A small program becomes a design question once it has more than a few values: which values you keep, and which you work out when needed. Both choices are decisions, and the program is easier to change when they are made on purpose.
About 2 min of reading
Design the values for a small task before coding it.
Store two facts and derive a third.
Before you run it, say out loud what it will print. Then open the answer.
Keep cents as the fact and show dollars as derived.
Before you run it, say out loud what it will print. Then open the answer.
Loop on a named state instead of a counted range.
Before you run it, say out loud what it will print. Then open the answer.
Track a best so far, starting from nothing.
Before you run it, say out loud what it will print. Then open the answer.
Before you write anything, put these lines back in the right order. Click a line to move it up one step. This support step is easier than writing code from scratch, and it is here to make the last example stick.
Read the first example one line at a time. A line that prints shows what appears on screen; the rest set things up quietly.
Everything before this was about one value at a time. This is the point where values meet a program, and where the question stops being what the computer does and becomes what you chose to keep. That question is the one that scales for as long as you write programs.
Split the values of a program into the ones that are facts and the ones that follow from facts. Only facts get changed. Every derived value is a function of the facts, so it can never disagree with them, and it can be recomputed whenever needed.
first = "Ada"
last = "Lace"
full = first + " " + last
print(full)first and last are the facts. full is worked out when needed. If a name changes, only the facts change, and every derived value follows.
cents = 1250
print(cents / 100)Storing the dollars as well would mean two values that must agree. They can drift apart, and a program with drifting values has answers that contradict each other. Deriving removes the possibility of drift.
remaining = 3
while remaining > 0:
print("turn")
remaining = remaining - 1remaining says what is left, which is a fact a reader can check. A counter counting up to a limit says the same thing less directly. Naming the state is what makes a loop readable.
best = None
for score in [4, 9, 7]:
if best is None or score > best:
best = score
print(best)best holds the best so far, and None records that nothing has been seen. The loop only ever changes one value, so the whole program is one fact updated in place.
Each one is defined here on this site. Open the entry for the full definition and the other words it points at.