`Counter`

Often in Python programs we want to record the number of uses, hits, or instances of something. We collect (count) frequencies.

`Counter` info

With Counter, from the collections module, we can do this with minimal set-up code. We access the Counter like a list, but can count strings or numbers.

Simple example

To begin, we import Counter and create a Counter named "x." We add to indexes with the counter, and print the Counter directly to the console.

Info The Counter stores the values we have incremented, and when we display its contents, we see what we have added.

from collections import Counter
x = Counter()

# Add 1 at index 0.
x[0] += 1
print(x)

# Add more to the counter, and display it.
x[2] += 1
x[3] += 1
x[2] += 1
print(x)Counter({0: 1})
Counter({2: 2, 0: 1, 3: 1})

Complex example

For a more complex example, we use the Counter with range loops. We add to the counter based on ranges of integers. The counts accumulate.

Detail We can display values by accessing each key, or we can display the entire counter by passing it to the print method.

from collections import Counter

# Create new counter.
freqs = Counter()

# Add to the counter based on ranges of numbers.
for i in range(0, 10):
    freqs[i] += 1

for i in range(0, 5):
    freqs[i] += 1

for i in range(0, 2):
    freqs[i] += 1

# Display counter contents.
for i in range(0, 10):
    print(f"Value at {i} is {freqs[i]}")

# The counter can be displayed directly.
print(freqs)Value at 0 is 3
Value at 1 is 3
Value at 2 is 2
Value at 3 is 2
Value at 4 is 2
Value at 5 is 1
Value at 6 is 1
Value at 7 is 1
Value at 8 is 1
Value at 9 is 1
Counter({0: 3, 1: 3, 2: 2, 3: 2, 4: 2, 5: 1, 6: 1, 7: 1, 8: 1, 9: 1})

Strings

We can do more things with a counter than just increment by one. We can decrement. Or we can assign values. We can also use string keys in the counter.

from collections import Counter

freqs = Counter()
print(freqs)

# Three birds are added.
freqs["bird"] += 1
freqs["bird"] += 1
freqs["bird"] += 1
print(freqs)

# One bird was removed.
freqs["bird"] -= 1
print(freqs)

# Four birds were counted.
freqs["bird"] = 4
print(freqs)Counter()
Counter({'bird': 3})
Counter({'bird': 2})
Counter({'bird': 4})

A summary

The collections module in Python contains useful collections like Counter. For recording the frequencies of numbers and strings (and other things) we can use Counter.

A major benefit

For Counter, a major benefit is that less initialization logic is needed. So we can spend more attention on counting rather than dealing with a dictionary.

Counter