C++ char search in a long string (random locations) - c++

Question

So basically I have a character such as 'g' and I want to find the instances of the char in a string such as 'george'. The twist is that I want to return the location of the character randomly.
I have it working with string.find which just returns the first instance of the location of the character, so in the above example it would be 0. But there is also a 'g'at 4.
I want my code to randomly return a location of the character in the string aka 0 or 4 instead of just returning the first instance of the letter. I was thinking of using a regex statement but I will admit I am not very confident in my regex skills.
Any guidance is greatly appreciated, thanks in advance :)

Answer 1

One solution could follow the following steps:
Find all occurrences of a character in a string, store them in a vector
Generate a random number using rand() function which should be between 0 and length of the vector -1.
Use the generated number to index an element from the match vector and return the result.

Answer 2

You could writhe a function that store into an array all the occurrences of char then pick a random index from that array.
something like this...
int findX(char x, char* s){
int *indexes = new int[strlen(s)]; // reserve
int count= 0;
int index = findFirst(x, s, 0);
while(index!=-1){
indexes[count++] = index;
index = findFirst(x, s, index );
}
if(count>0){
int randomIndex = generateRandom(count);
index = indexes[randomIndex];
}
else
index = -1;
delete []indexes;
return index;
}

Answer 3

One possible solution is to find all instances of the character in a loop (just iterate over all of the string and compare the characters). Save the positions of the letters in a vector.
Then randomly select one of the elements in the vector of positions to return.
For the random selection I suggest std::uniform_int_distribution.
If the data is read from a large file (and with "large" I mean multi-megabytes or larger) then instead of just a single loop over the string, consider using threads. Divide the string into smaller chunks, and have each thread go through its own chunk in parallel, adding to its own vector of positions. Then when all threads are done merge the position vectors into a single vector and randomly choose the position from that collected vector.
If the file is very large (multi-gigabytes) then if it's stored on a SSD have the threads read its chunk as well. Otherwise you could memory map the file contents, and have each thread just go through the mapped memory as a large array. Memory mapping such large files requires a 64-bit system though.

Answer 4

You can use the C++ pseudo random generation rand() function. Here are more details on how to use it: http://www.cplusplus.com/reference/cstdlib/rand/
You are encouraged to use C++11 random generators http://en.cppreference.com/w/cpp/numeric/random