How Many Sevens?
Required Materials
OnlineGDB (login required)
Word processing software
You’ve learned about lists and defining your own functions. Now, you will put those skills together by writing a program that will print how many times the number 7 appears in a list.
Step 1: Practice
But before we do that, let’s visualize a bit of code to be sure you understand how it works. We are going to run the code in a visualizer. A visualizer shows you not just the output of the code but also what is happening step by step.
Go to Python Tutor Visualizer.
Then, type in this code:
my_list = [1, 2, 3, 4, 5]
for x in range(len(my_list)):
print(my_list[x])
Now, click on the Visualize Execution button. You will see a screen that has your code on the left and a blank area on the right.
Press the Next > button to begin stepping through the program. On the right, you will see a visual representation of the fact that the code has created a list; you will also see the items in the list as well as their index numbers.
Press the Next > button until you have finished the program. Did you notice how the red and green arrows showed you which lines of the program had just executed and which ones were about to execute during each step of the way through the program?
Now, let’s review the program itself. The first line of this code creates a list called my_list . Then, the second line is the command to iterate over the list using a variable that we call x . We did this iteration by making the range equal to the length of the list. For each iteration, it printed the item from the list that had the same index as the iterating variable. Now, you should be comfortable with iterating over a list and accessing each item in the list as you iterate over the list by using the incrementing variable as the index number.
Step 2: Program
Now, we’re ready to program! Leave the visualizer and go to OnlineGDB to write your program. Here is the pseudocode for your program:
Define a function called “seven_checker” that will do the following things:
Iterate over a list.
Check to see whether the number 7 is in the list.
Print how many times the number 7 appears in the list.
Here is the Python code for the program:
def seven_checker(lst):
count = 0
for item in lst:
if item == 7:
count += 1
print("The number 7 appears", count, "times in the list.")
# Example usage
my_list = [1, 7, 3, 7, 5, 7]
seven_checker(my_list)