Related
#include<stdio.h>
int areaOfRectangle(int,int);
int perimeter(int,int);
int main()
{
int l,b;
scanf(" %d %d",&l,&b);
printf("%d %d %d %d",l,b,areaOfRectangle(l,b),perimeter(l,b));
return 0;
}
int areaOfRectangle(int a,int b)
{
int area;
area=a*b;
return area;
}
int perimeter(int c,int d)
{
int meter;
meter=2(c+d);
return meter;
}
why this error:called object is not a function or function pointer at line: meter=2(c+d)?
Also, can I use the same variable a,b to pass in perimeter function?
Your code meter=2(c+d);should be changes as meter=2*(c+d);
You can use the same variable a,b to pass in perimeter function, A parameter is just a local variable.
In this statement:
int perimeter(int c,int d)
{
int meter;
meter=2(c+d);
return meter;
}
You'll obviously get a syntax error since the compiler won't detect 2(c+d) as you're trying to implicitly multiply 2 with (c + d). Rather, if you explicitly define 2 * (c + d) then it'll no longer show you any error.
One side tip, you don't need to use any local variables inside a function if you just want to return a simple return statement, rather you may use:
return 2 * (c + d);
The declaration is redundant thence.
I want to create a function that performs a function passed by parameter on a set of data. How do you pass a function as a parameter in C?
Declaration
A prototype for a function which takes a function parameter looks like the following:
void func ( void (*f)(int) );
This states that the parameter f will be a pointer to a function which has a void return type and which takes a single int parameter. The following function (print) is an example of a function which could be passed to func as a parameter because it is the proper type:
void print ( int x ) {
printf("%d\n", x);
}
Function Call
When calling a function with a function parameter, the value passed must be a pointer to a function. Use the function's name (without parentheses) for this:
func(print);
would call func, passing the print function to it.
Function Body
As with any parameter, func can now use the parameter's name in the function body to access the value of the parameter. Let's say that func will apply the function it is passed to the numbers 0-4. Consider, first, what the loop would look like to call print directly:
for ( int ctr = 0 ; ctr < 5 ; ctr++ ) {
print(ctr);
}
Since func's parameter declaration says that f is the name for a pointer to the desired function, we recall first that if f is a pointer then *f is the thing that f points to (i.e. the function print in this case). As a result, just replace every occurrence of print in the loop above with *f:
void func ( void (*f)(int) ) {
for ( int ctr = 0 ; ctr < 5 ; ctr++ ) {
(*f)(ctr);
}
}
Source
This question already has the answer for defining function pointers, however they can get very messy, especially if you are going to be passing them around your application. To avoid this unpleasantness I would recommend that you typedef the function pointer into something more readable. For example.
typedef void (*functiontype)();
Declares a function that returns void and takes no arguments. To create a function pointer to this type you can now do:
void dosomething() { }
functiontype func = &dosomething;
func();
For a function that returns an int and takes a char you would do
typedef int (*functiontype2)(char);
and to use it
int dosomethingwithchar(char a) { return 1; }
functiontype2 func2 = &dosomethingwithchar
int result = func2('a');
There are libraries that can help with turning function pointers into nice readable types. The boost function library is great and is well worth the effort!
boost::function<int (char a)> functiontype2;
is so much nicer than the above.
Since C++11 you can use the functional library to do this in a succinct and generic fashion. The syntax is, e.g.,
std::function<bool (int)>
where bool is the return type here of a one-argument function whose first argument is of type int.
I have included an example program below:
// g++ test.cpp --std=c++11
#include <functional>
double Combiner(double a, double b, std::function<double (double,double)> func){
return func(a,b);
}
double Add(double a, double b){
return a+b;
}
double Mult(double a, double b){
return a*b;
}
int main(){
Combiner(12,13,Add);
Combiner(12,13,Mult);
}
Sometimes, though, it is more convenient to use a template function:
// g++ test.cpp --std=c++11
template<class T>
double Combiner(double a, double b, T func){
return func(a,b);
}
double Add(double a, double b){
return a+b;
}
double Mult(double a, double b){
return a*b;
}
int main(){
Combiner(12,13,Add);
Combiner(12,13,Mult);
}
Pass address of a function as parameter to another function as shown below
#include <stdio.h>
void print();
void execute(void());
int main()
{
execute(print); // sends address of print
return 0;
}
void print()
{
printf("Hello!");
}
void execute(void f()) // receive address of print
{
f();
}
Also we can pass function as parameter using function pointer
#include <stdio.h>
void print();
void execute(void (*f)());
int main()
{
execute(&print); // sends address of print
return 0;
}
void print()
{
printf("Hello!");
}
void execute(void (*f)()) // receive address of print
{
f();
}
Functions can be "passed" as function pointers, as per ISO C11 6.7.6.3p8: "A declaration of a parameter as ‘‘function returning type’’ shall be adjusted to ‘‘pointer to function returning type’’, as in 6.3.2.1. ". For example, this:
void foo(int bar(int, int));
is equivalent to this:
void foo(int (*bar)(int, int));
I am gonna explain with a simple example code which takes a compare function as parameter to another sorting function.
Lets say I have a bubble sort function that takes a custom compare function and uses it instead of a fixed if statement.
Compare Function
bool compare(int a, int b) {
return a > b;
}
Now , the Bubble sort that takes another function as its parameter to perform comparison
Bubble sort function
void bubble_sort(int arr[], int n, bool (&cmp)(int a, int b)) {
for (int i = 0;i < n - 1;i++) {
for (int j = 0;j < (n - 1 - i);j++) {
if (cmp(arr[j], arr[j + 1])) {
swap(arr[j], arr[j + 1]);
}
}
}
}
Finally , the main which calls the Bubble sort function by passing the boolean compare function as argument.
int main()
{
int i, n = 10, key = 11;
int arr[10] = { 20, 22, 18, 8, 12, 3, 6, 12, 11, 15 };
bubble_sort(arr, n, compare);
cout<<"Sorted Order"<<endl;
for (int i = 0;i < n;i++) {
cout << arr[i] << " ";
}
}
Output:
Sorted Order
3 6 8 11 12 12 15 18 20 22
You need to pass a function pointer. The syntax is a little cumbersome, but it's really powerful once you get familiar with it.
typedef int function();
function *g(function *f)
{
f();
return f;
}
int main(void)
{
function f;
function *fn = g(f);
fn();
}
int f() { return 0; }
It's not really a function, but it is an localised piece of code. Of course it doesn't pass the code just the result. It won't work if passed to an event dispatcher to be run at a later time (as the result is calculated now and not when the event occurs). But it does localise your code into one place if that is all you are trying to do.
#include <stdio.h>
int IncMultInt(int a, int b)
{
a++;
return a * b;
}
int main(int argc, char *argv[])
{
int a = 5;
int b = 7;
printf("%d * %d = %d\n", a, b, IncMultInt(a, b));
b = 9;
// Create some local code with it's own local variable
printf("%d * %d = %d\n", a, b, ( { int _a = a+1; _a * b; } ) );
return 0;
}
How is it possible to give a function (B) a function (A) as a parameter?
So that I can use function A in the function B.
Like the Variable B in the following example:
foo(int B) { ... }
By using function pointers. Look at the qsort() standard library function, for instance.
Example:
#include <stdlib.h>
int op_add(int a, int b)
{
return a + b;
}
int operate(int a, int b, int (*op)(int, int))
{
return op(a, b);
}
int main(void)
{
printf("12 + 4 is %d\n", operate(12, 4, op_add));
return EXIT_SUCCESS;
}
Will print 12 + 4 is 16.
The operation is given as a pointer to a function, which is called from within the operate() function.
write function pointer type in another function's parameter list looks a little weird, especially when the pointer is complicated, so typedef is recommended.
EXAMPLE
#include <stdio.h>
typedef int func(int a, int b);
int add(int a, int b) { return a + b; }
int operate(int a, int b, func op)
{
return op(a, b);
}
int main()
{
printf("%d\n", operate(3, 4, add));
return 0;
}
Lets say you have a function you want to call:
void foo() { ... }
And you want to call it from bar:
void bar(void (*fun)())
{
/* Call the function */
fun();
}
Then bar can be called like this:
bar(foo);
How can I create a "function pointer" (and (for example) the function has parameters) in C?
http://www.newty.de/fpt/index.html
typedef int (*MathFunc)(int, int);
int Add (int a, int b) {
printf ("Add %d %d\n", a, b);
return a + b; }
int Subtract (int a, int b) {
printf ("Subtract %d %d\n", a, b);
return a - b; }
int Perform (int a, int b, MathFunc f) {
return f (a, b); }
int main() {
printf ("(10 + 2) - 6 = %d\n",
Perform (Perform(10, 2, Add), 6, Subtract));
return 0; }
typedef int (*funcptr)(int a, float b);
funcptr x = some_func;
int a = 3;
float b = 4.3;
x(a, b);
I found this site helpful when I was first diving into function pointers.
http://www.newty.de/fpt/index.html
First declare a function pointer:
typedef int (*Pfunct)(int x, int y);
Almost the same as a function prototype.
But now all you've created is a type of function pointer (with typedef).
So now you create a function pointer of that type:
Pfunct myFunction;
Pfunct myFunction2;
Now assign function addresses to those, and you can use them like they're functions:
int add(int a, int b){
return a + b;
}
int subtract(int a, int b){
return a - b;
}
. . .
myFunction = add;
myFunction2 = subtract;
. . .
int a = 4;
int b = 6;
printf("%d\n", myFunction(a, myFunction2(b, a)));
Function pointers are great fun.
You can also define functions that return pointers to functions:
int (*f(int x))(double y);
f is a function that takes a single int parameter and returns a pointer to a function that takes a double parameter and returns int.
I want to create a function that performs a function passed by parameter on a set of data. How do you pass a function as a parameter in C?
Declaration
A prototype for a function which takes a function parameter looks like the following:
void func ( void (*f)(int) );
This states that the parameter f will be a pointer to a function which has a void return type and which takes a single int parameter. The following function (print) is an example of a function which could be passed to func as a parameter because it is the proper type:
void print ( int x ) {
printf("%d\n", x);
}
Function Call
When calling a function with a function parameter, the value passed must be a pointer to a function. Use the function's name (without parentheses) for this:
func(print);
would call func, passing the print function to it.
Function Body
As with any parameter, func can now use the parameter's name in the function body to access the value of the parameter. Let's say that func will apply the function it is passed to the numbers 0-4. Consider, first, what the loop would look like to call print directly:
for ( int ctr = 0 ; ctr < 5 ; ctr++ ) {
print(ctr);
}
Since func's parameter declaration says that f is the name for a pointer to the desired function, we recall first that if f is a pointer then *f is the thing that f points to (i.e. the function print in this case). As a result, just replace every occurrence of print in the loop above with *f:
void func ( void (*f)(int) ) {
for ( int ctr = 0 ; ctr < 5 ; ctr++ ) {
(*f)(ctr);
}
}
Source
This question already has the answer for defining function pointers, however they can get very messy, especially if you are going to be passing them around your application. To avoid this unpleasantness I would recommend that you typedef the function pointer into something more readable. For example.
typedef void (*functiontype)();
Declares a function that returns void and takes no arguments. To create a function pointer to this type you can now do:
void dosomething() { }
functiontype func = &dosomething;
func();
For a function that returns an int and takes a char you would do
typedef int (*functiontype2)(char);
and to use it
int dosomethingwithchar(char a) { return 1; }
functiontype2 func2 = &dosomethingwithchar
int result = func2('a');
There are libraries that can help with turning function pointers into nice readable types. The boost function library is great and is well worth the effort!
boost::function<int (char a)> functiontype2;
is so much nicer than the above.
Since C++11 you can use the functional library to do this in a succinct and generic fashion. The syntax is, e.g.,
std::function<bool (int)>
where bool is the return type here of a one-argument function whose first argument is of type int.
I have included an example program below:
// g++ test.cpp --std=c++11
#include <functional>
double Combiner(double a, double b, std::function<double (double,double)> func){
return func(a,b);
}
double Add(double a, double b){
return a+b;
}
double Mult(double a, double b){
return a*b;
}
int main(){
Combiner(12,13,Add);
Combiner(12,13,Mult);
}
Sometimes, though, it is more convenient to use a template function:
// g++ test.cpp --std=c++11
template<class T>
double Combiner(double a, double b, T func){
return func(a,b);
}
double Add(double a, double b){
return a+b;
}
double Mult(double a, double b){
return a*b;
}
int main(){
Combiner(12,13,Add);
Combiner(12,13,Mult);
}
Pass address of a function as parameter to another function as shown below
#include <stdio.h>
void print();
void execute(void());
int main()
{
execute(print); // sends address of print
return 0;
}
void print()
{
printf("Hello!");
}
void execute(void f()) // receive address of print
{
f();
}
Also we can pass function as parameter using function pointer
#include <stdio.h>
void print();
void execute(void (*f)());
int main()
{
execute(&print); // sends address of print
return 0;
}
void print()
{
printf("Hello!");
}
void execute(void (*f)()) // receive address of print
{
f();
}
Functions can be "passed" as function pointers, as per ISO C11 6.7.6.3p8: "A declaration of a parameter as ‘‘function returning type’’ shall be adjusted to ‘‘pointer to function returning type’’, as in 6.3.2.1. ". For example, this:
void foo(int bar(int, int));
is equivalent to this:
void foo(int (*bar)(int, int));
I am gonna explain with a simple example code which takes a compare function as parameter to another sorting function.
Lets say I have a bubble sort function that takes a custom compare function and uses it instead of a fixed if statement.
Compare Function
bool compare(int a, int b) {
return a > b;
}
Now , the Bubble sort that takes another function as its parameter to perform comparison
Bubble sort function
void bubble_sort(int arr[], int n, bool (&cmp)(int a, int b)) {
for (int i = 0;i < n - 1;i++) {
for (int j = 0;j < (n - 1 - i);j++) {
if (cmp(arr[j], arr[j + 1])) {
swap(arr[j], arr[j + 1]);
}
}
}
}
Finally , the main which calls the Bubble sort function by passing the boolean compare function as argument.
int main()
{
int i, n = 10, key = 11;
int arr[10] = { 20, 22, 18, 8, 12, 3, 6, 12, 11, 15 };
bubble_sort(arr, n, compare);
cout<<"Sorted Order"<<endl;
for (int i = 0;i < n;i++) {
cout << arr[i] << " ";
}
}
Output:
Sorted Order
3 6 8 11 12 12 15 18 20 22
You need to pass a function pointer. The syntax is a little cumbersome, but it's really powerful once you get familiar with it.
typedef int function();
function *g(function *f)
{
f();
return f;
}
int main(void)
{
function f;
function *fn = g(f);
fn();
}
int f() { return 0; }
It's not really a function, but it is an localised piece of code. Of course it doesn't pass the code just the result. It won't work if passed to an event dispatcher to be run at a later time (as the result is calculated now and not when the event occurs). But it does localise your code into one place if that is all you are trying to do.
#include <stdio.h>
int IncMultInt(int a, int b)
{
a++;
return a * b;
}
int main(int argc, char *argv[])
{
int a = 5;
int b = 7;
printf("%d * %d = %d\n", a, b, IncMultInt(a, b));
b = 9;
// Create some local code with it's own local variable
printf("%d * %d = %d\n", a, b, ( { int _a = a+1; _a * b; } ) );
return 0;
}