Why is this ranged based for loop causing issues with subtraction? - c++

Question

I'm having an issue with a ranged based for loop causing my values to go negative, and I've resolved the issue with a regular for loop but want to understand why it messed up in the first place. You can see from the sample output below that the initial values exist correctly, but then when attempting to subtract from them, they get reset to a default initialized value of 0 I guess?
Broken code:
#include <iostream>
#include <vector>
#define IS_TRUE(x) { if (!(x)) std::cout << __FUNCTION__ << " failed on line " << __LINE__ << std::endl; }
int maximumScore(std::vector<int>& nums, std::vector<int>& multipliers) {
std::vector<int> multRank;
multRank.resize(multipliers.size());
std::cout << "multRank: ";
//ISSUE IS IN THE LOOP BELOW
for (int n : multRank) {
n = multipliers.size();
std::cout << " " << n;
}
std::cout << std::endl;
for (auto i = 0; i < multipliers.size(); ++i) {
for (auto j = 0; j < multipliers.size(); ++j) {
int abs1 = std::abs(multipliers[i]);
int abs2 = std::abs(multipliers[j]);
if (abs1 > abs2) {
multRank[i] = multRank[i] - 1;
std::cout << multRank[i];
}
}
}
std::cout << std::endl << "multRank after: ";
for (int n : multRank) {
std::cout << " " << n;
}
std::cout << std::endl << std::endl;
return 0;
}
void test1()
{
std::vector<int> nums = { 1, 2, 3 };
std::vector<int> multipliers = { 3, 2, 1 };
int test = maximumScore(nums, multipliers);
IS_TRUE(test == 14);
}
int main()
{
std::cout << "Maximum Score from Performing Multiplication Operations\n";
test1();
}
Broken code output:
Maximum Score from Performing Multiplication Operations
multRank: 3 3 3
-1-2-1
multRank after: -2 -1 0
Repaired code:
#include <iostream>
#include <vector>
#define IS_TRUE(x) { if (!(x)) std::cout << __FUNCTION__ << " failed on line " << __LINE__ << std::endl; }
int maximumScore(std::vector<int>& nums, std::vector<int>& multipliers) {
std::vector<int> multRank;
multRank.resize(multipliers.size());
std::cout << "multRank: ";
//ISSUE WAS IN THE LOOP BELOW
for (auto i = 0; i < multipliers.size(); ++i) {
multRank[i] = multipliers.size();
std::cout << " " << multRank[i];
}
std::cout << std::endl;
for (auto i = 0; i < multipliers.size(); ++i) {
for (auto j = 0; j < multipliers.size(); ++j) {
int abs1 = std::abs(multipliers[i]);
int abs2 = std::abs(multipliers[j]);
if (abs1 > abs2) {
multRank[i]--;
std::cout << multRank[i];
}
}
}
std::cout << std::endl << "multRank after: ";
for (int n : multRank) {
std::cout << " " << n;
}
std::cout << std::endl << std::endl;
return 0;
}
void test1()
{
std::vector<int> nums = { 1, 2, 3 };
std::vector<int> multipliers = { 3, 2, 1 };
int test = maximumScore(nums, multipliers);
IS_TRUE(test == 14);
}
int main()
{
std::cout << "Calculate Rank\n";
test1();
}
Repaired code output:
Maximum Score from Performing Multiplication Operations
multRank: 3 3 3
212
multRank after: 1 2 3

Answer 1

The first range based for loop is not using references:
for (int n : multRank) {
n = multipliers.size();
std::cout << " " << n;
}
In this loop, n is a copy of the data in multRank. If you want to be able to modify the data in multRank, you want n to be a reference:
for (int& n : multRank) {
n = multipliers.size();
std::cout << " " << n;
}