Using rand() to generate numbers around a variable?(C++) - c++

Question

I am making a small text-based game in c++ called "House Evolution" for fun. The game consists of 'searching under the couch cushions' to gain credits. When you search, the game is supposed to generate a random number anywhere from creditRate-5 to creditRate+5. How would I go about doing this using the rand() function, no matter what number creditRate is? Here is example code:
#include <iostream>
#include <unistd.h>
#include <string>
#include <cstdlib>
#include <math.h>
int main()
{
int creditRate = 30; // Just for example.
int credits;
int searching;
while (1) {
// Yes, I know, infinite loop...
std::cout << "Credits: " << credits << std::endl;
std::cout << "Type any key to search for credits: " << std::endl;
std::cout << "Searching...\n";
usleep(10000000); // Wait 10 seconds
searching = rand(?????????); // Searching should be creditRate-5 to creditRate+5
std::cout << "You found " << searching<< " credits\n";
credits += searching;
}
}

Answer 1

The way I would go about it is using rand % 11, to get a range of 11 numbers and then adding it to credit rate -5 to cover the range from creditrate-5 to creditrate+5.
So:
searching = rand() % 11 + creditRate - 5;

Answer 2

Try:
searching = rand() % 11 + creditRate-5; That's because your range is 11 (remember, there are 11 numbers from -5 to 5, for example) and the lower limit is creditRate-5.

Answer 3

Use the <random> header instead of rand(), because <random> provides facilities to generate these distributions correctly instead of making you do it yourself.
#include <iostream>
#include <thread>
#include <random>
int main()
{
int creditRate = 30; // Just for example.
// Searching should be creditRate-5 to creditRate+5
std::uniform_int_distribution<> random_credit_amount(creditRate - 5, creditRate + 5);
int credits = 0;
// arrange a source of randomness
std::random_device r;
std::seed_seq seed{r(),r(),r(),r(),r(),r()};
std::mt19937 pRNG(seed);
while (true) {
// Yes, I know, infinite loop...
std::cout << "Credits: " << credits << '\n';
std::cout << "Type any key to search for credits: " << '\n';
std::cout << "Searching...\n";
std::this_thread::sleep_for(std::chrono::seconds(10)); // Wait 10 seconds
int searching = random_credit_amount(pRNG);
std::cout << "You found " << searching<< " credits\n";
credits += searching;
}
}
<random> even provides more advanced options than the typical uniform distribution. For example, instead of having every values from creditRate - 5 to creditRate + 5 be equally likely, you could have values closer to creditRate be more likely than values further away, using a 'normal' (a.k.a. 'bell curve') distribution:
// credits found should be near creditRate
std::normal_distribution<> random_credit_amount(creditRate, 5);
and then in the loop:
int searching = std::round(random_credit_amount(eng));
(You don't have to change the code in the loop at all, but it skews the distribution a bit. Performing proper rounding avoids the skew.)
Notice another change I made, replacing the non-standard usleep with the standard this_thread::sleep_for. Notice that this code makes the comment entirely redundant:
std::this_thread::sleep_for(std::chrono::seconds(10)); // Wait 10 seconds
And one can just as easily ask for sleep durations of microseconds or hours
std::this_thread::sleep_for(std::chrono::hours(2));
std::this_thread::sleep_for(std::chrono::microseconds(50));