Control constructs

if statements
- Boolean operators
for loops
- Controlling for loops
while loops

if statements

if statements are used to execute code when a condition is true.

is_rainy = True
if is_rainy:
    print('Bring your bumbershoot')

Bring your bumbershoot

These can be followed up by an else:

if is_rainy:
    print('Bring your bumbershoot')
else:
    print('No rain')

Bring your bumbershoot

Note: In Python, whitespace is used to define levels in the code. It is convention to use 4 spaces (not tabs) as the first level of indentation. The second level would be 4 more spaces (8 total) and so on.

If is_rainy to False, the code under the else statement is executed instead of the code under the if statement.

More conditions can be added with elif.

if is_rainy:
    print('Bring your bumbershoot')
elif is_sunny:
    print('Bring your hat')
else:
    print('No rain, no sun')

Bring your bumbershoot

Boolean operators

It is common to use boolean operators with if statements.

some_number = 9
print(some_number == 2)

False

Start IPython by typing ipython in a terminal, and try out a few more comparisons.

!=
<
>=

Note: Above I am using print to see the results of the comparison, but if you're in IPython, you don't need to do this.

Using them within an if statement look like this:

if some_number < 10:
    print('There are less than 10')

There are less than 10

for loops

for loops allow you to iterate over variables.

genes = ['DNJC..uh', 'FOXP2', 'MET7B']
for gene in genes:
    print('My favorite gene: ' + gene)

My favorite gene: DNJC..uh
My favorite gene: FOXP2
My favorite gene: MET7B

for and if statments can be nested.

for gene in genes:
    if gene != 'MET7B':
        print(gene +  ', what a dumb gene')

DNJC..uh, what a dumb gene
FOXP2, what a dumb gene

Controlling for loops

Once a for loop is running, you often want to change the behavior based on the current iteration. For example, if a certain value is encountered, skip to the next iteration. This can be done with continue, which means stop the current iteration of the loop right here and go to the next iteration. Using continue, we can produce the exact same results as the last block.

for gene in genes:
    if gene == 'MET7B':
        continue
    print(gene +  ', what a dumb gene')

DNJC..uh, what a dumb gene
FOXP2, what a dumb gene

Another common behavior is to break out of the loop completely if some condition is met. break is the command to do this.

lottery_numbers = [888, 301, 405, 772, 332]
winning_number = 405
for number in lottery_numbers:
    print(number)
    if number == winning_number:
        print('We have a winner')
        break

888
301
405
We have a winner

while loops

There are also while loops in python. These allow the code to continually be executed while a condition is met.

number = 1
while number < 4:
    print(number)
    number += 1

1
2
3

Note: The += is an assignment operator. It adds the value to the current value of the variable and then assigns the new value back to the variable. It is equivalent to number = number + 1. Similar operators exist for subtraction (-=), multiplication (*=), and division (\=).