Why constants

Since then, the metric system has undergone multiple evolutions and redefinitions, although it was always based, even indirectly, on physical constants. The redefinition of the seven base units will consequently be based on an explicit-constant formulation, which is a definition in which the unit is defined indirectly, and one that gives an exact value to a recognized fundamental constant.

This value can therefore not be obtained empirically through a measurement. Advances in instrumentation have already provided measures that are sufficiently precise to allow for conventionally establish exact values for some of them.

Seven constants for measuring everything. The CGPM has provided a very detailed roadmap on how the different fundamental constants involved in the new SI should be remeasured , 2 before fixing their numerical values definitively. For instance for the Planck constant, the Conference demanded two independent methods, each applied in a number of metrology laboratories throughout the world. The first was based on the Kibble balance also called a watt balance , which can balance masses with electromagnetic forces.

The second consists of counting the atoms of a sphere of silicon 10 centimeters in diameter in order to define the Avogadro constant, and to then deduce a value of the Planck constant by using other known fundamental constants.

Once again, the discussions were tense. That is a way of saying that one ultimately has to make a decision. More specifically, these adjustments are the work of the Committee on Data for Science and Technology CODATA , 3 which since has been tasked with keeping the list and value of fundamental constants in physics up to date. This coherence was deemed satisfactory by the CIPM in late during its th session, which duly noted that the conditions fixed to proceed with the revision of the SI had been met, opening the way for the adoption of the new system by the CGPM scheduled for November.

For example, high-energy physicists sometimes use Planck units, for which the values of the constants h , c , and G are all fixed by convention at 1. This considerably simplifies the writing of equations that describe the laws of physics… but provides units that are not very practical as soon as we leave the realm of the infinitely small. Constants and theories of physics Constants and theories of physics P. These fundamental constants, which cannot be calculated, also emphasize the limits of our physical theories.

The number and status of constants that physicists use reflect the evolution of theories in physics, by making explicit the unit of certain physical phenomena. Today the calorie has become obsolete. Sign in. Variables do not have to be assigned initial values. Variables once defined may be assigned a value within the instructions of the program.

Skip to content Overview A constant is a value that cannot be altered by the program during normal execution, i. Constants are used in two ways. They are: literal constant defined constant A literal constant is a value you type into your program wherever it is needed.

Examples include the constants used for initializing a variable and constants used in lines of code: 21 Defining Constants and Variables Named constants must be assigned a value when they are defined.

It helps to express yourself in code, and avoid comments. For a programming language in general, you don't really need them.

High level dynamic languages such as Ruby and JavaScript doesn't have them or at least not in a true constant sense. Variables are used instead, just like you suggested. The const keyword is often used for function parameters, particularly pointers, to suggest the memory the pointer points to will not be modified by the function. Look at the decleration for strcpy for instance:. It's a really easy way to trap a certain class of errors. If you declare a variable const , and accidentally try to modify it, the compiler will call you on it.

Constants are very necessary in regards to declaration and intialization of variable for any purpose such as at the starting of the loop, to check the condition within the if -else statement, etc. Therefore the end of the world happens and you also loose the answer to life. Plus const is stored elsewhere in memory, when compared to the storage of normal variables, and is more efficient. Stack Overflow for Teams — Collaborate and share knowledge with a private group.

Create a free Team What is Teams? Collectives on Stack Overflow. Learn more. Why use constants in programming? Asked 11 years, 5 months ago. Active 6 years, 6 months ago. Viewed 30k times. Improve this question. Adam N Adam N 1 1 gold badge 3 3 silver badges 3 3 bronze badges.

Fixed length saves memory where worst case copying to new array works — Niklas Rosencrantz. This question requires only one counter-example: why would you make PI a variable? Hans Passant: Are you sure Pi is a constant? So far it's only been measured in the proximity of Earth. Are you sure it's constant throughout the space-time continuum?

Agreed, a value of "3" is not unlikely close to the horizon of a black hole. Which requires a redefinition of the bool type too, have to add "file irretrievably lost". Developer Art: Pi is an abstract mathematical constant independent of physical space, as is every other mathematical constant. Show 4 more comments. Active Oldest Votes.

As to why constants are used instead of literal numbers: 1 It makes code more readable. Improve this answer. Uri Uri OP isn't asking about literals. I feel the first paragraph of my response answers that: no need to use a variable if it doesn't change over time. But I will clarify this further.