# Data Dictionary

Finally, we need to know what data our system is actually going to store. This could be in a Pandas dataframe, a dictionary, or just whatever format it comes in from your API. We need to know this before we start programming because we need a sense of what rules the data will need to follow. 

Otherwise, your shall we say... less than tech savvy users will cause error after error after error. Never underestimate an end user - they can and will break the rules! 

So, let's be specific with the data types and validation so we know how to stop this. 

## Example

Here's a **data dictionary** for storing rock data in the Rock Collection Program:

<table data-full-width="true"><thead><tr><th width="116">Variable</th><th>Data Type</th><th>Format for Display</th><th width="105">Size in Bytes</th><th>Size for Display</th><th>Description</th><th>Example</th><th>Validation</th></tr></thead><tbody><tr><td>name</td><td>String</td><td>Text</td><td>50</td><td>50</td><td>The name of the rock/mineral</td><td>"Granite"</td><td>Must be a non-empty string</td></tr><tr><td>hardness</td><td>Integer</td><td>Whole number</td><td>4</td><td>2</td><td>hardness scale value</td><td>6</td><td>1 ≤ hardness ≤ 10</td></tr><tr><td>composition</td><td>String</td><td>Text</td><td>100</td><td>100</td><td>Main mineral components</td><td>"Quartz, Feldspar"</td><td>Must be a valid string</td></tr><tr><td>colour</td><td>String</td><td>Text</td><td>20</td><td>20</td><td>Common rock colors</td><td>"Grey"</td><td>Must be a valid colour name</td></tr><tr><td>location_found</td><td>String</td><td>Text</td><td>20</td><td>100</td><td>Place where the rock was found</td><td>"Yosemite, USA"</td><td>Must be a valid location</td></tr></tbody></table>

## Need More Information?

{% content-ref url="../../../charts-and-algorithms/data-dictionaries" %}
[data-dictionaries](https://gosford-high-school.gitbook.io/preliminary-software-engineering/unit-1-programming-fundamentals/charts-and-algorithms/data-dictionaries)
{% endcontent-ref %}