Function parameter

Passing a function to another function

Fact:

Example:

void f() { printf("Hello\n"); } void g() { printf("Good-bye\n"); } void run( void h() ) // Function with a function as parameter ! { h(); // Run h()... } int main( int argc, char *argv[] ) { run( f ); // Call run with parameter f run( g ); // Call run with parameter g }

Output:

Hello Good-bye

Example Program: (Demo above code)
- Prog file: click here
How to run the program:

How is this implemented ???

Implementation:

A function is passed "by reference":
The function can be invoked with a special branch subroutine instruction (that I did not teach) that uses a register as operand:
Note: this version of bsr is not available in M68000.

Example: compiler the above program with gcc -S func-param1.c and look for this portion of code:

run: .LFB2: .cfi_startproc pushq %rbp .cfi_def_cfa_offset 16 movq %rsp, %rbp .cfi_offset 6, -16 .cfi_def_cfa_register 6 subq $16, %rsp movq %rdi, -8(%rbp) movq -8(%rbp), %rdx // Put starting address of function in rdx movl $0, %eax call *%rdx // Call the function (bsr) !!!

Defining function parameter type

"Type" of a function parameter

Warning:

A function passed as a parameter must match the type of the function parameter
I.e.:

Example: demos compiler error when parameters do not match

void f( char *s ) // f has a string parameter s { printf("%s\n", s); } void g( int s ) // g has an int parameter s { printf("%d\n", s); } int h( char * s ) // h has different return type { printf("%s\n", s); return 0; } /* ---------------------------------------------------------------- run has a function parameter of the form: void h( char * ) --- i.e., a string (char[]) Function f matches this function parameter declaration Function g does NOT match this function parameter declaration ---------------------------------------------------------------- */ void run( void h( char * ), char *a ) { h(a); } int main( int argc, char *argv[] ) { run( f, "Hello" ); run( g, "Hello" ); // Error: argument 1 (g) has the wrong type !!! run( h, "Hello" ); // Error: argument 1 (h) has the wrong type !!! }

Explanation:

The function parameter h of the function run has the type:
I.e.: it is a function with one parameter of the data type char *.

The function f matches the type:

void f( char *s ) { ... }

So there is no error in the function call:

run ( f, "Hello" ); // f matches the function parameter type

However, the function g has the type:

void g( int s ) // Parameter type of g is not char * { ... }

So there is a compiler error in the function call:

run ( g, "Hello" ); // g does not match the function parameter type

The function h causes an error because the return type mismatches....

Example Program: (Demo above code)
- Prog file: click here
How to run the program:

Using function parameters

Problem description:

Write the bi-section algorithm in C:
The bi-section algorithm was discussed in CS170: click here

The bi-section algorithm:

#define epsilon 0.00001 /* ---------------------------- We solve this equation... ---------------------------- */ double f( double x ) { return x*x*x + 2*x - 5 ; // Solve: x^3 + 2x - 5 = 0 } int main( int argc, char *argv[] ) { double a, b, m, y_m, y_a; a = 0; b = 10; // Start interval [a .. b] while ( (b-a) > epsilon ) { m = (a+b)/2; // Mid point of [a .. b] y_m = f(m); // y_m = f(m) y_a = f(a); // y_a = f(a) if ( (y_m > 0 && y_a < 0) || (y_m < 0 && y_a > 0) ) { // f(a) and f(m) have different signs: move b b = m; } else { // f(a) and f(m) have same signs: move a a = m; } printf("New interval: [%lf .. %lf]\n", a, b); // Print progress } printf("Approximate solution = %lf\n" , (a+b)/2 ); }

Example Program: (Demo above code)
- Prog file: click here
How to run the program:

The bi-section algorithm written as a function:

#define epsilon 0.00001 double f( double x ) { return x*x*x + 2*x - 5 ; // Solve: x^3 + 2x - 5 = 0 } double g( double x ) { return x*log(x) + 3*sin(x) - 7 ; // Solve: x*ln(x) + 3*sin(x) - 7 } /* ====================================================== BiSection algorithm: uses a function parameter !!! ====================================================== */ double BiSection( double f(double), double a, double b ) { double m, y_m, y_a; while ( fabs(b-a) > epsilon ) // fabs() = floating point (double) absolute { m = (a+b)/2; // Mid point y_m = f(m); // y_m = f(m) y_a = f(a); // y_a = f(a) if ( (y_m > 0 && y_a < 0) || (y_m < 0 && y_a > 0) ) { // f(a) and f(m) have different signs: move b b = m; } else { // f(a) and f(m) have same signs: move a a = m; } printf("New interval: [%lf .. %lf]\n", a, b); // Print progress } return (a+b)/2 ; } int main( int argc, char *argv[] ) { double r; r = BiSection( f, 0, 10 ); printf("x^3 + 2x - 5 = 0, x = %lf\n" , r ); r = BiSection( g, 0, 10 ); printf("x*ln(x) + 3*sin(x) - 7, x = %lf\n" , r ); }

Output:

x^3 + 2x - 5 = 0, x = 1.328273 x*ln(x) + 3*sin(x) - 7, x = 5.445571

Example Program: (Demo above code)
- Prog file: click here
How to run the program: