List¶

author: Parin Chaipunya affil: KMUTT

What is a list ?¶

A list is a collection of items that are putted together inside a pair of square brackets, separated by commas.

In [1]:
[1, 2.0, "three"] # without storing
Out[1]:
[1, 2.0, 'three']
In [2]:
L = [1, 2, "three"] # stroing into a variable
In [3]:
print(L) #printing the whole list.

[1, 2, 'three']
In [4]:
L = [1, 2, 3, [1, 2, 3]]
print(L)

[1, 2, 3, [1, 2, 3]]

Accessing an entry¶

Forward indexing¶

Entries of a list is indexed with integers starting with 0. An entry can be called by its parent list followed by its index within the square brackets.

Therefore, the above list L could be seen as L = [L[0], L[1], L[2]]. From the above list L, we print each entry in the following block.

In [5]:
print(L[0])
print(L[1])
print(L[2])

1
2
3

Backward indexing¶

An entry can be accessed with a backward index. Here, the last entry is indexed -1, the second last entry is indexed -2, etc.

In [6]:
print(L[-1])
print(L[-2])
print(L[-3])

[1, 2, 3]
3
2

List operations¶

One could play around with lists, which includes

  • adding entries to a list
  • removing an entry from a list.

Adding entries to a list¶

One could concatenate two lists using +.

In [7]:
L1 = [1, 2.0, "three"]
L2 = [4, "FIVE"]
LL = L1 + L2
print(LL)

[1, 2.0, 'three', 4, 'FIVE']
In [8]:
LLL = L2 + L1
print(LLL)

[4, 'FIVE', 1, 2.0, 'three']
In [9]:
print(LLL)

[4, 'FIVE', 1, 2.0, 'three']

One could repeat the list any number of times by using *.

In [10]:
3*L2  # the number that multiplies here must be an integer.
Out[10]:
[4, 'FIVE', 4, 'FIVE', 4, 'FIVE']

A remark here is that a list of numbers is not an equivalence of a vector in mathematics. As we see above, we cannot add or multiply a scalar to do algebra.

Removing entries from a list¶

If a specific value is to be removed from a list, we use a list's method .remove(). By default, it removes the first match.

In [11]:
L = [1, 2.0, "three", 2.0, 1]
print(L)

[1, 2.0, 'three', 2.0, 1]
In [12]:
L.remove(1)
print(L)

[2.0, 'three', 2.0, 1]

To remove an entry at a specific index, we may use del or to use a list's method .pop().

In [13]:
del L[1]
print(L)

[2.0, 2.0, 1]
In [14]:
L.pop(-1)
print(L)

[2.0, 2.0]

To remove all entries matching some conditions, we may use the following trick.

In [15]:
L = [1, 2, 3, 2, 1]
print(L)

[1, 2, 3, 2, 1]
In [16]:
[x for x in L if x != 2]
Out[16]:
[1, 3, 1]

Slicing¶

Slicing refers to the extraction of some entries (usually multiple ones) from a list. The main tool to use is :.

i:j¶

From a list L, we use the slice L[i:j] to extract a list [L[i], L[i+1], ..., L[j-1]]. Note that i:j starts at i and ends at j-1, excluding the j index.

In [17]:
L = [1, 2, 3, 4, 3, 2, 1]
print(L[2:5])

[3, 4, 3]

i: and :j¶

From a list L, the slice L[i:] generates the list [L[i], L[i+1], ..., L[-1]]. Likewise, the slice L[:j] generates the list [L[0], L[1], ..., L[j-1]].

In [18]:
print(L[2:])
print(L[:4])

[3, 4, 3, 2, 1]
[1, 2, 3, 4]