`Index`

The index() function in Python searches lists. We pass it a value, and it returns the index where that value is found. If no value is found, it throws an error.

With count, we count the matching elements in a list. Like index() this loops through and tests each element in a linear way. List provides no rindex method.

This example shows how the index() method searches a list. If no value is found, we get an Error. We can handle this in a try-except block (but this is not ideal).

Tip For programs where you need more control, consider using a for-loop instead of index().

Info With a for-loop, we can more elegantly handle cases where the value is not found. This avoids the ValueError.

# Input list.
values = ["uno", "dos", "tres", "cuatro"]

# Locate string.
n = values.index("dos")
print(n, values[n])

# Locate another string.
n = values.index("tres")
print(n, values[n])

# Handle nonexistent string.
try:
    n = values.index("?")
    # Not reached.
    print(n)
except:
    # Value not found.
    print("Not found")1 dos
2 tres
Not found

`Count`

This method does not return the number of elements in the list. Instead it counts a specific element. As an argument, pass the value of the element we wish to count.

Note Internally count() loops through all the elements and keeps track of the count. It then returns this value.

names = ['a', 'a', 'b', 'c', 'a']

# Count the letter a.
value = names.count('a')
print(value)
3

Benchmark, index

What is the fastest way to search a list? Given that searching a list is common done, learning the best way to do this is beneficial.

Version 1 We test the index() method. Index() raises an exception if no match is found.

Version 2 In this version of the code we use a for-loop with a range. This code does not raise an exception.

Result In this test, the index method is far faster than the for-loop that tests each string element.

So Index() is likely optimized at a low level in Python. I suggest you prefer index() for list searching.

import time

values = ["Augustus", "Tiberius", "Caligula", "Claudius"]

print(time.time())

# Version 1: index.
i = 0
while i < 10000000:
    v = values.index("Caligula")
    i += 1

print(time.time())

# Version 2: for-range.
i = 0
while i < 10000000:
    v = 0
    for n in range(0, len(values)):
        if values[n] == "Caligula":
            v = n
            break
    i += 1

print(time.time())1362429980.178
1362429984.018  index() =  3.84 s
1362429994.569  for     = 10.55 s

Performance, count

Consider the count() method. If we were to keep track of the items added to the list as we add them, we could compute counts without a loop. This would be faster.

With index and count, we have declarative (function-based) ways of testing the elements of a list. We can search for matching elements (and get an index, or the sum of matches).