I have an array of elements and I want to multiply every element in that arr with -1 and store it in another array.

int arr1[] ={1,0,1,1,1,0,0,1}
int outputArr[]= {-1,0,-1,-1,-1,0,0,-1};

How do I do it? I know I can iterate over the loop one at a time but I am looking for a function or something (if it exists ) so that I can preferably do that in a single line.

CodePudding user response：

You can use std::tranform to apply a unary operation to each element.

std::transform( std::begin( arr1 ), std::end( arr1 ), std::begin( output ), []( int val ) { 
    return val * -1; 
});

You could also use std::negate, but keep in mind this is not a multiplication, but instead applies operator-()

std::transform( std::begin( arr1 ), std::end( arr1 ), std::begin( output ), std::negate<int>() );

Note: std::begin and std::end will only work for the array if arr1 is defined in the same function (i.e. not a parameter to a function).

Also note that output must be (at least) the same size as arr1.

CodePudding user response：

1. Create an output array with the same size of the input array. You can use std::size for this since C 17.
2. Walk it, e.g. using std::transform, and, for each element n, just have transform return -n.

#include <algorithm>  // transform
#include <fmt/ranges.h>

int main()
{
    int arr1[] = {1,0,1,1,1,0,0,1};
    int outputArr[std::size(arr1)];
    std::ranges::transform(arr1, std::begin(outputArr), [](auto& n) { return -n; });
    fmt::print("arr1 = {}\n", arr1);
    fmt::print("outputArr = {}\n", outputArr);
}

// Outputs:
//
//   arr1 = [1, 0, 1, 1, 1, 0, 0, 1]
//   outputArr = [-1, 0, -1, -1, -1, 0, 0, -1]