Customized Functions

A function is a block where you input values and then you get an output (result). The structure of a function is the following:

```def FunctionName(arg1, arg2 , ...):
[Body]
...

return Output
```

You need to use a valid function name (FunctionName), it's not allowed to call it with number, or with a predefined name. The arguments (arg1, arg2, …) are the inputs of your function, you need to put them a valid name as well. Inside the body you develop your calculations or algorithm. Finally you need to return your output, you do it using a reserved word return.

In python the indentation is a big deal, putting this way, if you are a programmer, it's like a comma in C. So don't forget to use it as well as the colon.

Before to start. I create a file called: myfirtsfunction.py in whichever editor that you prefer (some alternatives here).

Here goes the example.

```def sumar(a,b): # input arguments a, b
c = a + b # the sum is assigned to c
return c # c is returned
```

It returns the sum between two numbers:

```>>> sumar(3,54)
57
```

Here goes another example. Let's round a number without using the predefined python function (round).

```def roundnum(num):
num = num + 0.5 # Round the integer part
num = str(num) # Convert real to string
index = num.find('.') # Get the index of the period
num = num[:index] # Get the integer part as a string yet
return int(num) # Convert string to integer
```

Output:

```>>> print roundnum(5.6)
6
>>> print roundnum(5.3)
5
```

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