Understanding Constants in C
Constants are fixed values that cannot be modified after their definition. In C programming, constants improve code readability and maintainability by avoiding repeated literal values.
Defining Constants
There are two main ways to define constants in C:
- Using
#definepreprocessor directive (macro) - Using the
constkeyword
Using #define Directive
The #define directive substitutes a name with its associated value during the preprocessing stage before compilation. The syntax is:
#define NAME value
For example:
#define PI 3.14159
Anywhere PI appears in the code, the preprocessor replaces it with 3.14159.
Advantages of #define
- Improves readability by replacing magic numbers with meaningful names
- Makes changing constants easy in one place
Important Best Practices with #define
- Do not add a semicolon at the end: Adding
;after a macro definition causes compilation errors. - Use uppercase letters for macro names: This avoids conflicts with variable names and enhances code clarity.
- Macros inside double quotes are not replaced: String literals remain unchanged by the preprocessor.
Common Pitfalls
If a macro name conflicts with a variable, the preprocessor may replace all occurrences leading to errors, e.g.,
#define me 89
int me = 74; // Error: 89 = 74
Using uppercase macro names like ME prevents such conflicts.
Macro as Functions
Macros can also mimic functions. Example:
#define ADD(X, Y) ((X) + (Y))
When called as ADD(4, 3), it expands to (4) + (3), returning 7.
Multi-line Macros
Use backslash \ to continue macros over multiple lines:
#define COMPARE(X, Y) \
if ((X) > (Y)) { \
printf("X is greater than Y"); \
} else { \
printf("X is less than Y"); \
}
Macro Expansion and Evaluation
Macros are expanded first, then the expression is evaluated following C operator precedence. For example:
#define MACRO (4 + 3)
int result = 5 * MACRO; // expands to 5 * (4 + 3) = 5 * 7 = 35
However, without parentheses:
#define MACRO 4 + 3
int result = 5 * MACRO; // expands to 5 * 4 + 3 = 20 + 3 = 23
Always use parentheses in macro definitions to prevent unexpected results. For a deeper understanding of such nuances, see Understanding Advanced printf Usage and Integer Behaviors in C Programming.
Predefined Macros in C
C provides predefined macros for compile-time information such as the current date and time:
__DATE__, current compilation date__TIME__, current compilation time
Example:
printf("Compiled on %s at %s", __DATE__, __TIME__);
Summary
- Constants defined with
#defineimprove readability and ease maintenance. - Avoid semicolons at the end of macro definitions.
- Prefer uppercase letters for macro names to prevent naming conflicts.
- Macros can function like inline functions and support multi-line operations.
- Use parentheses in macros to ensure correct operator precedence.
- Utilize predefined macros
__DATE__and__TIME__to access compile-time metadata.
For foundational concepts on variables and constants in C, you might also find Understanding Variables in C Programming: Declaration, Initialization, and Usage useful.
Following these guidelines will help write robust, clear, and maintainable C programs involving constants and macros.
now we are going to have a discussion on constants in C what is a constant as the name suggests some think that never
change of course once defined cannot be modified later in the code yes now how do we define constants we can
define constants in our code using hash define or using Const keyword let's see how to use hash define syntax for has
defined would be hash define then name and then value sometimes name is also called as macro okay job of preprocessor
not compiler is to replace name with value now this is the job of preprocessor like preprocessor does the
job of replacing the header files with their actual content you know this thing that hash include stdio.h is going to
get replaced by preprocessor this name over here is also going to get replaced by this value available over here and
this is the job of preprocessor let's see one example here suppose I define a pie with 3.14159 now this is the name
and this is the value hash define name name is pie and value is 3.14159 right I am simply going to print this pie
instead of using this 3.14159 constant over here I'm simply using pi and print it up to five decimal points let's see
what is going to be the output here you can see this is going to be our output 3.14159 right we would be able to print
this pi using just this P I name over here right with the help of this name we would be able to print this value
3.14159 and this is the advantage of using hash define suppose in our code we have to write 3.14159 many number of
times then writing this constant is quite cumbersome therefore instead of this constant we
can write the spy which is quite readable as well as easy to write therefore we are using hash define now
here are some takeaways to take along with you please don't add semicolon at the end if you notice it carefully that
we are not adding any semicolon at the end so we should not add semicolon at the end here you can see in this example
I'm adding a semicolon at the end now this is one thing that you need to avoid at any cost this will produce an error
so please avoid adding semicolon at the end now this is the common practice and this is the common mistake we do almost
all the time because we usually end our statements with semicolon like this printf function we are ending with
semicolon after this return 0 we are ending with semicolon but here we are not going to add semicolon at the end we
need to avoid this choosing capital letters for name is a good practice for example here you can see I am NOT using
capital letters for me now what is the disadvantage you may get it is possible that I declare a variable with the same
name right and I assigned it a value 74 and I'm just simply trying to print this value now we got an error out of this
because our macro is also with same name right now preprocessor does its job and replace this value with 89 wherever it
finds it out here also it replace this with 89 and replace this also with 89 now this is the disadvantage because as
you can see this is equivalent to this statement right when the preprocessor replaced this value with 89 and then
this is what we got 89 is equals to 74 now this is something we cannot do right we cannot assign a constant to a
constant and we got an error now it is better to write capital letters for the hash define macros this will avoid
confusions later on suppose you have a big file with lots of lines of codes and some where you declare a variable with
the same name as macro then in that case you got an error and finding out the error is very difficult so we need to
avoid such practice better is to write capital letters for the names of the macros you're defining
over here now whatever inside double quotes won't get replaced if you think in this example this value is also going
to get replaced with 89 then you are wrong because this is inside double quotes and it is treated as string
preprocessor won't replace this particular value with this 89 it is only going to replace this value with 89 now
the output for this is value is 89 not 89 is 89 whatever is there in double quotes is not going to replaced by the
preprocessor remember this thing we can use macros like functions for example here you see I use this add X comma Y
just like a function you know and after that I perform some operation which is X plus y what I want to do is I want to
perform the addition of two numbers for entry I simply call this macro and this is going to get replaced with this X
plus y which is nothing but equals to 4 plus 3 this gives us the output as 7 dashing of two numbers is 7 we can write
multiple lines also using slash now here we can see that I am writing multiple lines over here using slash here what I
want to do is I want to know this thing that whether X is greater than Y or Y is greater than X if X is greater than Y
then I'm going to print X is greater than Y if it is not then X is lesser than Y in this case I am providing 5 and
6 as the values of x and y this is the if/else construct we are going to talk about if-else in later sections here you
can see inside if if the condition is evaluated to true then this statement is evaluated else
this statement is evaluated here you can see I provided 5 and 6 as the values 2 x and y 5 is not greater than 6 therefore
this condition is false this means else condition is evaluated which is 5 is lesser than 6 and this is
going to be our output here 5 is than six right first expansion then evaluation let's see what does it mean
here you can see I have written a result of expression a star B plus C is person times D this person that D is going to
get replaced with the second argument of the printf we know this thing already here I'm performing the operation five
multiplied with this macro over here here you can see this macro is going to get replaced with four plus three which
is equals to seven and we simply multiplied with five which results in 35 right wrong this is not done five is
multiplied with macro first the replacement is going to get done first expansion then evaluation here the
expansion simply means that this macro is expanded to X plus y and it is getting replaced over here which is
nothing but 5 into 4 plus 3 and then the whole expression is evaluated which is 5 into 4 plus 3 result of 5 into 4 plus 3
according to the board mass rule we know this thing that 5 into 4 is evaluated first which gives us 20 and then we add
3 to it which gives us 23 now the answer is not 35 answer is 23 some predefined macros like date/time
can print current date and time if we use this macros they are going to print the current date and current time please
note down this thing that these are double underscores not single same for the time as well now these are the
predefined macros or standard macros which prints the current date and current time okay friends this is it for
now see you in the next lecture bye bye [Applause] [Music]
You can define constants with #define by specifying a name and value without a semicolon, like #define PI 3.14159. This tells the preprocessor to replace all occurrences of PI with 3.14159 before compilation, improving code readability and maintainability.
Adding a semicolon at the end of a #define macro causes compilation errors because the preprocessor substitutes the macro directly into the code, and the extra semicolon might not fit syntactically. Always omit the semicolon to prevent such issues.
Use uppercase letters for macro names, such as NAME or MAX_SIZE, to distinguish them from variables. This prevents the preprocessor from unintentionally replacing variable names that match macro names, which can lead to compilation errors.
Define function-like macros with parameters, for example: #define ADD(X, Y) ((X) + (Y)). Use parentheses around parameters and the entire expression to ensure correct operator precedence during macro expansion and avoid unexpected results.
Multi-line macros use a backslash \ at the end of each line except the last to continue the macro definition. For example: #define COMPARE(X, Y) \ if ((X) > (Y)) { \ printf("X is greater than Y"); \ } else { \ printf("X is less than Y"); \ }. This allows writing complex macros spanning multiple lines.
C provides predefined macros like DATE and TIME that contain the current compilation date and time as string literals. You can use them in your code, e.g., printf("Compiled on %s at %s", DATE, TIME), to display compile-time information.
Without parentheses, operator precedence can cause macros to expand into unexpected code, leading to wrong results. For example, #define MACRO 4 + 3 without parentheses can cause expressions like 5 * MACRO to evaluate incorrectly. Using parentheses like #define MACRO (4 + 3) ensures correct evaluation.
Heads up!
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