Changing Variables in VEXcode Pro V5 – VEX Library

The VEX Visual Studio Code Extension has replaced VEXcode Pro V5, which is now end-of-life.

VEXcode Blocks and VEXcode Text remain actively developed and supported for all VEX platforms.

Changing a variable is a complex question when it comes to object-oriented programming languages like C++. In this article, you'll learn the fundamentals of changing variables, which include the const and lvalue concepts.

What is this "const" before the data type?

const int PI = 3.14159;

“ const” is a very important qualifier. You should use this qualifier for variables that should never change.

L-value or R-value concept

The following shows some examples:

int X  = 1,  Y = 3;	
X = Y + 1;    // Good		
Y + 1 = X;   // INCORRECT!

The key is for L-value:

Variable to be modified must be put on the left side of the “=” operator.
Operation expression should be on the right side of the “=” operator.

R-Value:

Basically, any other expressions.

Caution: While the following is syntactically correct, it is bad:

#include "vex.h"

using namespace vex;

int main() {
  // Initializing Robot Configuration. DO NOT REMOVE!
  vexcodeInit();

  int X = 1, Y = 10;
  X = (Y = (X+5)) = Y*2; // BAD! Compiler gives a warning.
  Brain.Screen.print("x = %d ; y = %d ", X, Y);

}

Output: X = 20; Y = 20;

Besides, when the compiler produces a warning, you should always review it carefully. Never a good idea to ignore it.