I have finished it, but to don't violate the rules of this website --"Don't give away code" , I can not give you all my source code. The following is the framework of program. hope it can help you.
...

The various combinations of the flags and the corresponding meaning as below:

ios_base Flags Meaning
in Reads (file must exist)
out ...

int len = strlen(sentenceHolder1);
for(int i = 0; i < len; i++)
{
if( sentenceHolder1[i] == look_for_character )
sentenceHolder1[i] = sentenceHolder1[0]; //I think this will be OK
}...