Artwork: An introduction to
programming primitives
• Draw a picture [1 minute]
– It can be concrete, abstract, whatever you want.
– Don’t show it to anyone yet. You will show it later.
• Find a partner far away from you & sit together [1 minute]
• Each A and B (partners), do this:
– A: Without showing B your picture,
tell B how to replicate the picture exactly.
– B: Following A’s instructions, draw the picture.
• After 2 to 3 minutes, compare the original and the copy.
• Repeat, with A and B reversed.
Fundamentals of Software
Development 1
Slide 1
Artwork – Observations
• Class discussion:
– What are your observations from this exercise?
• Your observations may include:
– Giving adequate instructions is hard!
– Good instructions must be precise.
– The instructions can be executed more than once.
• The 2nd execution may yield the same picture as the 1st.
• Or perhaps not, if the instructions interact with the environment:
– perhaps you use a different pencil
– perhaps you had too much coffee and your hand starts shaking.
– These are all characteristics of computer programs, too!
Fundamentals of Software
Development 1
Slide 2
What is a Program?
• A program is a set of instructions that a computer can
follow, one by one.
– If the computer knows how to do the instruction,
the computer simply carries out the instruction.
– If there is an instruction that the computer
does not know how to do,
the instruction needs to be rewritten in more detail.
• These are called high-level instructions.
• Exercise: What are some high-level instructions from
the copy-my-picture exercise?
– See the next slide for some answers.
Fundamentals of Software
Development 1
Slide 3
High-Level Instructions
• Suppose the picture is a picture of a person.
– Draw the face is a high-level instruction.
How can it be rewritten?
– Draw the face might become:
• Draw the outline of the face. Draw two eyes. Draw eyebrows. Draw
the nose. Draw the mouth.
• These are high-level instructions too!
How might draw two eyes be rewritten?
– Draw two eyes might become:
• Draw one eye near the top left of the outline. Draw the other eye
near the top right of the outline.
• How might draw one eye be rewritten?
Fundamentals of Software
Development 1
Slide 4
Designing a Program, Part 1
• These high-level instructions show that you can use
top-down design to build programs like the copymy-picture program.
• Step 1:
– Ask yourself “What do I do next?”
– until you have a sequence of instructions that accomplishes
the desired result.
• Step 2:
– Ask yourself “How do I do each of these things?”
– until each instruction is something the computer knows
how to do.
Fundamentals of Software
Development 1
Slide 5
Programming Primitives
• We call the things that the computer knows how to do
programming primitives.
• What general-purpose programming primitives did you use in
the draw-my-face exercise?
• Most computer languages provide:
– Combining by sequencing: do this, then that, then ...
– Combining by grouping and giving the group a name (high-level
instructions).
• This is called procedural abstraction (aka encapsulating in methods)
– Loops
• E.g. (for drawing a circle): Repeat 360 times: draw a short line and change
direction by 1 degree.
– Conditionals
• E.g.: If you have finished the circle, stop; else ...
Fundamentals of Software
Development 1
Slide 6
Summary, so far
• A programmer answers the questions:
– What do I do next?
– How do I do each of these things?
• Programming primitives include:
– Sequencing, procedural abstraction, conditionals, loops.
• This is only one part of the programming problem.
The second part is:
Coordinating the activities
of many interdependent participants
in a computational community
Fundamentals of Software
Development 1
Slide 7