My function does not always work because of the second argument - c++

Question

My trouble is that my function not always work, and i don't know why.
Here's my code:
#include <stdio.h>
#include <string.h>
#include <cstring>
#include <iostream>
#include <math.h>
#include <stdlib.h>
#include <string>
using namespace std;
void fill_zeros(std::string& fill_zeros_str, int fill_zeros_num){
if (fill_zeros_str.length()<fill_zeros_num){
int i = 0;
for (i=0;i<=((int)fill_zeros_num)-((int)fill_zeros_str.length());i++){
fill_zeros_str = "0"+fill_zeros_str;
}
}
}
int main(){
string testString = to_string(2);
fill_zeros(testString,7);
cout << testString << "\n";
return 0;
}
The second argument of fill_zeros (fill_zeros_num) does not work all the time, and I don't know why.

Answer 1

Because in
for (i=0;i<=((int)fill_zeros_num)-((int)fill_zeros_str.length());i++)
the length of fill_zeros_str changes as you add zeros(decrease by one), and you are also adding one to i(so, the start is adding by one, and the end is decreasing by one). So the best way is to define a length at the beginning of the function to store the string length.

Answer 2

Your loop is modifying the std::string on each iteration, which affects its length(). The loop is re-evaluating the length() on each iteration.
You need to calculate the number of zeros wanted and save that value to a local variable first, and then use that variable in your loop. Also, your loop needs to use < instead of <=.
Try this:
void fill_zeros(std::string& str, size_t min_length){
if (str.length() < min_length){
size_t fill_zeros_num = min_length - str.length();
for (size_t i = 0; i < fill_zeros_num; ++i){
str = "0" + str;
// or: str.insert(0, "0");
// or: str.insert(0, '0');
// or: str.insert(str.begin(), '0');
}
}
}
Live Demo
However, there is a much simpler way to implement fill_zeros() that doesn't involve using a manual loop at all:
void fill_zeros(std::string& str, size_t min_length){
if (str.length() < min_length){
str = std::string(min_length - str.length(), '0') + str;
// or: str.insert(0, std::string(min_length - str.length(), '0'));
// or: str.insert(str.begin(), std::string(min_length - str.length(), '0'));
// or: str.insert(0, min_length - str.length(), '0');
// or: str.insert(str.begin(), min_length - str.length(), '0');
}
}
Live Demo
Alternatively:
#include <sstream>
#include <iomanip>
void fill_zeros(std::string& str, size_t min_length){
if (str.length() < min_length){
std::ostringstream oss;
oss << std::setw(min_length) << std::setfill('0') << str;
str = oss.str();
}
}
Live Demo
In which case, you could simply get rid of fill_zeros() altogether and apply the I/O manipulators directly to std::cout in main() instead:
#include <iostream>
#include <iomanip>
int main(){
std::cout << std::setw(7) << std::setfill('0') << 2 << "\n";
return 0;
}
Live Demo