6.3. More on range
¶
In one example from the last section, we used range
to make the loop run
four times:
1 2 3  for num in range(4):
print(num)
print("Hello" * num)

For each iteration, the variable num
took on a new value (0, 1, 2, or 3).
What if we wanted the loop to use the sequence 1, 2, 3, 4 instead?
6.3.1. Set Start and End Values¶
Whenever we use range(value)
in a for
statement, Python always begins
counting with 0. To start counting at a different number, we need to include
that value inside the ()
in addition to a stop value.
Instead of range(value)
, a more detailed version of the keyword is
range(start_value, end_value)
. The starting value is included in the
counting for the loop, but the end value is NOT.
If we replaced line 1 in the code above with for num in range(1, 5)
, then
the loop variable num
would take values of 1, 2, 3, and 4.
Try It
What should be the start and end values in range
to print the following?
HelloHello
HelloHelloHello
HelloHelloHelloHello
Cool, range(start, stop)
allows us to count upwards from any number we
want.
What if we want to count DOWN from one value to another? Also, what if we want to change the loop variable by more than a single unit each iteration?
6.3.2. Set a Step Value¶
Suppose we want our loop variable to only be a set of even numbers (e.g. 0, 2, 4, 6…). We want to begin counting at 0 and then increase the loop variable by 2 units instead of 1.
To make this happen, we need to add one more value inside range
. This is
called the step value, range(start_value, end_value, step_value)
.
Examples
To count from 0 to 20 by 2’s, use:
for num in range(0, 21, 2)
To count up by 5’s, use:
for num in range(0, 21, 5)
We can even count DOWN from a higher number to a lower one. The step value just needs to be negative:
for num in range(50, 39, 1) # Counts from 50 down to 40
Note
For range()
, the start and step values are OPTIONAL.
6.3.3. Try It!¶
In the editor below, change the values inside of range
to accomplish the
following:
Print the numbers 0  5.
Print the numbers 33  45, including 45.
Print only the odd numbers from 0  20.
Print the numbers 25, 35, 45…95.
Print the numbers from 3 to 10.
Print by 3’s from 15 to 21.
6.3.4. Use Variables in range
¶
To make a for
loop run, we must tell Python exactly how many times we want
the loop body to repeat. However, sometimes this number changes each time the
program runs. Variables to the rescue!
Tip
Whenever possible, use variables instead of specific numbers inside
range()
.
Paste these statements into the editor above (before the loop), and use the
variable names in range
:
1 2 3  start_value = int(input("Enter the FIRST number to print: "))
end_value = int(input("Enter the LAST number to print: "))
step_value = int(input("Enter the step value for the loop: "))

Repeat each of the items in the Try It section above. Enter the start, stop, and step values to print the desired output.
Warning
A common mistake for new coders is to forget that the end value in
range
is NOT assigned to the loop variable at any time.
After you pasted in the input
statements and ran the program, did you have
to type 0, 6, 1
to get the numbers 0  5 to show in the console? The
input
statement implies that we want our typed end value to show up, but
using the variable in range
skips that number.
How do we fix this?
6.3.5. Use Expressions in range
¶
Not only can we use variables inside range
, we can also use expressions,
which we practiced in the Data and Variables chapter.
For the program above, replace the for
statement with this:
for num in range(start_value, end_value+1, step_value):
For tasks 1  4, the expression end_value + 1
makes sure that the value we
type will be included in the loop. With the negative stop values in tasks 5 and
6, we need to use end_value  1
.
Try It
Run the following program. Enter different words to see how the behavior changes.
When Python executes the for
statement, the expression len(word)
returns the length of the string. So if word = "Hi"
, then
range(len(word))
acts just like range(2)
.
6.3.6. Check Your Understanding¶
Question
In the command range(3, 10, 2)
, the second argument (10
) specifies that
range
should:
 generate a set of values that stops at 9 (including 9).
 generate a set of values that starts at 10 (including 10).
 generate a set of values starting at 3 that stops at 10 (including 10).
 generate a set of values using every 10th number between 3 and 10.
Question
What command correctly generates the values 2, 5, 8
in that order?
 range(2, 5, 8)
 range(2, 8, 3)
 range(2, 10, 3)
 range(8, 1, 3)
Question
What happens if you give range only one argument, like range(14)
?
 It will generate a set of values starting at 1 and ending with the number in the ().
 It will generate a set of values starting at 1 up to but NOT including the number in the ().
 It will generate a set of values starting at 0 and ending with the number in the ().
 It will generate a set of values starting at 0 up to but NOT including the number in the ().