Arrays: Difference between revisions

Line 6,741:

array[0] = 2

print array[0]</syntaxhighlight>

print(array[0])</syntaxhighlight>

A simple, single-dimensional array can also be initialized thus:

<syntaxhighlight lang="python">~~myArray~~ = [0] * size</syntaxhighlight>

<syntaxhighlight lang="python">my_array = [0] * size</syntaxhighlight>

However this will not work as intended if one tries to generalize from the syntax:

However, this will not work as intended if one tries to generalize from the syntax:

<syntaxhighlight lang="python">~~myArray~~ = [[0]* width] * height # DOES NOT WORK AS INTENDED!!!</syntaxhighlight>

<syntaxhighlight lang="python">my_array = [[0] * width] * height # DOES NOT WORK AS INTENDED!!!</syntaxhighlight>

This creates a list of "height" number of references to one list object ... which is a list of width instances of the number zero. Due to the ~~differing~~ semantics of immutables (strings, numbers) and mutables (dictionaries, lists), a change to any one of the "rows" will affect the values in all of them. Thus we need to ensure that we initialize each row with a newly generated list.

This creates a list of "height" number of references to one list object... which is a list of width instances of the number zero. Due to the different semantics of immutables (strings, numbers) and mutables (dictionaries, lists), a change to any one of the "rows" will affect the values in all of them. Thus we need to ensure that we initialize each row with a newly generated list.

To initialize a list of lists one could use a pair of nested list comprehensions like so:

<syntaxhighlight lang="python">~~myArray~~ = [[0 for x in range(width)] for y in range(height)]</syntaxhighlight>

<syntaxhighlight lang="python">my_array = [[0 for x in range(width)] for y in range(height)]</syntaxhighlight>

That is equivalent to:

<syntaxhighlight lang="python">~~myArray~~ = list()

<syntaxhighlight lang="python">my_array = list()

for x in range(height):

~~myArray~~.append([0] * width)</syntaxhighlight>

my_array.append([0] * width)</syntaxhighlight>

To retrieve an element in an array, use any of the following methods:

Line 6,771:

# Use the array like a stack. Note that using the pop() method removes the element.

array.pop() # Pop last item in a list

array.pop(0) # Pop first item in a list

# Using a negative element counts from the end of the list.

item = array[-1] # Retrieve last element in a list.

</syntaxhighlight>

Line 6,781:

try:

# This will cause an exception, which will then be caught.

print array[len(array)]

print(array[len(array)])

except IndexError as e:

# Print the exception.

print e

print(e)

</syntaxhighlight>