An introduction to the use of Iteration in Python
Using loops in computer programming allows us to automate and repeat similar tasks multiple times. In this tutorial, we’ll be covering Python’s for loop.
A for loop implements the repeated execution of code based on a loop counter or loop variable. This means that for loops are used most often when the number of iterations is known before entering the loop, unlike while loops which are conditionally based.
In Python, for loops are constructed like so:
for [iterating variable] in [sequence]: [do something]
The something that is being done will be executed until the sequence is over.
Let’s look at a for loop that iterates through a range of values:
for i in range(0,5): print(i)
When we run this program, the output looks like this:
0 1 2 3 4
This for loop sets up i as its iterating variable, and the sequence exists in the range of 0 to 5.
Then within the loop we print out one integer per loop iteration. Keep in mind that in programming we tend to begin at index 0, so that is why although 5 numbers are printed out, they range from 0-4.
You’ll commonly see and use for loops when a program needs to repeat a block of code a number of times.
One of Python’s built-in immutable sequence types is range(). In loops, range() is used to control how many times the loop will be repeated.
When working with range(), you can pass between 1 and 3 integer arguments to it:
for i in range(6): print(i)
In the program above, the stop argument is 6, so the code will iterate from 0-6 (exclusive of 6):
0 1 2 3 4 5
Next, we’ll look at range(start, stop), with values passed for when the iteration should start and for when it should stop:
for i in range(20,25): print(i)
Here, the range goes from 20 (inclusive) to 25 (exclusive), so the output looks like this:
20 21 22 23 24
The step argument of range() is similar to specifying stride while slicing strings in that it can be used to skip values within the sequence.
With all three arguments, step comes in the final position: range(start, stop, step). First, let’s use a step with a positive value:
for i in range(0,15,3): print(i)
In this case, the for loop is set up so that the numbers from 0 to 15 print out, but at a step of 3, so that only every third number is printed, like so:
0 3 6 9 12
We can also use a negative value for our step argument to iterate backwards, but we’ll have to adjust our start and stop arguments accordingly:
for i in range(100,0,-10): print(i)
Here, 100 is the start value, 0 is the stop value, and -10 is the range, so the loop begins at 100 and ends at 0, decreasing by 10 with each iteration. We can see this occur in the output:
100 90 80 70 60 50 40 30 20 10
When programming in Python, for loops often make use of the range() sequence type as its parameters for iteration.
Write a program that prints your name 5 times using a FOR loop.
Write a program that asks the user for a number and prints what they type that number of times in a FOR loop.
Create a program that loops from 100 to 200 and prints the loop number each time.
Thanks [name] from [whichclass][location], Mr McG will get your message…"
[message]"…and will get back to you ASAP via [email]. Catch you soon.
Have a great day!!
K McGuinness - 2018
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