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Exercise 3 - Loops and conditionals

Exercise 3 - Loops and conditionals

Andrew Valentine - andrew.valentine@anu.edu.au

Much of the power in programming comes from being able to repeat calculations, and/or change how our program works depending on the values of certain variables. To achieve this, we need to employ ‘loop expressions’ and ‘conditional expressions’.

Conditionals

Conditional expressions allow us to alter the behaviour of our program depending on the circumstances. To do this, we employ an if...elif...else construct, which takes the form:

if <condition>:
    [...]
elif <another condition>:
    [...]
else:
    [...]

Here, each <condition> is a ‘logical’ expression - something which is either ‘true’ or ‘false’. Each [...] denotes a block of code that is only executed if the condition is met. We can have as many elif (‘else if’) blocks as we wish, and we can omit the elif and/or else blocks entirely. At most, one block will be executed: each is tried in the order they appear in the program, until one is found for which <condition> is True. Notice that the else block does not have a condition - this is executed if none of the other conditions are met.

To make this clearer, here’s a real example, inside a function:

def condExample(x):
    if x<0:
        print("x is negative")
    elif x<=1:
        print("x is between 0 and 1 (inclusive)")
    else:
        print("x is greater than 1")

➤ Try it out! Does everything behave as you would expect? Try deleting the elif and/or else clauses; how does this affect the output?

# Try it here!

Logical expressions

Logical expressions are calculations that result in either True or False. As we have already seen, they often arise by comparing the value of two variables (or a variable and a constant), such as x > 0. The comparison operators are:

Operator

Meaning

>

Greater than

<

Less than

>=

Greater than or equal to

<=

Less than or equal to

==

Equal to

!=

Not equal to

You can check you understand how these work by testing expressions in a Python cell: for example, entering

3 > 5

should evaluate to False.

➤ Try all the operators above.

# Try it here!

Note: an important point is that the constants True or False are Booleans, and not text. So "True" (string) is very different of True (Boolean). A possible mistake is to confound them and use "True" instead of True. In general, as soon as you want to use something based on a true/false behavior, use Boolean constants in your program.

To build more complicated expressions, you can use the Boolean operators and, or and not. An expression of the form A and B is only True if both A and B are separately True. On the other hand, A or B is True if either (or both) of A and B are themselves True. The not operator flips True to False, and vice versa. You can use parentheses to group expressions if necessary.

For example:

x > 3 and (y == 2 or not (y > 3 and x+y ==4) )

Many logical conditions can be expressed in multiple forms, for example x > 3 is identical to not x <= 3. In general, you should use the simplest form that is appropriate to your circumstances.

N.B. In some other languages, the symbols & and | are used for and and or. In Python, these symbols are ‘bitwise’ operators, and they will not give the results you expect. We will avoid using them in this course.

➤ Try using the and, or and not operators.

# Try it here!

Another logical operator you may encounter is is. This is used to test whether two variable names refer to the same entity. This is stronger than simply testing for equality. For example:

a = 300
b = 300
a is b

will return False, whereas a is a returns True. This may seem rather pointless, but it turns out to be useful when writing more advanced programs. It is also commonly encountered in conjunction with a null default value within a function:

def printHello(name=None):
    if name is None:
        print("Hello, what is your name?")
    else:
        print("Hello "+name)

To some extent, this is a matter of idiom, rather than necessity.

➤ In the previous exercise, you wrote a function to calculate someone’s fortnightly pay, given an annual salary. Extend this to allow the user to choose to have weekly or monthly figures, instead.

# Try it here!

Loops

Loops allow you to repeat a series of calculations a number of times, or until certain conditions are met. In Python, there are two main loop formats. The first is a while loop:

while <condition>:
    [...]

Here, <condition> is a logical expression (just like those discussed above). If this evaluates to True, the block of code ([...]) is executed in its entirety. The condition is then evaluated again, and the entire process repeats until it becomes False. For example, here is a loop that keeps doubling a number until it exceeds some threshold:

x = 1
while x<100:
    x+=x
print(x)

Normally, the entire indented block of code below the while statement is executed before the condition is checked again. However, two commands can be used to alter this:

  • The break keyword terminates the loop, jumping to the first statement after the indented block

  • The continue keyword skips over any remaining code within the indented block, but returns to re-evaluate the <condition>, and if this is True will execute the indented block as before. These two commands are almost invariably used in conjunction with an if statement.

For example:

x = 1
while x < 100:
    x+=5
    if x == 71: break
    if x%2==0: continue # Skip even x
    print("In loop: x=", x)
print("After loop: x=", x)

If you run this code, you will see that it never prints an even number (since the print statement is after the continue in this case, and so doesn’t get executed); moreover, the loop terminates at x=71, due to the break statement.

# Try it here!

Sometimes, it may be appropriate to use the following style:

while True:
    [...]
    if <condition>: break
    [...]

Here, the loop condition is always True, creating an infinite loop. However, the use of a break allows us to escape from the loop.

There is another form of loop, the for loop. We will meet this in a later practical.

# Try it here!

➤ Caution: infinite loops… if you have no break statement, the program will execute the loop until you provide an explicit “stop” signal. In Jupyter notebooks, this is the stop button, and in a terminal the ctrl+c command (for all operating systems). Infinite loops will run endlessly if not manually stopped (by sending a termination signal), and are a well known problem since the dawn of computer programming. Nowaday, it is not such a big deal but old systems used to froze when running such loops… So be careful and always check that your loop is not infinite!

Suppose you start with an empty basket and you want to pick apples on a tree. You will make 100 picking moves.

You start by picking 2 apples at a time, but after doing this 50 times you get bored. You then pick 5 apples each time, and after doing this a further 25 times, you start picking 10 apples.

➤ Write a function containing a loop to return the number of apples in your basket.

# Try it here!

Suppose you take out a \(\$\)500,000 mortgage to buy a house. You make a repayment of $2,000 each month. However, each month the bank charges interest at a rate of 0.3% of the outstanding balance. How many months will it take you to pay off the debt? How much does it cost you?

➤ Write a function to calculate this information for any loan amount, interest rate and monthly repayment.

# Try it here!