What computing is

Welcome to AP CSP

• This is AP Computer Science Principles — the big ideas of computing.
• You already learned Python in the basics course. Here we use it for AP ideas.
• Some lessons are reading only. Others let you run code. Read each step, then press Continue.

Every program has a purpose

• A program is a set of instructions a computer follows.
• Every program is made for a reason — to solve a problem or help people.
• Example purposes: send a message, play a song, find the shortest road.

Input → process → output

• Most programs work in three steps.
• Input: data goes in (a number you type, a tap, a file).
• Process: the program does work on that data.
• Output: a result comes out (text on the screen, a sound, a saved file).

INPUT            PROCESS                 OUTPUT
"5"     ->   add 5 and 3      ->        "8"

A tiny example

• This program takes an input (a name), does a small process, and shows an output.
• You do not need to write it now — just read it and find the three steps.

name = input("Your name: ")     # input
greeting = "Hello, " + name     # process
print(greeting)                 # output

Design is a cycle

• Good programs are not written perfectly the first time.
• We use an iterative cycle: do it again and again, a little better each time.
• Plan → write → test → improve, then repeat.

   plan ──> write ──> test ──> improve
    ^                              │
    └──────────────────────────────┘

Errors are normal

• An error (a bug) is a mistake that makes the program do the wrong thing.
• In the basics course you learned to debug: read the message, find the line, fix it.
• Finding and fixing bugs is a normal part of every step in the cycle.

Computing is collaborative

• Most real programs are built by a team, not one person.
• People share ideas, split the work, and check each other's code.
• Different people bring different views, which makes the program better.

Common mistakes

• Computing is problem-solving with data, not just typing code.
• An algorithm is a clear, finite sequence of steps.

What is next

• This course follows the 5 Big Ideas of AP CSP: Creative Development, Data, Algorithms & Programming, Computer Systems & Networks, and the Impact of Computing.
• Next lesson we look at bits and binary — how computers store data with 0s and 1s.
• First, try the small task below — take an input, process it, and print the output, just like the example above.