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Control Flow 2

Loops

while statement: the conditional loop

You can make a block of code execute over and over again using a while statement. The code in a while clause will be executed as long as the while statement’s condition evaluates to True.

In code, a while statement always consists of the following:

  • The while keyword
  • A condition (that is, an expression that evaluates to True or False)
  • A colon (:)
  • Starting on the next line, an indented block of code

You can see that a while statement looks similar to an if statement. The difference is in how they behave. At the end of an if statement, the program execution continues after the if statement. But at the end of a while statement, the program execution jumps back to the start of the while statement.

Let’s look at an if statement and a while loop that use the same condition and take the same actions based on that condition.

Example

i = 0
if i < 5:
    print("Hello World")
    i = i + 1

Output

Hello World

Flowchart

Example

i = 0
while i < 5:
    print("Hello World")
    i = i + 1

Output

Hello World
Hello World
Hello World
Hello World
Hello World

Flowchart

for statement: the sequence loop

for c in "Hello":
    print(c)

Output

H
e
l
l
o

The "List" and "Tuple" Sequence Data Types

So far we established string data type, which consists of a sequence of Unicode symbols. Going a step further, we can generalize the concept of sequences and obtain two new data types: Lists and Tuples.

Lists

Similar to a string, a list is a sequence of values, however instead of being limited to single unicode symbols, the elements can be of any size and data type. Lists may contain items of different data types, but usually the items all have the same type.

The literal expression of the List data type consists of comma-separated values, wrapped in square brackets ([, ])

For example, let a, b and c be expressions of any data type. A List containing a, b and c in this order looks like this:

[a, b, c]

Just like other expressions, we can use variables to store a List, e.g. a shopping list:

groceries = ["Apples", "Oranges", "Cheddar Cheese", "Milk", "Bread"]

Just like letters/symbols in a String, each item in our List is assigned to an Index:

Element "Apples" "Oranges" "Cheddar Cheese" "Milk" "Bread"
Index 0 1 2 3 4

We can use the same sequence operations we learned earlier with strings:

>>> ["zero", "one", "two"] + ["three", "four"]
["zero", "one", "two", "three", "four"]

Tuples

A Tuple ist almost identical to a List, however it is immutable. So unlike a List, you cannot add or remove elements to or from a tuple after the fact.

Creating a tuple is done the same way, but instead of using square ([, ]), we use parantheses ((, )):

>>> foo = ("the meaning of life", 42)
>>> foo[0]
'the meaning of life'
>>> foo[1]
42

The range() expression

Very commonly, you need to execute a block of code a specific number of times. For example we want to output all numbers from 0 to 10. With our current knowledge, we have two ways to accomplish:

  1. Using a while loop with a counter variable:

    i = 0
while i <= 10:
    print(i)
    i = i + 1

  2. Using a for loop with a List Literal Expression:

    for i in [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]:
    print(i)
    i = i + 1

But there is a better way in Python to do this: The range() expression. In fact, there are three different ways to use range():

Syntax Description Example Result
range(stop) Generates numbers from 0 up to, but not including, stop range(5) [0, 1, 2, 3, 4]
range(start, stop) Generates numbers from start up to, but not including, stop range(2, 7) [2, 3, 4, 5, 6]
range(start, stop, step) Generates numbers from start up to, but not including, stop with a step of step range(1, 10, 2) [1, 3, 5, 7, 9]

Using it in conjunction with the for loop, we can achieve the same goal with the following:

for i in range(0, 11):
    print(i)

Output

0
1
2
3
4
5
6
7
8
9
10

Instead of just counting up numbers, we can also simply repeat a task x times:

for i in range(0, 5):
    print("Hello World")

Output

Hello World
Hello World
Hello World
Hello World
Hello World

Loop Control using continue and break

continue

continue statements are exclusively used inside loops. When the program execution reaches a continue statement, the program execution immediately jumps back to the start of the loop and reevaluates the loop’s condition. (This is also what happens when the execution reaches the end of the loop.)

for i in range(1,10):
    if i == 7:
        continue
    print(i)

Output

0
1
2
3
4
5
6
8
9

Notice how the output does not contain 7. Since we the if condition evaluates to True for i = 7, the continue statement is executed. As such, we jump back to the beginning of the loop and continue with the next number (8). This results in the print(i) statement being skipped.

The same works for while loops. Using continue inside a while loop skips over the rest of the code block and goes back to the while loop's condition, checks it, and executes the code block again if the condition evaluated to True. If the condition evaluates to False, we exit the loop and the rest of the program is executed as usual.

The following script creates the same output as shown above in the for loop example:

i = 0
while i < 10:
    if i == 7:
        i = i + 1
        continue
    print(i)
    i = i + 1

break

Like continue statements, break statements are used inside loops. When the execution reaches a break statement, it immediately stops and exits the loop. Unlike continue, the execution does not jump back to beginning of the loop, but instead skips over the rest of the code block and leaves the loop entirely.

A classical example is a while True loop, which would run indefinitely, since the condition is always true. However, with the break statement we can use an if statement inside the while loop to exit the loop once we reach the number 7.

i = 0
while True:
    if i == 7:
        break
    print(i)
    i = i + 1

Output

0
1
2
3
4
5
6

Tip

If you happen to run into an infinite loop, you can kill the program by pressing Ctrl+C inside the Terminal.