Notes on Python for Informatics Chapter 1: Why should you learn to write programs?

Source

What does a programmer do:

• knows a programming language (i.e. Python) - "words", "sentences", "paragraphs", etc.
• "tells a story": combine the text elements is a way that provides a general "idea" to the "reader". In programming, our program is the “story” and the problem you are trying to solve is the “idea”.

Computer hardware architecture

The high-level definitions of these parts are as follows:

• The Central Processing Unit (or CPU) is that part of the computer that is built to be obsessed with “what is next?”. If your computer is rated at 3.0 Gigahertz, it means that the CPU will ask “What next?” three billion times per second. You are going to have to learn how to talk fast to keep up with the CPU.
• The Main Memory is used to store information that the CPU needs in a hurry. The main memory is nearly as fast as the CPU. But the information stored in the main memory vanishes when the computer is turned off.
• The Secondary Memory is also used to store information, but it is much slower than the main memory. The advantage of the secondary memory is that it can store information even when there is no power to the computer. Examples of secondary memory are disk drives or flash memory (typically found in USB sticks and portable music players).
• The Input and Output Devices are simply our screen, keyboard, mouse, microphone, speaker, touchpad, etc. They are all of the ways we interact with the computer.
• These days, most computers also have a Network Connection to retrieve information over a network. We can think of the network as a very slow place to store and retrieve data that might not always be “up”. So in a sense, the network is a slower and at times unreliable form of Secondary Memory

The building blocks of programs

These constructs are part of every programming language from machine language up to the high-level languages.

• input:
  Get data from the “outside world”. This might be reading data from a file, or even some kind of sensor like a microphone or GPS. In our initial programs, our input will come from the user typing data on the keyboard.
• output:
  Display the results of the program on a screen or store them in a file or perhaps write them to a device like a speaker to play music or speak text.
• sequential execution:
  Perform statements one after another in the order they are encountered in the script.
• conditional execution:
  Check for certain conditions and execute or skip a sequence of statements.
• repeated execution:
  Perform some set of statements repeatedly, usually with some variation.
• reuse:
  Write a set of instructions once and give them a name and then reuse those instructions as needed throughout your program.

General types of errors:

• Syntax errors:
  These are the first errors you will make and the easiest to fix. A syntax error means that you have violated the “grammar” rules of Python. Python does its best to point right at the line and character where it noticed it was confused. The only tricky bit of syntax errors is that sometimes the mistake that needs fixing is actually earlier in the program than where Python noticed it was confused. So the line and character that Python indicates in a syntax error may just be a starting point for your investigation.
• Logic errors:
  A logic error is when your program has good syntax but there is a mistake in the order of the statements or perhaps a mistake in how the statements relate to one another. A good example of a logic error might be, “take a drink from your water bottle, put it in your backpack, walk to the library, and then put the top back on the bottle.”
• Semantic errors:
  A semantic error is when your description of the steps to take is syntactically perfect and in the right order, but there is simply a mistake in the program. The program is perfectly correct but it does not do what you intended for it to do. A simple example would be if you were giving a person directions to a restaurant and said, “... when you reach the intersection with the gas station, turn left and go one mile and the restaurant is a red building on your left.”. Your friend is very late and calls you to tell you that they are on a farm and walking around behind a barn, with no sign of a restaurant. Then you say “did you turn left or right at the gas station?” and they say, “I followed your directions perfectly, I have them written down, it says turn left and go one mile at the gas station.”. Then you say, “I am very sorry, because while my instructions were syntactically correct, they sadly contained a small but undetected semantic error.”.

Terminology:

• high-level language:
  A programming language like Python that is designed to be easy for humans to read and write.
• machine language / low-level language:
  A programming language that is designed to be easy for a computer to execute; also called “machine code” or “assembly language.” The lowest level language for software which is the language that is directly executed by the central processing unit (CPU).
• compile:
  To translate a program written in a high-level language into a low-level language all at once, in preparation for later execution.
• interpret:
  To execute a program in a high-level language by translating it one line at a time.
• portability:
  A property of a program that can run on more than one kind of computer.
• program:
  A set of instructions that specifies a computation.
• source code:
  A program in a high-level language.
• bug:
  An error in a program.
• interactive mode:
  A way of using the i.e. Python interpreter by typing commands and expressions at the prompt.
• parse:
  To examine a program and analyze the syntactic structure.
• prompt:
  When a program displays a message and pauses for the user to type some input to the program.
• semantics:
  The meaning of a program.