Python Projects for Beginners a ten-Week Bootcamp Approach to Python Programming

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Python Projects for Beginners A Ten Week Bootcamp Approach to Python

Bu sahifa navigatsiya:

• Changing Values in a List
• Variable Storage

Index Location	Value at Location	Data Type
Can Be Indexed Again
0	5	Integer	no
1	‘book’	string	Yes
2	[34, ‘hello’]	List	Yes
3	true	Boolean	no

Notice that strings can also be index further. If you wanted to only print out the “b” in “book,” you would simply write the following:
>>> print( data[ 1 ][ 0 ] ) # will output 'b'

Changing Values in a List

When you work with lists you need to be able to alter the value of the items within the list. It’s like re-declaring a normal variable to a different value, except you access the index first:

# changing values in a list through index data = [5, 10, 15, 20] print(data) data[0] = 100 # change the value at index 0 - (5 to 100) print(data)

Go ahead and run that cell. Before we altered the value at index 0, it outputs [5, 10, 15, 20]. Once we accessed the zero index and changed its value to 100, however, the list ended up changing to [100, 10, 15, 20].

Variable Storage

When variables are declared, the value assigned is put into a location in memory. These locations have a specific reference ID. It’s not often you’ll need to check the ID of a variable, but for educational purposes, it’s good to know how storage works. We would use the id() function to check the storage location in memory for a variable:
>>> a = [ 5, 10 ]
>>> print( id(a) ) # large number represents location in memory
When a list is stored in memory, each item is given its own location. Changing the value using index notation will change the value stored within that memory block. Now, if a variable’s value is another variable, like so:
>>> a = [5, 10]
>>> b = a
Changing the value at a specific index will change the value for both lists. Let’s see an example:

# understanding how lists are stored a = [5, 10] b = a print( "a: { }\t b: { }".format(a, b) )
print( "Location a[0]: { }\t Location b[0]: { }".format( id(a[0]), id(b[0]) ) ) a[0] = 20 # re-declaring the value of a[0] also changes b[0] print( "a: { }\t b: { }".format(a, b) )

Go ahead and run that cell. We’re going to get several outputs here. The first is printing out the values of both list variables to show that they have the same values. The second print statement will output the location in memory for each list’s first item. Then lastly, after we change the value of the first item within our “a” list, the value in our “b” list also changes. This is because they share the same memory location.
ChApter 4 LIsts And Loops

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