CCS/TM4C123GH6PM: Is a float not allowed to be evaluated using a variable within its expression. "expression must have a constant value"

Phil Eli

Prodigy 100 points

Part Number: TM4C123GH6PM

Tool/software: Code Composer Studio

I am doing float point calculation but I am getting an error when I compile the code.

float PI = 3.14159265358979323846f;

float GyroMeasError = PI * (60.0f / 180.0f);

float beta = sqrt(3.0f / 4.0f) * GyroMeasError;

The error occurs for the lines where the GyroMeasError and the beta are calculated.

error #28: expression must have a constant value

error #60: function call is not allowed in a constant expression

Thanks,

Fil

over 8 years ago

0 Robert Adsett over 8 years ago

Guru 27665 points

Initializers must be constant expressions. Neither of the bolded declarations have constant initializers, the first has a variable in the initializer, the second has a function call.

Robert

0 Phil Eli over 8 years ago in reply to Robert Adsett

Prodigy 100 points

Thank you Robert this makes sense. I had to reconfigure my complier settings to allow C++ suport to get it to work.

0 Robert Adsett over 8 years ago in reply to Phil Eli

Guru 27665 points

Or you could just do the initialization separate from the declaration. That could well result in clearer code in any case.

Robert

0 Phil Eli over 8 years ago in reply to Robert Adsett

Prodigy 100 points

Good suggestion I will try that instead.

Thanks,
Filmon

0 Gregory Holcomb over 8 years ago in reply to Phil Eli

Prodigy 50 points

If you change to:

#define PI = 3.14159265358979323846f

Then your GyroMeasError initialization should be valid.

I'm unsure how CCS handles the const parameter but it is a good idea to qualify constants as a compiler can sometimes give you smaller/faster code, intention can be more clear, and GyroMeasError might compile with:

const float PI = 3.14159265358979323846f;

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CCS/TM4C123GH6PM: Is a float not allowed to be evaluated using a variable within its expression. "expression must have a constant value"