Month: June 2014

vectors and for_each

A friend of mine asked how to use the newer for_each and is it similar to the foreach in php, so here is the example that I gave and thus doing it online as well.

To start with the foreach (php version here) does loop over data and so does the for_each in c++, but the main difference is that the for_each in c++ is basically a for loop

for (; first!=last;++first)
   functionCall(*first);

where the functionCall is the parameter passed in to do something with that part of the data.

The great thing with c++ is that you are able to use either a method, structure etc as long as it is able to output a variable then we are good.

So here is a example, please note that I am loading up a vector e.g. only functions that have a single int make sense.

void printNumber (int i) {
	cout << "Print the number from a function : " << i << "\n";
}
 
for_each(v.begin(), v.end(), printNumber);

here we are using the for_each (where the v is a vector) passing in the start and end of the vector with a function as the last parameter.

and here

// create a object that can use the operator() to output value passed in.
struct structToOutput {
	void operator() (int i) {
		cout << "Struct to output : " << i << "\n";
	}
} structToOutputObject;
 
for_each(v.begin(), v.end(), structToOutputObject);

will output the values differently but in essence still able to access the values.

Here is the code in full

#include <iostream>
#include <vector>
#include <algorithm>
 
using namespace std;
 
void printNumber (int i) {
	cout << "Print the number from a function : " << i << "\n";
}
 
// create a object that can use the operator() to output value passed in.
struct structToOutput {
	void operator() (int i) {
		cout << "Struct to output : " << i << "\n";
	}
} structToOutputObject;
 
int main(int argc, char* argv[])
{
	// lets load up some dummy data.
	vector<int> v;
	for (int i =0; i< 10; i++)
		v.push_back(i);
 
	// run through the vector using a standard function with parameter
	for_each(v.begin(), v.end(), printNumber);
	// output using a operator() method
	for_each(v.begin(), v.end(), structToOutputObject);
	return 0;
}

with the output being

Print the number from a function : 0
Print the number from a function : 1
Print the number from a function : 2
Print the number from a function : 3
Print the number from a function : 4
Print the number from a function : 5
Print the number from a function : 6
Print the number from a function : 7
Print the number from a function : 8
Print the number from a function : 9
Struct to output : 0
Struct to output : 1
Struct to output : 2
Struct to output : 3
Struct to output : 4
Struct to output : 5
Struct to output : 6
Struct to output : 7
Struct to output : 8
Struct to output : 9

How to compile with g++

g++ <filename> -o <outfilename> -std=c++11

auto and lambda

The C++ 11 standards are starting to take more and more cue’s from other languages where you are able to create functions on the fly (lambda). With also adding the “auto” keyword which will allow the program to automatically figure out what type of variable will be returned from lambda function.

So here the break up of the lambda function is

<return type> <[optional local variables to be used]>(optional parameters);

The optional local variables are abit like the “use” within PHP (use)

So the full code is

#include <iostream>
#include <vector>
#include <algorithm>
 
using namespace std;
 
int main(int argc, char* argv[])
{
	int num = 30;
	int anothernum = 0;
 
	auto func = [&anothernum](int i) {
			cout << "Parameter passed in :" << i << "\n";
			anothernum = anothernum + 10; // alter the value passed in.
			return 50;	// return a value and use the auto keyword.
	};
	int ret = func(num);
	cout << "Return from func :" << ret << "\n";
	cout << "Another num :" << anothernum << "\n";
	return 0;
}

and the output would be

Parameter passed in : 30
Return from func : 50
Another num : 10;

To compile with g++ I used these parameters

g++ <filename> -o <outfilename> -std=c++11