1. Functions in C and C++
CS-2303 System Programming Concepts Hugh C. Lauer (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
Functions in C and C++

2. Definition – Function
A fragment of code that accepts zero or more argument values, produces a result value, and has zero or more side effects.
A means of encapsulating a subset of a program or a system
To hide details
To be invoked from multiple places
To share with others
A function in C is equivalent to a method in Java, but without the surrounding class
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3. Functions – a big Topic Examples Function definition
Function prototypes & Header files
Pre- and post-conditions
Scope rules and storage classes
Implementation of functions
Recursive functions
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4. Reading Assignment Chapter 4 of K&R CS-2303, A-Term 2012
Functions in C and C++

5. Common Functions in C #include <math.h> #include <stdio.h>
sin(x) // radians
cos(x) // radians
tan(x) // radians
atan(x)
atan2(y,x) /* atan(y/x), even when x == 0 */
exp(x) // ex
log(x) // loge x
log10(x) // log10 x
sqrt(x) // x  0
pow(x, y) // xy
...
#include <stdio.h>
printf()
fprintf()
scanf()
sscanf()
...
#include <string.h>
strcpy()
strcat()
strcmp()
strlen()
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6. Common Functions (continued)
In K&R
<assert.h> // for diagnostics, loop invariants, etc.
<stdarg.h> // for parsing arguments
<time.h> // time of day and elapsed time
<limits.h> // implementation dependent numbers
<float.h> // implementation dependent numbers
<setjmp.h> // beyond scope of this course
<signal.h> // beyond scope of this course
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7. Common Functions (continued)
See also the man pages of your system for things like
<pthread.h> // concurrent execution
<socket.h> // network communications
... // many, many other facilities
Fundamental Rule: if there is a chance that someone else had same problem as you, …
… there is probably a package of functions to solve it in C!
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8. Functions in C
resultType functionName(type1 param1, type2 param2, …) { …
body }
If no result, resultType should be void
Warning if not!
If no parameters, use void between ()
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9. Functions in C
resultType functionName(type1 param1, type2 param2, …) { …
body
} //functionName
If no result, resultType should be void
Warning if not!
If no parameters, use void between ()
It is good style to end a function with a comment showing its name
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10. N = f(pi*pow(r,2), b+c) + d;
Using Functions
Let int f(double x, int a) be (the beginning of) a declaration of a function.
Then f(expr1, expr2) can be used in any expression where a value of type int can be used – e.g.,
N = f(pi*pow(r,2), b+c) + d;
CS-2303, A-Term 2012
Functions in C and C++

11. Definitions
Parameter:– a declaration of an identifier within the '()' of a function declaration
Used within the body of the function as a variable of that function
Initialized by the caller to the value of the corresponding argument.
Argument:– an expression passed when a function is called; becomes the initial value of the corresponding parameter
Note: Changes to parameters within the function do not propagate back to caller!
All parameters “call by value,” — as in Java
CS-2303, A-Term 2012
Functions in C and C++

12. N = f(pi*pow(r,2), b+c) + d;
Using Functions
Let int f(double x, int a) be (the beginning of) a declaration of a function.
Then f(expr1, expr2) can be used in any expression where a value of type int can be used – e.g.,
N = f(pi*pow(r,2), b+c) + d;
These are parameters
These are arguments
CS-2303, A-Term 2012
Functions in C and C++

13. Parameters and Arguments
Parameters are variables (or constants) that can be used inside of functions
Arguments are expressions that are evaluated at call time and use to initialize parameters
CS-2303, A-Term 2012
Functions in C and C++

14. Note
Functions in C do not allow other functions to be declared within them
Like C++, Java
Unlike Algol, Pascal
All functions defined at “top level” of C programs
(Usually) visible to linker
Can be linked by any other program that knows the function prototype
Can “see” anything declared previously in same program (or in an included interface)
CS-2303, A-Term 2012
Functions in C and C++

15. Note on printf parameters
int printf(char *s, ...) { body } // printf
In this function header, “…” is not a professor’s shorthand
(as often used in these slides)
…but an actual sequence of three dots (no spaces between)
Meaning:– the number and types of arguments is indeterminate
Use <stdarg.h> to extract the arguments
Also scanf(char *s, …);
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16. Questions?
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17. Function Prototypes
There are many, many situations in which a function must be used separate from where it is defined –
before its definition in the same C program
In one or more completely separate C programs
This is actually the normal case!
Therefore, we need some way to declare a function separate from defining its body.
Called a Function Prototype
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18. Function Prototypes (continued)
Definition:– a Function Prototype in C is a language construct of the form:–
return-type function-name (parameter declarations) ;
I.e., exactly like a function definition, except with a ';' instead of a body in curly brackets
Essentially, the method of a Java interface.
aka function header
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19. Purposes of Function Prototype
So compiler knows how to compile calls to that function, i.e.,
number and types of arguments
type of result
… without knowing anything about how it is defined!
As part of a “contract” between developer and programmer who uses the function
As part of hiding details of how it works and exposing what it does.
A function header serves as a “black box.”
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20. Questions?
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Functions in C and C++

21. “Contract” between Developer and User of a Function
Function Prototype
The pre- and post-conditions
I.e., assertions about what is true before the function is called and what is true after it returns.
A logical way of explaining what the function does
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22. Definitions
Pre-condition:–a characterization or logical statement about
the values of the parameters, and
values of relevant variables outside the function
prior to calling the function
Post-condition:–a logical statement or characterization about
the result of the function in relation to the values of the parameters and pre-conditions, and
changes to values of variables outside the function
after the function returns
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23. Example 1 double sin (double angle);
Pre:– angle is expressed in radians
Post:– result is the familiar sine of angle
Note: this function does not use or change any other variables
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24. Example 2 int printf (string, arg1, arg2, …)
Pre:– string is terminated with '\0' and contains conversion specifiers
Pre:– a buffer maintained by the file system contains zero or more unprinted characters from previous calls.
Post:– args are substituted for conversion codes in copy of string; resulting string is added to buffer
Post:– if '\n' is anywhere in buffer, line is “printed” up to '\n'; printed characters are cleared from buffer
Post:– result is number of characters added to buffer by printf
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25. Example 3
float total = 0; int count = 0; int GetItem(void) { float input; int rc; printf("Enter next item:- "); if ((rc = scanf("%f", &input)) != EOF && (rc > 0)) { total += input; count++; }; // if return rc; } // GetNewItem
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26. Example 3
Pre:– total is sum of all previous inputs, or zero if none
Pre:– count is number of previous inputs, or zero if none
Post:– if valid input is received total = totalprev + input, count = countprev + 1
float total = 0; int count = 0; int GetItem(void) { float input; int rc; printf("Enter next item:- "); if ((rc = scanf("%f", &input)) != EOF && (rc > 0)) { total += input; count++; }; // if return rc; } // GetNewItem
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27. Important Pre- and post-conditions are analogous to loop invariants
I.e., they describe something about the data before and after a function is called and the relationship that the function preserves
Often are used together with loop invariants
… to show that loop invariant is preserved from one iteration to the next
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28. Questions?
Next Topic
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