Lesson 6 - Loops in Python

In the previous exercise, Solved tasks for Python lesson 4-5, we've practiced our knowledge from previous lessons.

In the previous tutorial, Solved tasks for Python lesson 4-5, we learned about conditions in Python. In today's lesson, we're going to introduce you all to loops. After today's lesson, we'll have covered almost all of the basic constructs to be able to create reasonable applications.

Loops

The word loop suggests that something is going to be repeated. If we wanted a program to do something 100 times, we certainly wouldn't write the same code 100x. Instead, we'll put it in a loop. There are several types of loops. We'll explain how to use them, and of course, make practical examples.

The while loop

The while loop is the simplest loop in Python. It simply repeats the commands in the block while the condition is True. It can contain an optional else: branch which will be executed when the condition is no longer True. The syntax of the loop is the following:

while (condition):
    # commands
else:
    # commands

Let's create a simple example. Most of us know Sheldon from The Big Bang Theory. For those who don't, we'll simulate a situation where a guy knocks on his neighbor's door. He always knocks 3 times and then yells: "Penny!". Our code, without a loop, would look like this:

{PYTHON}
print("Knock")
print("Knock")
print("Knock")
print("Penny!")

However, using loops, we no longer have to copy the same code over and over:

{PYTHON}
i = 0
while (i < 3):
    print("Knock")
    i += 1
print("Penny!")

Console application
Knock
Knock
Knock
Penny!

We've introduced an auxiliary variable named i. The loop will run through 3 times. At the very beginning, i is set to zero, the loop then prints "Knock" and increases i by one. It continues in the same way with values one and two. Once i hits three, the condition i < 3 is no longer True and the loop terminates. Loops have the same indentation rules as conditions. Now, we can simply change value 3 to 10 in the loop declaration. The command will execute 10x without adding anything else to it. Surely, you now see that loops are a very powerful tool.

Now, let's put the variable incrementation to use. We'll print the numbers from one to ten. We want our output to be separated by spaces rather than lines. Therefore, we'll specify the end parameter for the print() function and set the space character to it.

{PYTHON}
i = 1
while (i <= 10):
    print(i, end = ' ')
    i += 1

The result:

Console application
1 2 3 4 5 6 7 8 9 10

As you can see, the i variable has a different value after each iteration (iteration is one step of the loop). Notice that this time, the loop doesn't start from zero because we want the initial value to be 1 and the last one to be 10. Also, keep in mind that almost everything starts from zero in programming (we'll find out why later).

Sequences

To be able to work with other loops, we'll have to get familiar with sequences first. Sequences in Python are container data structures, i.e. they're variables that can contain multiple items. We've already met and used a container structure (strings). Strings are sequences of characters.

The for loop

If we wanted to work with all of the elements in a sequence, we'd use the for loop. This loop always performs a fixed number of steps which is equal to the number of the elements in a sequence. Its syntax is the following:

for item in sequence:
    # commands
else:
    # commands

The for loop will go over all the items in a sequence. In each iteration (each step of the loop), the current item will be copied to the variable (the item variable in the example) and execute the commands.

Let's try it out on a string sequence. We'll print the string variable's characters out on separate lines:

{PYTHON}
word = "Hello"
for character in word:
    print(character)

The result:

Console application
H
e
l
l
o

The range() function

You might have noticed that the for loop's syntax is a bit nicer than the while loop's. We can also use it easily to loop over numeric indexes which we generate using the range() function. Let's print the numbers from 1 to 10 again, this time, using the for loop:

{PYTHON}
for i in range(1, 11):
    print(i, end = ' ')

The result:

Console application
1 2 3 4 5 6 7 8 9 10

As you can see, the range() function generated a sequence of numbers from 1 to 10. There are 3 ways to call the range() function:

• range(n) - Generates a sequence of numbers from 0 to n - 1.
• range(m, n) - Generates a sequence of numbers from m to n - 1 (This is why we specified 11 in the example above).
• range(m, n, i) - Generates a sequence of numbers from m and every i-th number to n - 1.

The for loop would be even faster, in this case, than the while loop (if used for the same purpose). The while loop is used for slightly different things. Typically, we call a method that returns a logical True/False value in the while loop's parentheses. We could rewrite the original for-loop example to use the while loop like this:

{PYTHON}
for i in range(3):
    print("Knock")
print("Penny!")

Console application
Knock
Knock
Knock
Penny!

Nesting loops

Now, let's print a simple multiplication table that contains multiples of numbers from 1 to 10. All we need to do is to declare a loop that goes from 1 to 10 and multiply the control variable by the current multiplier:

{PYTHON}
print("Here's a simple multiplication table using loops:")
for i in range(1, 11):
    print(i, end = " ")
print()
for i in range(1, 11):
    print(i * 2, end = " ")
print()
for i in range(1, 11):
    print(i * 3, end = " ")
print()
for i in range(1, 11):
    print(i * 4, end = " ")
print()
for i in range(1, 11):
    print(i * 5, end = " ")
print()
for i in range(1, 11):
    print(i * 6, end = " ")
print()
for i in range(1, 11):
    print(i * 7, end = " ")
print()
for i in range(1, 11):
    print(i * 8, end = " ")
print()
for i in range(1, 11):
    print(i * 9, end = " ")
print()
for i in range(1, 11):
    print(i * 10, end = " ")

Console application
Simple multiplication table using loops:
1 2 3 4 5 6 7 8 9 10
2 4 6 8 10 12 14 16 18 20
3 6 9 12 15 18 21 24 27 30
4 8 12 16 20 24 28 32 36 40
5 10 15 20 25 30 35 40 45 50
6 12 18 24 30 36 42 48 54 60
7 14 21 28 35 42 49 56 63 70
8 16 24 32 40 48 56 64 72 80
9 18 27 36 45 54 63 72 81 90
10 20 30 40 50 60 70 80 90 100

Notice the empty print() calls which are used to skip a line. The program works nicely, but we still wrote a lot. Honestly, we could break it down even more since all it does is repeat 10 times while increasing the multiplier. What we'll do is nest the loops (put one inside the other):

{PYTHON}
print("Here's a simple multiplication table using nested loops:")
for j in range(1, 11):
    for i in range(1, 11):
        print(i * j, end = " ")
    print()

Makes a big difference, doesn't it? Obviously, we can't use i in both loops since they are nested and would count as a re-declaration. The variable j for the outer loop goes through from 1 to 10. During each iteration of the loop, another inner loop with the variable i is executed. As we already know, it writes the multiples and is multiplied by the variable j. After each time the inner loop terminates, we also have to break the line using print(). You may try to align the printed rows using the ljust() method so the numbers will be printed nicely in columns.

Let's make one more program where we'll practice working with an outer variable. The application will be able to calculate an arbitrary power for an arbitrary number. Of course, we could just use the ** operator, but we'll calculate it on our own for completeness' sake:

{PYTHON}
print("Exponent calculator")
print("===================")
a = int(input("Enter the base: "))
n = int(input("Enter the exponent: "))
result = a
for i in range(n - 1):
    result = result * a

print("Result: %d" % (result))
print("Thank you for using our exponent calculator")

I'm sure we all know how powers (exponents) work. Just to be sure, let me remind that, 2^3 = 2 * 2 * 2. Therefore, we compute a^n by multiplying the number a by the number a for n - 1 times. Of course, the result must be stored in a variable. Initially, it'll have a value of a and this value will be gradually multiplied during the loop. We can see that our variable result in the loop body is normally accessible. If, however, we create a variable in a loop body, this variable will be no longer accessible after the loop terminates.

Console application
Exponent calculator
==========================
Enter the base: 2
Enter the exponent: 3
Result: 8
Thank you for using our exponent calculator

Now, we know what some practical uses of the for loop are. Remember that it has a fixed amount of iterations.

Enhancing our calculator

Let's take our calculator from previous lessons and improve it a little bit. We'll add the ability to enter more math problems. The program will not end immediately. Instead, it'll ask the user whether they wish to calculate another math problem. Here is the original version of the code:

{PYTHON}
print("Welcome to calculator!")
a = float(input("Enter the first number: "))
b = float(input("Enter the second number: "))
print("Choose one of the following operations:")
print("1 - addition")
print("2 - subtraction")
print("3 - multiplication")
print("4 - division")
option = int(input(""))

if (option == 1):
    result = a + b
elif (option == 2):
    result = a - b
elif (option == 3):
    result = a * b
elif (option == 4):
    result = a / b
if option > 0 and option < 5:
    print("result: %f" % (result))
else:
    print("Invalid option")
print("Thank you for using our calculator.")

Now, we'll put almost all the code into a while loop. Our condition will be that the user enters "yes", and we'll check the content of the variable goOn. This variable is set to "yes" in the beginning because the program has to begin somehow. Then, we'll assign the user's choice to it:

print("Welcome to calculator!")
goOn = "yes"
while (goOn == "yes"):
    a = float(input("Enter the first number: "))
    b = float(input("Enter the second number: "))
    print("Choose one of the following operations:")
    print("1 - addition")
    print("2 - subtraction")
    print("3 - multiplication")
    print("4 - division")
    option = int(input(""))

    if (option == 1):
        result = a + b
    elif (option == 2):
        result = a - b
    elif (option == 3):
        result = a * b
    elif (option == 4):
        result = a / b
    if option > 0 and option < 5:
        print("result: %f" % (result))
    else:
        print("Invalid option")
    goOn = input("Would you like to make another calculation? [yes/no]")
print("Thank you for using our calculator.")

Console application
Welcome to our calculator
Enter the first number: 12
Enter the second number: 128
Choose one of the following operations:
1 - addition
2 - subtraction
3 - multiplication
4 - division
1
Result: 140.000000
Would you like to make another calculation? [yes/no]
yes
Enter the first number
-10.5
Enter the second number:

Our application can now be used multiple times and is almost complete. Note: this is one of the few codes that cannot be run online since our user bot would have to repeat inputs. In the next lesson, Solved tasks for Python lesson 6, we'll show you how to work with arrays. You've already learned quite a lot! Nothing better than a little noggin exercise, right?

In the following exercise, Solved tasks for Python lesson 6, we're gonna practice our knowledge from previous lessons.

 

Did you have a problem with anything? Download the sample application below and compare it with your project, you will find the error easily.