Python for Everyone: The PreTeXt Interactive Edition

A for loop executes the same block of code “for each item in a sequence.”

In general, the length of the list determines the number of times that the loop repeats. The value of the variable x is selected from the list. Each time the loop runs, the next value from the list is assigned to x.

Before your for statement, you can assign your list to a variable and your program will run the same way:

In addition, for loops can be used with strings:

With strings, a for statement iterates over each character in the string. The length of the string determines how many times the body of the loop will run.

for loops can also handle many other data types, like tuples and dictionaries. Experiment on your own to see this in action!

The Python range function will generate a list of numbers. The range function can take up to three numbers as arguments.

The first line of output describes the range as a function, whereas the second line shows the actual range of values as a list by using the list function.

If the argument of the range function specifies a single number, like range(x), the first number listed will be 0, the last number listed will be x - 1, and there will be x numbers in the list.

If the argument of the range function specifies two numbers, like range(x, y), the first number listed will be x, the last number listed will be y - 1, and there will be y - x numbers in the list.

If the argument of the range function specifies three numbers, like range(x, y, z), the first number listed will still be x, just like the two parameter version. The third argument represents how much to increment the number by each time. To calculate how many numbers will be in the list, take the result of (y - x) / z and round it up to the nearest whole number.

The arguments to range must be integers, so range does not work with strings. However, if you wanted to print the letters A to Z in a loop, you could do something like this:

A more general looping structure is the while statement.

In the above code, the variable i is incremented by 1 each time the loop body is executed. Because the value of i steadily grows, the “loop condition” (the Boolean expression after the while) eventually becomes false when i = 3, which causes the loop body to stop executing.

A while loop has three parts that control the number of times it executes. The first part initializes the variable or condition, the second part tests whether the end has been reached, and the third part updates the variable or condition.

When writing a while loop, it's helpful to answer a few questions before you start:

For example, consider the code below. The add(n) function prompts the user for n numbers and returns the sum of these values. For example, when add(5) is called, the user is asked to input five numbers. If the user inputs 3, 1, 5, 2, and 4, the function would return the value 15.

Before the loop begins, the i variable, which counts how many times the loop runs, must be initialized. However, the total variable must also be initialized outside of the while loop, or else it would reset to 0 each time the loop ran.

The loop repeats n times, so the Boolean expression that must be true for the loop to continue is i < n.

Finally, for the loop to eventually end, i must be incremented, so we include the statement i = i + 1.

Making sure you answer these questions helps you write better (and less buggy) while loops.

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