Loops allow us to iterate over a sequence of items, facilitating operations such as summing a list of values, modifying a list of names, or conducting searches in a specific manner. In Python, loops are a fundamental concept and are utilized frequently in data science. Python primarily offers two types of loops: for and while. This lesson focuses on the for loop, highlighting its utility in iterating over collections like lists and dictionaries, while the next lesson will introduce the while loop.

A for loop sequentially iterates over each element in a specified sequence.

During each iteration of a for loop, a loop variable (also known as an iterator) is automatically assigned the value of the current item in the sequence. The loop continues until every item in the sequence has been used. Here is the basic syntax for a for loop:

for i in sequence: # run code # close indent to finish for loop

We can use a for loop with any object that can be sequenced, including lists, dictionaries, and dataframes. The loop is initiated with the for keyword, followed by the specification of the iterator - in this case i - and the sequence to iterate over - in this case sequence - with code to run for each iteration within curly brackets. Following this, indented lines of code delineate the actions to be performed during each iteration, utilizing the current value of the iterator. The loop cycles through each element in the sequence, executing the specified actions each time, and concludes when there are no more elements to process.

Looping over lists

Let's see this in action with a list:

Or if we would like to iterate over a range of integers we may more succinctly use the range function to generate a list of numbers for us. For example range(1, 4) will make a list of integers from 1 through 3, which is the same as writing [1,2,3], or range(1, 101) would be the same as [1,2,3,...,98,99,100]

In these examples we have iterated over the elements in the list, but we can also iterate over both the indices and elements in the list using the enumerate function. This function returns a tuple of the index and element at that index, which we can unpack into two variables. Note that the iterator starts counting from 0, as is standard in Python.

Looping over strings

A string has a similar structure to a list, in that it is a sequence of characters. As such, we can even iterate over a string in the same way we would a list! In this case, the iterator will be assigned each character in the string in turn.

We further spice things up by printing out the characters of the string in a staircase format by multiplying a space string by the iterator before the character. Furthermore, instead of using the enumerate function, we initialize an index variable ourselves, starting at 0, then incrementing it by one each iteration using the += operator.

Looping over dictionaries

We can also iterate over dictionaries. Caution should be taken however, as elements in these are unordered, meaning we may not always iterate over them in the same order every time. Iterating over a dict will return only the keys in the dict. Using a method inherit to all dicts items, we can unpack both the key and value at once. In this case, we have two options. If we use only one variable, then it will become a tuple of the unpacked elements. If we provide two variables, the interpreter will unpack the key and value into each respectively.

Skipping and exiting for loops

While iterating over a sequence in a loop, there may be instances where we wish to skip certain values or exit the loop before it naturally concludes. The continue statement allows us to skip specific iterations, often employed in tandem with a conditional statement to dictate which elements should be bypassed. Whenever the continue statement is executed, the loop foregoes the remaining lines in the current iteration and returns to the beginning to start the next iteration. For instance, in the example we'll look at, the for loop will skip the rest of the commands for the current iteration when val > price_limit and commence the next cycle.

On the other hand, if the intention is to exit the loop prematurely, this can be achieved using the break command. Like the continue command, it is often utilized alongside a conditional statement to determine the specific scenario where the loop should terminate. When the break statement is encountered, the loop ends instantly, and the program moves to execute the line of code outside of the loop. In our forthcoming example, the for loop will terminate altogether when the condition sum(purchasable) >= purchase_limit is met, avoiding any further iterations.