Calculating percent chance - c++

Question

So my objective is to create a random password generator of length n (n >= 5 && n <= 15) that adds in only two numbers at random locations.
(e.g. 7S4js 86dJxD h6Zqs9K)
I have this working... or so I want to believe. What I want to know is will my code ALWAYS work at determining whether or not a number should be inserted.
'newPassword': Returns a string of length 'len', using 'nums' numbers.
std::string newPassword(int len, int nums)
{
std::string password = "";
// Required numbers
int req = nums;
for (int i = 0; i < len; i++)
{
bool needNum = req > 0;
bool chance = rand() % len > req;
bool useNum = needNum && chance;
if (useNum)
req--;
char c = nextChar(useNum);
password += c;
}
return password;
}
'nextChar': Returns a random character. The character will be a number if 'isNum' is true.
char nextChar(bool isNum)
{
char c;
if (!isNum)
{
// 50% chance to decide upper or lower case
if (rand() % 100 < 50)
{
c = 'a' + rand() % 26;
}
else
{
c = 'A' + rand() % 26;
}
}
else
{
// Random number 0-9
c = '0' + rand() % 10;
}
return c;
}
So specifically, will the 'chance' variable in 'newPassword' work all the time?

Answer 1

rand() is an obsolete and terrible way to generate random numbers. The c++11 <random> header provides much higher quality facilities for dealing with all kinds of random stuff.
Your way of choosing the letter or a digit will not always work. I would approach it in a different way: generate the needed number of letters and digits and then shuffle the string. It might not be the most efficient way, but given your requirements for password length, I'd value code clarity more.
#include <string>
#include <random>
#include <algorithm>
std::string generatePassword(int length, int nDigits)
{
std::string password;
password.resize(length);
std::mt19937 generator{std::random_device{}()};
// Generate capital/lowercase letters
std::uniform_int_distribution<char> letterGen(0, 2 * 26 - 1);
auto digitsBeginIter = std::generate_n(password.begin(), length - nDigits,
[&letterGen, &generator]() {
auto l = letterGen(generator);
return l < 26 ? 'a' + l : 'A' + (l - 26);
});
// Generate the digits
std::uniform_int_distribution<char> digitGen('0', '9');
std::generate_n(digitsBeginIter, nDigits,
[&digitGen, &generator]() { return digitGen(generator); });
// Shuffle the string
std::shuffle(password.begin(), password.end(), generator);
return password;
}