continues to the next iteration of the loop.

Lauro

I've been thinking for the past three or four days, why would I ever need a 'continue' statement and then I run into this post which in turn helps me with some QA — many thanks! – Grogshop

same here, when we write python script (for cron job) and iterating on large values and some exception occurs then cron job will be stopped and you only be able to know that the next day, it really helps a lot in those case, Yes we can write send mail feature on exception but still the execution will be stopped. – Lauro

This illustrates what continue does but it's not overly useful, when you could do if letter != 'D': print 'Current Letter:', letter – Odom

I would have chosen Pray / remove r, or Fast / remove s. – Boling

Answer 2

I like to use continue in loops where there are a lot of contitions to be fulfilled before you get "down to business". So instead of code like this:

for x, y in zip(a, b):
    if x > y:
        z = calculate_z(x, y)
        if y - z < x:
            y = min(y, z)
            if x ** 2 - y ** 2 > 0:
                lots()
                of()
                code()
                here()

I get code like this:

for x, y in zip(a, b):
    if x <= y:
        continue
    z = calculate_z(x, y)
    if y - z >= x:
        continue
    y = min(y, z)
    if x ** 2 - y ** 2 <= 0:
        continue
    lots()
    of()
    code()
    here()

By doing it this way I avoid very deeply nested code. Also, it is easy to optimize the loop by eliminating the most frequently occurring cases first, so that I only have to deal with the infrequent but important cases (e.g. divisor is 0) when there is no other showstopper.

Logging

So "continue" is used mostly for readability by reducing indentation. I am wondering if "continue" adds any expressive power. – Thallic

Using continue in this fashion is similar to using GOTO. However, this is the right way to use GOTO. – Knucklebone

@Thallic I don't believe there is any use of continue (or break, for that matter) that cannot be reproduced without it, although not having it can be expensive in run-time. – Sultry

Answer 3

Usually the situation where continue is necessary/useful, is when you want to skip the remaining code in the loop and continue iteration.

I don't really believe it's necessary, since you can always use if statements to provide the same logic, but it might be useful to increase readability of code.

Oophorectomy

note that using if <condition>: continue rather than if not <condition>: ... avoids a level of indentation that would otherwise be needed if it was written without it. – Fouquet

I was about to post a similar qn to JohnG but then found that he had already asked what I wanted to know. Reading through all the responses here has helped, but I still need to be sure about one thing - so is it that whenever we use a continue statement, we are essentially jumping out of a conditional testing section and allowing the iteration of the loop to proceed on to the next iteration? It isn't apparent to me how this would be better than using else. Is it just all about improved readability and run-time performance? – Manard

"I don't believe it's necessary" ... well, python creators thought otherwise. – Ethnocentrism

Answer 4

import random  

for i in range(20):  
    x = random.randint(-5,5)  
    if x == 0: continue  
    print 1/x  

continue is an extremely important control statement. The above code indicates a typical application, where the result of a division by zero can be avoided. I use it often when I need to store the output from programs, but dont want to store the output if the program has crashed. Note, to test the above example, replace the last statement with print 1/float(x), or you'll get zeros whenever there's a fraction, since randint returns an integer. I omitted it for clarity.

Inwards

Answer 5

Some people have commented about readability, saying "Oh it doesn't help readability that much, who cares?"

Suppose you need a check before the main code:

if precondition_fails(message): continue

''' main code here '''

Note you can do this after the main code was written without changing that code in anyway. If you diff the code, only the added line with "continue" will be highlighted since there are no spacing changes to the main code.

Imagine if you have to do a breakfix of production code, which turns out to simply be adding a line with continue. It's easy to see that's the only change when you review the code. If you start wrapping the main code in if/else, diff will highlight the newly indented code, unless you ignore spacing changes, which is dangerous particularly in Python. I think unless you've been in the situation where you have to roll out code on short notice, you might not fully appreciate this.

Music

Answer 6

def filter_out_colors(elements):
  colors = ['red', 'green']
  result = []
  for element in elements:
    if element in colors:
       continue # skip the element
    # You can do whatever here
    result.append(element)
  return result

  >>> filter_out_colors(['lemon', 'orange', 'red', 'pear'])
  ['lemon', 'orange', 'pear']

Trouper

But what does the continue statement add here? It could be eliminated by using element not in colors, and the code would be just as readable. – Jonquil

Answer 7

Let's say we want to print all numbers which are not multiples of 3 and 5

for x in range(0, 101):
    if x % 3 ==0 or x % 5 == 0:
        continue
        #no more code is executed, we go to the next number 
    print x

Stacistacia

You could also just use if x %3 == 0 or x % 5 == 0:, pass, else:, print x – Worthwhile

Correct way is to either use continue or the not operator. Why unnecessarily use pass? – Stacistacia

I like your way better (hence the upvote). I came to this page trying to sort out if there is ever a need to use continue. My conclusion is that there is never a need, but in some cases (like this one) the code is more readable using continue. It's a very good example. – Worthwhile

Answer 8

It is not absolutely necessary since it can be done with IFs but it's more readable and also less expensive in run time.

I use it in order to skip an iteration in a loop if the data does not meet some requirements:

# List of times at which git commits were done.
# Formatted in hour, minutes in tuples.
# Note the last one has some fantasy.
commit_times = [(8,20), (9,30), (11, 45), (15, 50), (17, 45), (27, 132)]

for time in commit_times:
    hour = time[0]
    minutes = time[1]

    # If the hour is not between 0 and 24
    # and the minutes not between 0 and 59 then we know something is wrong.
    # Then we don't want to use this value,
    # we skip directly to the next iteration in the loop.
    if not (0 <= hour <= 24 and 0 <= minutes <= 59):
        continue

    # From here you know the time format in the tuples is reliable.
    # Apply some logic based on time.
    print("Someone commited at {h}:{m}".format(h=hour, m=minutes))

Output:

Someone commited at 8:20
Someone commited at 9:30
Someone commited at 11:45
Someone commited at 15:50
Someone commited at 17:45

As you can see, the wrong value did not make it after the continue statement.

Cherlynchernow

if can only do what continue does if all the code is contained in a block. continue skips code that is even outside of the if block. – Recusancy

why is hour 24-inclusive but minute is 60-exclusive? – Resistor

no particular reason, just an example, feel free to edit – Cherlynchernow

Answer 9

Here is a very good visual representation about continue and break statements

source

Abebi

good attempt. I trust this require bit more edits such as conditions. – Landscapist

Answer 10

For example if you want to do diferent things depending on the value of a variable:

my_var = 1
for items in range(0,100):
    if my_var < 10:
        continue
    elif my_var == 10:
        print("hit")
    elif my_var > 10:
        print("passed")
    my_var = my_var + 1

In the example above if I use break the interpreter will skip the loop. But with continueit only skips the if-elif statements and go directly to the next item of the loop.

Interlunar

your example will never print anything, as the first condition is always true. assuming an initial value for my_var of 0. – Fouquet

I simply adapted an example of an old code of mine. The original value of the var is not 0 and the increasing part was added for convenience as its original place wasn't there. Since it is an example and its value doesn't really matters I guess it's ok. – Interlunar

It's not OK. It's not a good advertisement for using continue. – Ramiah

And the first elif should be an if. The code just doesn't give the appearance that you know what you are doing. – Ramiah

this is a bad example – Thiol