Loops in programming allow executing a block of code multiple times over and over until a certain condition is met. They are essential in automating repetitive tasks, like creating, reading, and processing lists and other iterable objects. Python has two primary types of loops, for loops and while loops. Furthermore, Python also provides mechanisms like break and continue keywords for controlling the loop behaviours.

Let’s see how to define and use loops with the break and continue keywords.

For Loops

You need the following to define a for loop in Python:

  • a variable,
  • in keyword, and
  • an iterable object, like a list or tuple.
for variable in iterable:
    # code block to execute

The number of iterations of a for loop, the number of times the loop is executed, is determined by the length of the iterable. The loop automatically assigns a value for the variable using the next value from the iterable starting with the first item at the first iteration. The variable then can be used in the indented block of code. The indented code is repeatedly executed during each execution of the loop.

E.g., the code below iterates over the items in a shopping basket and prints them.

Generally, we don’t need to keep track of the index or count the length of the iterable when using loops since the for loop keeps track of the items in an iterable for us.

Range

Sometimes you may need to explicitly control a loop to run it for a certain number of iterations or you may just need a sequence of integers in your loop. Python’s range allows us to do just that. range is a type of iterable that defines a range of numbers as its name suggests. For example, range(10) defines a range of numbers 0–9 with length 10.

  • range(stop): a range of int numbers from 0 to stop-1.
  • range(start, stop): a range of integers from start to stop-1, where stop must be larger than or equal to start.
  • range(start, stop, step): same as the above, but with a custom step size (default step is 1). For a negative step size, stop should be less than start.

Ranges can be easily converted to lists and tuples by calling the list() and tuple() constructors. E.g., list(range(5)) creates a list of numbers 0 to 4.

Dictionaries, Enumerate, and Zip

Iterating over list and tuple objects gives us their items’ values. When you iterate over dict objects you iterate over their keys.

To access the values, you can use the keys as indices with info[key]. Alternatively, you can use .values() or .items() methods to access the values, or key-value pairs, respectively.

for value1, value2, ... in iterable allows iterating over nested iterables, like list of lists, tuple of tuples, etc. In the above, dict items is a nested iterable of key-value pairs, [(key1, value1), (key2, value2), ...].

You can use the latter feature of the for loop together with Python’s enumerate and zip functions to iterate over multiple iterables.

  • enumerate(iterable) : enumerates the iterable by adding a counter for each item in an iterable.

Let’s enumerate the items in the shopping basket from the first example.

  • zip(iterable1, iterable2, ...): combines multiple interables into a single iterable with items from multiple input iterables iterable1, iterable2, etc. The length of the zipped iterable object is determined by the input with the shorted length.

You can also use zip together with enumerate to simultaneously count and zip items during iteration, for id, (val1, val2, ...) in enumerate(zip(...)):.

Controlling Loops with continue and break Keywords

Python provides control over the loop using continue and break keywords.

  • continue: continues to the next iteration and ignores the rest of the code block below.
  • break : breaks out of the loop. Stops the loop and continues to the code outside the loop.

While Loops

A while loop is similar to the for loop above. Instead of executing for a given number of iterations like the for loop, the while loop executes while a given condition is true.

Similar to the for loop, you can use break and continue to control the while loop’s execution.

Exercises

E1: Help me fix the code below.

  • Run the code and see what it does.
  • Fix the code so that it prints the name of the item with its index in the basket.
  • The code should print the total number of items in a list in last line.

E2: What happens when you comment out or remove one of the lines that increments the counter counter += 1 inside the while loop below?

  • Run the code.
  • Reload this webpage to stop the execution of the code cell if gets stuck.
  • Uncomment the first line with the counter and comment out the second counter += 1. Run the code again.

As you may have noticed, the while loop never terminates and continues on until you reload the page and stop the Python interpreter. This is an example of an infinite while loop. We get it when the while loop does not terminate.

Usually you want to terminate your loops, but in some cases infinite loops may be useful. E.g., this website is served using an infinite loop. The same is true for your browser. It is running continuously in an infinite loop while waiting for your input actions!