Question

Write the code to produce a for loop that counts down from 20 to 0 by 2’s and prints the number each time it goes through the loop. (python)

Answer 1

Explanation:

Count-controlled for loop (Three-expression for loop)

This is by far the most common type. This statement is the one used by C. The header of this kind of for loop consists of a three-parameter loop control expression. Generally it has the form:

for (A; Z; I)

A is the initialisation part, Z determines a termination expression and I is the counting expression, where the loop variable is incremented or dcremented. An example of this kind of loop is the for-loop of the programming language C:

for (i=0; i <= n; i++)

This kind of for loop is not implemented in Python!

Numeric Ranges

This kind of for loop is a simplification of the previous kind. It's a counting or enumerating loop. Starting with a start value and counting up to an end value, like for i = 1 to 100

Python doesn't use this either.

Vectorized for loops

They behave as if all iterations are executed in parallel. This means, for example, that all expressions on the right side of assignment statements get evaluated before the assignments.

Iterator-based for loop

Finally, we come to the one used by Python. This kind of a for loop iterates over an enumeration of a set of items. It is usually characterized by the use of an implicit or explicit iterator. In each iteration step a loop variable is set to a value in a sequence or other data collection. This kind of for loop is known in most Unix and Linux shells and it is the one which is implemented in Python.

Example of a simple for loop in Python:

>>> languages = ["C", "C++", "Perl", "Python"]  

>>> for x in languages:

...     print(x)

...  

C

C++

Perl

Python

>>>