Prog error: for replacing spaces with "%20" - c++

Question

below is the prog i am compiling for replacing spaces with "%20" but when I run it output window shows blank and a message "arrays5.exe has occurred a prob"
#include <iostream>
#include<cstring>
using namespace std;
void method(char str[], int len) //replaces spaces with "%20"
{
int spaces, newlen,i;
for (i=0;i<len;i++)
if(str[i]==' ') spaces++;
newlen=len+spaces*2;
str[newlen]=0;
for (i=len-1;i>=0;i--)
{
if(str[i]==' ')
{
str[newlen-1]='0';
str[newlen-2]='2';
str[newlen-3]='%';
newlen=newlen-3;
}
else
{
str[newlen-1]=str[i];
newlen=newlen-1;
}
}
}
int main()
{
char str[20]="sa h ";
method(str,5);
cout <<str<<endl;
return 0;
}
Please help me finding the error.Thanks

Answer 1

spaces is uninitialised before you increment it.
You should give it an initial, default value.
An uninitialised variable will have a value which is undefined by the specification. This value could be 0, if you're lucky but it is highly likely that this value will be anything in the range of values which the datatype may represent.
Your program will compile and run fine when spaces is initialised properly.

Answer 2

I'm not fixing your problem, but providing a better solution. If you're using C++, then you should use the STL. You've got lots of classes and methods that do all of the job for you.
You could rewrite your 25 lines long method into this 4 lines long method(example included):
#include <iostream>
#include <string>
using namespace std;
std::string method(std::string str)
{
size_t index;
while((index = str.find(' ')) != std::string::npos)
str = str.replace(index, 1, "%20");
return str;
}
int main()
{
std::string str("sa h ");
str = method(str);
cout <<str<<endl; // outputs sa%20h%20
return 0;
}

Answer 3

I would suggest you use std::string, and use the .replace method. The reason your code doesn't work is because you're overwriting the input string in an odd way so I don't know if your expected output would be correct, however, the actual error you have is that you're potentially rewriting at index locations -3, -2, and -1. Consider the case where your first space is at index zero.
In C++, it's usually better to avoid char* unless you have a clear reason for doing so. As a matter of good style (this is somewhat subjective), I would suggest that you do NOT modify your input arguments directly, but instead return the result.
Ie, your method prototype should be:
std::string method(std::string str)
There is no-longer a need to pass the length of the string, because std::string takes care of that.