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Table of contents

1. Introduction
2. Variables
3. Data structures
4. Conditional statements and loops ← (Notebook)
5. Some exercises
6. Introduction to functions
   1. File manipulation
7. From 0D to 1D
   1. Adding lateral diffusion
8. From 1D to 2D
9. Playing with the model

3. Conditional statements and loops

3.1 Conditional statements

Another important part of coding are the conditional statements.

Conditional statements allow you to perform a given set of instruction(s) if a statement is true.

If necessary, it is possible to run a different set of instructions when the statement is false.

More information about conditional statements can be found there for example

This is usually called an if/else statement:

a_number = eval(input('Please enter a number and press enter: '))
if a_number == 2:
    print('the number is equal to 2')
else:
    print('the number is not equal to 2')

Note the function input which allows the user to ask for an input. Note also the function eval which allows you to evaluate the input from the user. In place of eval could be used the function int which would transform the input into an integer. If that was the case, the Python interpreter could not take as an input a decimal value like 0.9 for example. To do so the function float could have been used. Now, with eval, one can even enter an operation such as 1+1 and it will be evaluated and then treated as 2 in that case.

You can try to play with the function eval with the example above.

If multiple conditions need to be checked, the elif statement can be used:

a_number = eval(input('Please enter a number and press enter: '))
if a_number < 2:
    print('the number is stricly smaller than 2')
elif a_number == 2:
    print('the number is equal to 2')
elif 2 < a_number < 10:
    print('the number is strictly between 2 and 10')
else:
    print('the number is larger or equal to 10')

To go a little bit further:

if/else statements can be used within a line of code to assign values for example:

previous_number = eval(input('Please enter a number and press enter: '))
a_number = 1 if 10 <= previous_number else 'Strictly smaller than 10'
print(f'a_number --> {a_number}')

# The previous line is equivalent to the following ones:
if 10 < previous_number:
    a_number = 1
else:
    a_number = 'Strictly smaller than 10'

3.2 Loops

Loops are probably the core of coding! They are the reason why computers are so useful!

In Python two types of loops exist: the for loop and the while loop. The difference between the two kinds of loops can be small but basically you can almost alway make one with the other though making some while loops using for loops is sometimes a bit convoluted (but these are thoughts for another time).

A for loop in Python allows you to iterate over the items of any sequence (list, str for example). Note that it is different from loops in C, C++ or Pascal for example.

A while loop allows you to iterate as long as a given condition is True.

The syntax for a for loop is the following:

for item in sequence:
    # do_something

The syntax for a while loop is the following:

while condition:
    # do_something

Here is an example of a for loop:

words = ['Hello,', 'how', 'are', 'you?']
for w in words:
    print(w, end=' ')

Here the loop iterates over the items of the sequence words (which is a list) and prints them.

The equivalent with a while loop would look like that:

i = 0
while i<len(words):
    print(words[i], end=' ')
    i = i + 1

One can easily see that in that context, the while loop is a bit more convoluted.

Now, here is an example where the while loop is better.

stopping_value = 35
i = 0
number_sum = 0
while number_sum < stopping_value:
    i += 1
    number_sum += i
print(f'{i = }, {number_sum = }')

The equivalent for loop would be the following:

number_sum = 0
for i in range(stopping_value+1): # Here we assume that the maximum value
                                  # necessary to stop is the stopping value itself
    number_sum += i
    if stopping_value <= number_sum:
        break
print(f'{i = }, {number_sum = }')

Note that in the case of the for, it is necessary to use the break statement to stop the loop according to a given condition.

To go a little bit further (not required): Note that if you exchange lines 5 and 6 in the while loop you do not get the same result, can you find out why?

i = 0
number_sum = 0
while number_sum < stopping_value:
    number_sum += i
    i += 1
print(f'{i = }, {number_sum = }')

More on for loops can be found there

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