The_Official_Raspberry_Pi_-_Beginner’s_Book_Vol1,_2018 (1)

98 THE OFFICIAL RASPBERRY PI BEGINNER'S GUIDE

Indentation is a powerful part of Python, and one of the most common reasons for a

program to not work as you expected. When looking for problems in a program, a process

known as debugging, always double-check the indentation – especially when you begin

nesting loops within loops.

Python also supports infinite loops, which run without end. To change your program from

a definite loop to an infinite loop, edit line 2 to read:

while True:

If you click the Run icon now, you’ll get an error: name 'i' is not defined. This is

because you’ve deleted the line which created and assigned a value to the variable i. To fix

this, simply edit line 3 so it no longer uses the variable:

print("Loop running!")

Click the Run icon, and – if you’re quick – you’ll see the ‘Loop starting!’ message followed by

a never-ending string of ‘Loop running’ messages (Figure 5-4). The ‘Loop finished!’ message

will never print, because the loop has no end: every time Python has finished printing the ‘Loop

running!’ message, it goes back to the beginning of the loop and prints it again.

5 Figure 5-4: An infinite loop keeps going until you stop the program

COUNT FROM ZERO

Python is a zero-indexed language – meaning it starts counting from

0, not from 1 – which is why your program prints the numbers 0

through 9 rather than 1 through 10. If you wanted to, you could change

this behaviour by switching the range (10) instruction to range (1, 11) –

or any other numbers you like.