If statements

Learning outcomes¶

• Use comparison operators to write conditions.

• Use “if statements” to execute code based on these conditions.

Prerequisites¶

• First steps

• Variables

Conditions¶

Comparison operators¶

Our goal in this lesson is to learn to control whether a particular piece of code is executed by Python or is ignored, based on conditions set by us. Since these conditions are binary (execute the code or don’t), they should always come down to a boolean value of True or False.

The essential way to write a condition is to place two values on either side of a comparison operator. For example, using the ‘lesser than’ comparison operator <:

print(1 < 3)
print(3 < 1)

you should see the code above output True, then False.

The common comparison operators you may encounter are the following:

print(num_nitrogen < num_fluorine)
print(num_fluorine > num_nitrogen)
print(num_fluorine >= num_nitrogen)
print(num_fluorine == num_fluorine)

Although most of these comparison operators have an opposite one available, it is sometimes helpful for clarity to invert a condition. To achieve this, precede a comparison by not:

boiling_point = 100.0
temp = 73.3

print("Is the water boiling?", not temp < boiling_point)

Comparisons, and boolean values in general, can be combined together to make more complicated expressions. We achieve this using boolean operators

This allows us to make combined statements of the form:

a = 1
b = 2
c = 3
print(a < b and b <= c)

These combined statements can start looking quite messy, but we the help of parentheses, we can impose an order of operation and make things neater:

a = 1
b = 2
c = 3
d = 4
print((a < b or b <= c) and (b < d))

Without any parentheses, the default operations is to evaluate all the comparison operators from left to right, and then all of the boolean opeartors from left to right.

If statements and indentation¶

We are prepared to implement our first condition inside our code. Look at the following example closely and run it in your code editor:

molecule_a = "CH4"
molecule_b = "CH4"
if molecule_a == molecule_b:
    # This part of the code is run only if the if statement is true
    print("Yes, these molecules have the same formula.")
    print(f"This formula is {molecule_a}.")
print("End of first test.")

if molecule_a != molecule_b:
    # The condition has changed, in this statement
    print("No, these molecules don't have the same formula.")

First, note the the syntax of the line if molecule_a == molecule_b:. The word if is reserved in Python, so should appear in a different colour in your editor. There is then a space and what follows is anything that resolves to a Boolean; here, a comparison. The line must always end with a :.

Then, some code appears with 4 leading spaces (which you can write by presing <Tab>):

    # This part of the code is run only if the if statement is true
    print("Yes, these molecules have the same formula.")
    print(f"This formula is {molecule_a}.")

The leading spaces are called an indentation. Indented code directly following an if statement is only executed if the statement is fulfilled. To return to normal operations (where every line of code is executed), we remove the indentation, as in the line print("End of first test.").

Try changing molecule_b to a different formula and see how the output changes.

workfunction_Ni = 5.08 # eV
photon_energy = 6.17 # eV

if photon_energy >= workfunction_Ni:
    print(f"The photon has energy {photon_energy} eV. It will release an electron from the nickel surface.")

Else and elif statements¶

Often, we want to execute one piece of code if a condition is fulfilled, but a different piece if it is not. This can be achieved by two if statements and some careful writing of conditions, but it is easier and clearer to use the else keyword.

Here is the same example as earlier but using the else keyword:

molecule_a = "CH4"
molecule_b = "CH4"
if molecule_a == molecule_b:
    # This part of the code is run only if the if statement is true
    print("Yes, these molecules have the same formula.")
    print(f"This formula is {molecule_a}.")
else:
    # Otherwise, this section is run
    print("No, these molecules don't have the same formula.")

Try running the code as it is, and then changing molecule_b.

The general syntax has us writing else: at the same indentation as if (so in this case, no indentation) and directly below the if block. The else: statement is then followed by its own block.

We can be even more precise and combine a condition with an else statement, turning it into an elif statement:

molecule_a = "CH4"
molecule_b = "CH4"

# Is this water?
if molecule_b == "H2O":
    print("Molecule B is water.")
# Is molecule B the same as A?
elif molecule_a == molecule_b:
    print("Yes, these molecules have the same formula.")
    print(f"This formula is {molecule_a}.")
else:
    # Otherwise, this section is run
    print("No, these molecules don't have the same formula and molecule B is not water.")

The syntax for elif is identical to else except that it is followed by a boolean. We may chain as many elif statements as we want and decide or not to finish them with a catch-all else.

Remember, however, that as soon as an if or elif statement is fulfilled, the indentend block is executed and the rest of the chain of conditions is completely skipped.

peak_transmittance = 97 # %

if peak_transmittance < 0 or peak_transmittance > 100:
    print("Peak absorbance cannot be less than zero or greater than 100. There may be an error in your instrument.")
elif peak_transmittance >= 95 and peak_transmittance <= 100:
    print(f"The peak has an absorbance of {peak_transmittance}% and can be considered background noise.")
else:
    print(f"The peak has an absorbance of {peak_transmittance}% and can be considered significant.")

Summary¶

• You can use the following comparison operators: ==, !=, >, <, >=, <=, and in.

• And these boolean operators: and, or, and not.

• You can chain statements together.

• Python evaluates operators in the order: maths and comparisons, then logic. You can clarify your intentions by using round brackets.

• if...elif...else is used to create code that only executes if a certain condition is met.

• Use a colon after every if/elif/else condition, and indent any following code that should execute if that condition is met.

• When an if statement is True, elif and else statements directly following it will not execute.

• Use multiple if statements if you want every statement checked. Consider how you can minimise the number of if statements to have a more efficient and less cluttered code.