In cpp, i++ and ++i are two different increment operators, known as post-increment and pre-increment, respectively. The main difference between them lies in the order of operations and the return value.

i++ (Post-Increment)

The post-increment operator i++ returns the current value of the variable before incrementing it. Specifically:

  1. It returns the current value of the variable.
  2. Then, it increments the variable’s value by 1.

Example:

int i = 1;
int a = i++;  // a = 1, i = 2

In this example, a is assigned the current value of i (1), and then i is incremented to 2.

++i (Pre-Increment)

The pre-increment operator ++i increments the variable’s value by 1 before returning the new value. Specifically:

  1. It increments the variable’s value by 1.
  2. Then, it returns the incremented value.

Example:

int i = 1;
int a = ++i;  // a = 2, i = 2

In this example, i is first incremented to 2, and then a is assigned the new value of i (2).

Summary

  • i++: Returns the original value, then increments. The operation sequence is: return value -> increment.
  • ++i: Increments first, then returns the new value. The operation sequence is: increment -> return value.

Performance Considerations

In certain situations, especially in loops, using pre-increment (++i) can be more efficient than post-increment (i++), particularly for complex iterator objects. This is because post-increment needs to save the object’s original state to return the original value, whereas pre-increment does not. However, for basic types like int, modern compilers typically optimize away the differences, making the performance impact negligible.

Usage Recommendations

  1. If you simply need to increment a variable without using its old value, prefer pre-increment (++i).
  2. If you need to use the value of the variable before incrementing, use post-increment (i++).

A small example

construct the following cpp code:

src = '
#include <Rcpp.h>

// [[Rcpp::export]]
void test_increment(){
  const int n = 5;
  
  // use i++
  for(int i = 0; i < n; i++){
    Rcpp::Rcout << "using post increment: " << i << "---" << i++ << "\\n";
  }
  
  // use ++i
  for(int i = 0; i < n; ++i){
    Rcpp::Rcout << "using pre increment: " << i << "---" << ++i << "\\n";
  }
}
'
Rcpp::sourceCpp(code = src)
test_increment()

## using post increment: 0---0
## using post increment: 2---2
## using post increment: 4---4
## using pre increment: 0---1
## using pre increment: 2---3
## using pre increment: 4---5