Control flow in Python
Now, let's take a look at if statements.
The if statement
The if statement in Python is similar to that found in other programming languages (such as Java). It's the backbone of the logical flow of most programs. Here's an example:
y = 6
if y % 2 == 0:
print('Even')
The output is:
Even
Explore the control logic of the if statement
What do you see if you change y to an odd number?
Did you notice the indentation for the print function under the if statement? That indentation is important, because it's how Python delimits the scope of a control flow. It indicates what is to be contingently run or looped over. Python uses indentation instead of the braces ("{}") used in other languages.
To cover more contingencies without having to construct a follow-on if statement, you can add an else statement:
y = 7
if y % 2 == 0:
print('Even')
else:
print('Odd')
The output is:
Odd
Use elif to insert another logical test into an if statement:
y = 1
if y % 2 == 0:
print('Even')
elif y == 1:
print('One')
else:
print('Odd')
The output is:
One
Notice that, in the previous example, the if statement exited after finding the first logical test that was true. The y variable was set to 1, which is indeed an odd number. But, the if statement executed and exited after finding that y == 1 instead of continuing to the else clause at the end of the statement.
Try it yourself
Try changing the value of y in the preceding example. Do you get the output that you expect?
Hint (expand to reveal)
y = 3
if y % 2 == 0:
print('Even')
elif y == 1:
print('One')
else:
print('Odd')
Odd
y = 4
if y % 2 == 0:
print('Even')
elif y == 1:
print('One')
else:
print('Odd')
Even
The for loop: Basics
It's often necessary in programs to iterate over some set of items. A for loop is useful such a task. For example, it provides a way to iterate over the items of a list:
colors = ['red', 'yellow', 'blue']
for color in colors:
print(color)
The output is:
red
yellow
blue
Sometimes, you'll want to iterate over a list by using the list index rather than items from that list. For example, you might want to access items from another list at the same time. In this case, you can combine list-object methods and for loops:
comp_colors = ['green', 'purple', 'orange']
for i in range(len(comp_colors)):
print(colors[i], comp_colors[i])
The output is:
red green
yellow purple
blue orange
We've seen len() before, but range() is new to us. In its simplest form, range(stop) produces a sequence of integers from 0 up to a number that's 1 less than the number you passed into it. Here's an example:
for j in range(5):
print(j)
The output is:
0
1
2
3
4
The range function can take up to three parameters: range(start, stop[, step]). This odd-looking notation simply means that if you pass in a single argument, range() will take that argument to be the stop value. It will interpret two arguments as the start and stop values, and three arguments as star, stop, and step.
Explore the control flow of a for loop
How would you use range and a for loop to print the sequence of numbers from 10 to 20?
How about counting by threes from 17 to 41?
Sometimes you need to break out of afor loop. Python borrows the break statement from the C language for this purpose. To see break in action, consider two nested for loops:
for n in range(2, 10):
for x in range(2, n):
if n % x == 0:
print(n, 'equals', x, '*', n//x)
break
else:
print(n, 'is a prime number')
The output is:
2 is a prime number
3 is a prime number
4 equals 2 * 2
5 is a prime number
6 equals 2 * 3
7 is a prime number
8 equals 2 * 4
9 equals 3 * 3
Note
In the preceding example, the else statement belongs to the for loop, not to the if statement.
Try it yourself
Try changing the preceding code example by removing the break statement. What output does it now produce?
Hint (expand to reveal)
for n in range(2, 10):
for x in range(2, n):
if n % x == 0:
print(n, 'equals', x, '*', n//x)
else:
print(n, 'is a prime number')
2 is a prime number
3 is a prime number
4 equals 2 * 2
4 is a prime number
5 is a prime number
6 equals 2 * 3
6 equals 3 * 2
6 is a prime number
7 is a prime number
8 equals 2 * 4
8 equals 4 * 2
8 is a prime number
9 equals 3 * 3
9 is a prime number
The for loop: continue statement
As part of the control flow of your program, you might want to continue to the next iteration of your for loop. The continue statement (also borrowed from C) can help:
for num in range(2, 10):
if num % 2 == 0:
print("Found an even number:", num)
continue
print("Found an odd number:", num)
The output is:
Found an even number: 2
Found an odd number: 3
Found an even number: 4
Found an odd number: 5
Found an even number: 6
Found an odd number: 7
Found an even number: 8
Found an odd number: 9
Try it yourself
What happens when you replace the continue statement in the preceding example with a break statement?
Hint (expand to reveal)
for num in range(2, 10):
if num % 2 == 0:
print("Found an even number:", num)
break
print("Found an odd number:", num)
Found an even number: 2
The while loop
If we combined the functionality of the if statement and the for loop, we'd get the while loop. A while loop iterates for as long as some logical condition remains true. Consider the following code example, which computes the initial subsequence of the Fibonacci sequence. (In the Fibonacci series, the sum of two elements defines the next.)
a, b = 0, 1
while b < 100:
print(b, end=', ')
a, b = b, a+b
The output is:
1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89,
Go ahead and play with the number of iterations for the while loop. Notice that this example also uses Python's multiple-assignment capability for variables.
Takeaway
Control flows are what make a program a program, as opposed to a single sequence of operations. Mastering the logical flow of information in Python will enable you to automate tasks that would be impossibly complex or time consuming to do manually.