1. Symbolic Constants in C
Professor Hugh C. Lauer CS-2303, System Programming Concepts
(Slides include materials from The C Programming Language, 2nd edition, by Kernighan and Ritchie, Absolute C++, by Walter Savitch, The C++ Programming Language, Special Edition, by Bjarne Stroustrup, and from C: How to Program, 5th and 6th editions, by Deitel and Deitel)
CS-2303, A-Term 2012
Symbolic Constants in C

2. Specifying Symbolic Constants in C
Two ways
Textual substitution
Declaration of const data object
CS-2303, A-Term 2012
Symbolic Constants in C

3. Constant – Textual Substitution
See page
14 & 89 in K&R
#define NAME replacement-text
E.g.,
#define PI
#define LOWER 0 #define UPPER 300 #define STEP 20
A preprocessor directive!
It is traditional in C for textual substitution names to be all UPPER CASE
CS-2303, A-Term 2012
Symbolic Constants in C

4. Constant – Textual Substitution
See page
14 & 89 in K&R
#define NAME replacement-text
E.g.,
#define PI
#define LOWER 0 #define UPPER 300 #define STEP 20
When a textual substitution constant is used in a program, the compiler simply substitutes the replacement text on the fly
CS-2303, A-Term 2012
Symbolic Constants in C

5. Symbolic Constants in C
Constant Declaration
const double pi = ;
const double c = e+8; /* speed of light in meters/sec */
Defines a value of the declared type with the declared name
I.e., creates storage to hold this value
Must be initialized
May never be left side of an assignment
CS-2303, A-Term 2012
Symbolic Constants in C

6. Symbolic Constants in C
Constants in C and C++
Vast majority of C-programs define constants by textual substitution and “#define”
Reason:– It was the only kind of constant definition available in K&R and in early versions of the C-compiler
Has been expanded to a “macro” facility over the years
Most C++-programs define constants using const
Makes for cleaner, clearer programs
CS-2303, A-Term 2012
Symbolic Constants in C

7. Symbolic Constants in C
Questions?
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CS-2303, A-Term 2012
Symbolic Constants in C