Software Approach, Top Down Design

One way of coding is to simply have a rough idea of what you want to do and then start typing away to produce one hug source file.
This is a very bad idea.
The resulting code is likely to have many issues in it that include

• Poorly documented code
• very hard to maintain and update
• Spaghetti code
• tangled mess of jumps and loops all over the code, very hard for someone to understand. even by the original coder a few days later!
• Duplication of code
• wasting memory and running speed
• Very hard to develop by  more than one person
• there is only one file 
• Very hard to debug
• it either works or it doesn't

There are more issues, but hopefully the list shows that it is best to have a methodical, professional approach to coding which this mini-web will cover.