# Python Math: nth Catalan Number

## Python Math: Exercise-25 with Solution

Write a Python program for nth Catalan Number.

In combinatorial mathematics, the Catalan numbers form a sequence of natural numbers that occur in various counting problems, often involving recursively-defined objects. They are named after the Belgian mathematician Eugène Charles Catalan (1814–1894).

Using zero-based numbering, the nth Catalan number is given directly in terms of binomial coefficients by

The first Catalan numbers for n = 0, 1, 2, 3, … are

1, 1, 2, 5, 14, 42, 132, 429, 1430, 4862, 16796, 58786, 208012, 742900, 2674440, ....

**Pictorial Presentation**

The C5 = 42 noncrossing partitions of a 5-element set (below, the other 10 of the 52 partitions)

Image Credits: Watchduck

**Sample Solution**:-

**Python Code:**

```
def catalan_number(num):
if num <=1:
return 1
res_num = 0
for i in range(num):
res_num += catalan_number(i) * catalan_number(num-i-1)
return res_num
for n in range(10):
print(catalan_number(n))
```

Sample Output:

1 1 2 5 14 42 132 429 1430 4862

**Flowchart:**

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## Python: Tips of the Day

**Getting rid of unwanted characters:**

You can get rid of whitespaces or any specific character using strip methods in Python. You can use either plain strip for both sides, lstrip for the left side and rstrip for the right side only.

str="+++Welcome python+++" str.strip() print(str)

Output:

+++Welcome python+++

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