What is a program?

What you will be able to do

After this lesson you can

• say what a program is, and why the order of its lines matters,
• write a program of several print() lines,
• explain what every part of a print() line does, and
• add a comment to a program.

Instructions for a computer

Think about a recipe. It is a list of steps in a fixed order: wash the rice, add water, boil, wait. A person can guess what you mean if a step is a little unclear. A computer cannot. It does exactly what the list says, in the order of the list, and it never guesses.

A program is such a list of instructions. We write it in a programming language. Python is one. It reads almost like English, so it is a good first language, and many scientists, students and companies all over the world use it.

Your first program

• The computer starts at the first line and goes down, one line at a time, until the end.
• Each line is one instruction. Here, every instruction is a call of print().
• The output has three lines, in the same order as the program.

Taking print() apart

• print is the name of a function. A function is a ready-made helper that does one job. This one shows something on the screen.
• The brackets ( ) hold what we give to the function.
• The quotation marks " " mark the start and the end of a piece of text. The text between them is shown exactly as you typed it. The quotation marks themselves are not shown.
• 2 + 3 has no quotation marks, so Python works it out and shows the answer, 5.
• The comma between the two pieces tells print() that there are two things to show. It puts one space between them.

The order matters

Python runs the lines from the top to the bottom. If you change the order of the lines, you change the output.

Swap two lines in the code box and run it. The two lines of output swap places too.

Comments

A line that starts with # is a comment. Python skips it. A comment is a note from you to a person who reads the program, and that person is often you, a month later.

Look at the first line of each program above. It is a comment. It says what the program does. Writing a comment is a good habit from the first day.

Common mistakes

• Forgetting the quotation marks around text. print(Hello) is an error, because Python looks for something called Hello.
• Writing Print or PRINT. The name is print, in small letters.
• Forgetting the brackets. print "Hello" is an error in Python 3.
• Expecting Python to guess. If the lines are in the wrong order, the output is in the wrong order. Python does what you wrote, not what you meant.

Practice

Start with two quick questions. Each New numbers gives you a fresh one, so you can practise as long as you like.

Then the labs and the problems:

1. Warm-up. Open the lab Say it with print(). Write the lines, and let the checker tell you if the output is exactly right.
2. Build. Open the problem Print a message, and then write a program that shows your own name, class and school, on three lines.
3. Stretch. Open the problem Which lines does Python skip? Decide which lines are comments, before you read the solution.

Remember these

• A program is a list of instructions. The computer does them from the top to the bottom, exactly as written.
• print() shows things on the screen. Text goes inside quotation marks.
• A sum without quotation marks is worked out. A line that starts with # is a comment, and Python skips it.
• The order of the lines is the order of the output.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. How many lines does the first program of this lesson show?

2. Does Python show the quotation marks?

3. What is a comment for?

4. If you swap two print() lines, what changes?