Variables and Types

Store numbers, give them names, and use them to control the robot.

What is a variable?

A variable is a labeled box that holds a value. You can read it, change it, do math with it. The label is the name you choose. The value is what's inside.

int   driveSpeed = 50;        // whole number (percent)
double turnPower  = 35.5;     // decimal number
bool  isHolding  = true;     // true or false

The three types you'll use most

• int, whole numbers. Speed percentages, encoder ticks, time in milliseconds.
• double, decimal numbers. Sensor readings, distance in inches, PID error.
• bool, true or false. Flags like "has the robot stopped yet?"

Using variables in real code

Compare these two versions of the same program. Both drive forward at 50% for 1.5 seconds. Which is easier to tweak later?

Bad, magic numbers:

LeftMotor.spin(forward, 50, percent);
RightMotor.spin(forward, 50, percent);
wait(1.5, seconds);
LeftMotor.stop();
RightMotor.stop();

Better, variables with meaningful names:

int main() {
  vexcodeInit();   // IDE-supplied setup, leave this alone

  int    speedPct = 50;
  double driveSec = 1.5;

  LeftMotor.spin(forward, speedPct, percent);
  RightMotor.spin(forward, speedPct, percent);
  wait(driveSec, seconds);
  LeftMotor.stop();
  RightMotor.stop();
}

Naming rules

• Start with a letter. Use camelCase: driveSpeed, not drive_speed or DRIVESPEED.
• Be specific. x is too vague; distanceInches tells you what and the unit.
• Constants (values that never change), use const and ALL_CAPS: const int MAX_SPEED = 100;

Curated references

• videoCoding in VEX IQ Part 1: Drive Code
• docsVEX Library: Coding with VEX IQ Smart Motors

int    speedPct = 50;
int    driveSec = 1.5;

LeftMotor.spin(forward, speedPct, percent);
RightMotor.spin(forward, speedPct, percent);
wait(driveSec, seconds);
LeftMotor.stop();
RightMotor.stop();

const int MAX_SPEED = 80;

int main() {
  MAX_SPEED = 100;
  LeftMotor.spin(forward, MAX_SPEED, percent);
}