Last Updated on 2021-04-29 by Clay
Use Symbolic Constants is a way to improve the "source code readability". Just like the classic conversion between Celsius and Fahrenheit, the lower, upper, and step of the boundary value and the updated value are actually meaningful numbers, especially the temperature boundary value.
Such a magical but important value, we will call it magic number. In the program, if third-party developers who do not understand the situation cam understand the importance of this value at a glance, we can write it as a Symbolic Constant.
How to use Symbolic Constant
The setting of symbolic constants needs to be written at the beginning of the program, under the description of #include
library, etc.
#define NAME REPLACEMENT_TEXT
After that, the compiler will automatically treat it as the value of REPLACEMENT_TEXT every time it parses the name of NAME. You might want to ask: Is this different from using variables?
Variables can be reassigned, but symbolic constants cannot.
Suppose we set X to 100.
#define X 100
We can't set X to another value again:
# Error
X = 200;
This will report an error, because in the understanding of the compiler, this program is as follows:
100 = 200;
Obviously wrong.
Next, we rewrite again the code for converting between Celsius and Fahrenheit, which was rewritten using the for-loop in the previous note.
#include <stdio.h> #define LOWER 0 #define UPPER 300 #define STEP 20 int main() { float F, C; // For-loop for (F=LOWER; F<=UPPER; F+=STEP) { C = (5.0 / 9.0) * (F - 32.0); printf("%3.0f %6.1f\n", F, C); } return 0; }
Output:
0 -17.8
20 -6.7
40 4.4
60 15.6
80 26.7
100 37.8
120 48.9
140 60.0
160 71.1
180 82.2
200 93.3
220 104.4
240 115.6
260 126.7
280 137.8
300 148.9
Finally, explain again the symbolic constants:
- The purpose is to improve readability
- It is constant, not a variable, and cannot be assigned.
References
- https://www.codesansar.com/c-programming/symbolic-constants.htm
- https://stackoverflow.com/questions/5062019/what-is-the-point-of-symbolic-constants