Making decisions with conditionals

Imagine holding the collect button after a game piece is already in the intake. We probably don’t want the motor to keep pulling it in. How can our code handle that? A conditional lets it ask a question and choose what to do with the answer.

Choosing which instructions run¶

Start with if condition:. If the condition is true, Python runs the indented lines beneath it. Otherwise it skips them. The colon introduces the block; indentation shows where that block belongs. Use four spaces for each level.

Add else: for what should happen when the condition is false. An else doesn’t take a new condition. If there are more than two possibilities, elif means “otherwise, if ...”. In a single if/elif/else chain, Python chooses the first true branch, or the final else if none match. It does not run every branch.

Be a little careful with two separate if statements: Python considers both of them. The second can change an output the first just chose. When you want exactly one decision to win, keep those alternatives in one if/elif/else chain.

Read it before running it¶

Put this worked example in student-work/foundations/conditionals.py. Read it once and write down what you think it prints. Then save and run python student-work/foundations/conditionals.py using your environment’s Python.

enabled = True
has_piece = False
request = 0.6

# Choose an output; disabled behavior takes priority.
if not enabled:
    output = 0.0
elif has_piece:
    output = 0.0
else:
    output = request

print("Intake output:", output)

Here the disabled question is false, the possession question is false, and the final branch sets output to 0.6. The unindented print runs afterward regardless of the branch selected. Try all four combinations of enabled and has_piece. Write the selected branch next to each result.

Now set request = -0.4 while holding a piece. This version blocks release too. That is a missing requirement, not a Python error. We need to distinguish forward acquisition from reverse release.

Try it yourself¶

enabled = True
has_piece = True
request = -0.4
if not enabled:
    output = 0.0
elif has_piece and request > 0:
    output = 0.0
else:
    output = request
print("Intake output:", output)

Before moving on¶

Without opening the solution, write down which branch handles an enabled intake holding a piece with a request of -0.3. Then explain what changes when enabled becomes False. Run both cases and compare the outputs with your predictions.

Explain why the disabled check is first. Show the exact boundary at request 0.0, and explain what would happen with two unrelated if statements that overwrite output.