# Developing Functions¶

Let us understand how to develop functions using Python as programming language.

• Function starts with `def` followed by function name.

• Parameters can be of different types.

• Required

• Keyword

• Variable Number

• Functions

• Functions which take another function as an argument is called higher order functions.

Let us perform few tasks to understand how to develop functions in Python.

• Sum of integers between lower bound and upper bound using formula.

```def sumOfN(n):
return int((n * (n + 1)) / 2)
```
```sumOfN(10)
```
```55
```
```def sumOfIntegers(lb, ub):
return sumOfN(ub) - sumOfN(lb -1)
```
```sumOfIntegers(5, 10)
```
```45
```
• Sum of integers between lower bound and upper bound using loops.

```def sumOfIntegers(lb, ub):
total = 0
for e in range(lb, ub + 1):
total += e
```
```sumOfIntegers(1, 10)
```
```55
```
```sumOfIntegers(5, 10)
```
```45
```
• Sum of squares of integers between lower bound and upper bound using loops.

```def sumOfSquares(lb, ub):
total = lb * lb
for e in range(lb + 1, ub + 1):
total += e * e
```
```sumOfSquares(2, 4)
```
```29
```
• Sum of the even numbers between lower bound and upper bound using loops.

```def sumOfEvens(lb, ub):
total = lb if lb % 2 == 0 else 0
for e in range(lb + 1, ub + 1):
total += e if e%2==0 else 0
```sumOfEvens(2, 4)
```6