Conditional logic (if / else)

Learn

What you’ll learn

• if, else if and else
• Comparison operators
• Combining conditions with and, or and not

if / else if / else

if (distance() < 15) {
  motors(-50, -50);        // very close: back up
} else if (distance() < 40) {
  motors(-40, 40);         // close: turn
} else {
  motors(60, 60);          // clear: drive
}

The robot checks the conditions in order and runs the first one that’s true. Put the most specific or most urgent condition first.

Comparisons

Operator	Means
< / >	less than / greater than
<= / >=	less or equal / greater or equal
== (C++) / === (JavaScript)	equal to
!= / !==	not equal to

Combining conditions

Operator	Means	Example
&&	and: both must be true	lineLeft() && lineRight()
||	or: at least one is true	distanceLeft() < 10 || distanceRight() < 10
!	not: flips true and false	!lineLeft()

Tip

Robots run their conditions 50 or more times a second. Each time round, the robot re-decides based on fresh sensor readings.

Examples

Example: three-way decision

int light = analogRead(A0);
if (light < 200) Serial.println("dark");
else if (light < 700) Serial.println("normal");
else Serial.println("bright");

Example: order matters

// WRONG: the second branch can never run
if (distance() < 40) motors(-40, 40);
else if (distance() < 15) motors(-50, -50);

// RIGHT: most urgent first
if (distance() < 15) motors(-50, -50);
else if (distance() < 40) motors(-40, 40);

Example: and / or

if (lineLeft() && lineRight()) motors(0, 0);          // junction: stop
if (distanceLeft() < 8 || distanceRight() < 8) motors(-40, -40); // scraping a side

Practice

Try it

Decisions

▶ Run↺ Reset

Your code (JavaScript)function loop() { if (distance() < 15) { motors(-50, -50); } else if (distance() < 40) { motors(-40, 40); } else { motors(60, 60); } }Load starter code

Robot commands

• motors(left, right): wheel power, −100 to 100
• lineLeft(), lineRight(): true over the black line
• distance(): cm to the nearest thing ahead (max 200)
• distanceLeft(), distanceRight(): cm to the sides
• linePosition(): where the line is, −1 (left) to 1 (right), or null
• millis(): milliseconds since Run
• print(value): show a message below
• setup() runs once; loop() runs 50 times a second

Cautious driverEasy

Add another level: drive slowly (30) between 40 and 70 cm. Add else if (distance() < 70) motors(30, 30); before the final else.

Side guardMedium

If either side is closer than 8 cm, steer away from that side.

Show an answer

if (distanceLeft() < 8) motors(60, 20);        // wall on the left: steer right
else if (distanceRight() < 8) motors(20, 60);  // wall on the right: steer left

Put these before the forward branch.

Truth tableHard

Write a table for A && !B for all four combinations of A and B. A=false,B=false → false; A=false,B=true → false; A=true,B=false → true; A=true,B=true → false.

Quick check

1. With if / else if, which branch runs?

All true branchesThe first true oneThe last true oneA random one

2. What does && mean?

orandnotequals

3. !lineLeft() is true when…

The left sensor sees the lineThe left sensor does NOT see the lineBoth sensors see itAlways

Check my answers