Print, If and For

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Print

print is useful function:

>>> type(print)
<class 'builtin_function_or_method'>

It prints out the values that you pass it, such as strings or numbers.

>>> print(10)
10
>>> my_name = "Matthew"
>>> print(my_name)
Matthew

This can be useful in the Notebook to show values as we execute things within a cell.

If

We have already seen assignment statements (Assignment). We saw that they have the form: LHS = RHS, where the LHS is a variable name and the RHS is an expression.

We saw expressions on their own at the end of Notebook cells, to show us a value.

Now we add a couple of essential statements you will use all the time: if statements and for statements.

Here is an if statement:

>>> x = 3
>>> if x == 3:
...     print("x does equal 3")
...
x does equal 3

There are a few new things here. This if statement has the form:

• if followed by
• an expression (x == 3) followed by
• : (a colon) followed by
• an indented block of code (`` print(“x does equal 3”)``)

Remeber that an expression evaluates to a result. The trick of the if statement, is that it will only run the indented block of code above, if the expression evaluates to True. Try it. Run the code above. Now try:

>>> x = 4
>>> if x == 3:
...     print("x does equal 3")
...

It doesn’t print anything because x == 3 is False.

Next notice that Python knows which code to run, when x == 3 is True, because of the indentation. It runs all the code that is indented, when x == 3 is True, and ignores that code otherwise. There can be several lines in the indented block, but they all have to be indented the same amount:

>>> x = 3
>>> if x == 3:
...     print("Here in the if block")
...     print("x does equal 3")
...     print("Still in if block")
...
Here in the if block
x does equal 3
Still in if block

Now an exercise. For this exercise, you will need the modulo operator: %. In an expression it returns the remainder of dividing the number on the left by the number on the right. Here are some examples:

>>> 3 % 2
1
>>> 4 % 2
0
>>> 13 % 3
1
>>> 13 % 13
0

The exercise is to write an if statement that prints “yes” if the number x is exactly divisible by 7, and prints nothing otherwise. Test your code by setting various values of x, and running the cell.

What happens if you forget the colon : at the end of the if line?

What happens if you forget to indent the block?

For loops

As we’ve seen by now, we often want to repeat the same thing multiple times. The way we usually do this in Python is using a for loop. A for loop looks like this:

>>> for i in range(10):
...     print(i)
...
0
1
2
3
4
5
6
7
8
9

There are several new things here. Notice range(10). This looks like a function, and it looks like it returns 10 numbers, starting at 0 and continuing through to 9. That’s good enough for our purposes.

Next we see that the for statement is rather like the if statement. It has form:

• for followed by
• a variable name (in this case i, called the loop variable) followed by:
• an expression that retuns a series of things (range(10) returns a series of numbers from 0 through 9) followed by:
• : (a colon) followed by
• an indented block of code (`` print(i)``)

For statments are just a little more complicated than if statements, and it is worth going through the logic carefully. Read the for statement above like this:

• in the first iteration of the loop, set i to have value 0. Then print i;
• in the second iteration of the loop, set i to have value 1. Then print i;
• in the third iteration of the loop, set i to have value 2. Then print i;

and so on.

Let’s try some exercises.

Use a for loop to add up all the numbers between 0 and 10.

Use a for loop to count how many numbers from 0 through 99 that are divisible by both 7 and 4.