double_reverse_string_goodexercise.txt

/*
 Reverse the words of a string in the place. i.e This is great -> great is This
wxf: great exercise.
*/

#include <iostream>
#include <string.h>
using namespace std;

void reverse(char *str, int start, int end){
    char temp;
    while(start<end){
        temp = str[start];
        str[start] = str[end];
        str[end]= temp;
        start++;
        end--;
    }

}

int main(){
    cout<<"Input a sentense:"<<endl;
    char sentence[30];
    gets(sentence);
    
    /*reverse the whole string*/
    reverse(sentence,0,strlen(sentence)-1);
    
    /*reverse the substrings*/
    int start=0, end;
    for (int i=0; i<strlen(sentence); i++){
        if (sentence[i]==' '|| sentence[i]=='\0' ){
            end=i-1;
            reverse(sentence,start,end);
            start=i+1;
        }
    }
    
    cout<<sentence<<endl;
}

REMARK:
1. Hard one! And great. learned a lot from it.
2. IDEA: 
step1: use the function reverse() to reverse the whole sentence(i.e. from 0 to sentence.lengt()-1)
step2: reverse every word in the sentence.
3. In the code, I use 
char sentence[30]
gets()
strlen(sentence)
Note:gets():Reads characters from the standard input (stdin) and A terminating null character is automatically appended after the characters copied to str.
In addition, the reason I use char sentence[] is that it's convenient to pass as a parameter to the function and make change on it.
4. Question: how should I change the content in a string if I pass a string as a parameter to change it
At first I use a string to store the input but I finally found out 
1)I cannot change the content of the string. Solution: use a reference in the function's argument.
2)By using cin>>sentence; I cannot read in the whole sentence with space. I can only read in the first word until I hit the space. Solution: use getline(cin, string).
3) Using getline(cin,string) does not add a '\0' automatically, you have to change the stop condition a little bit.


#include <iostream>
#include <string.h>
using namespace std;

void reverse(string &str, int start, int end){
    char temp;
    while(start<end){
        temp = str[start];
        str[start] = str[end];
        str[end]= temp;
        start++;
        end--;
    }

}

int main(){
    cout<<"Input a sentense:"<<endl;
    string sentence;
    //cin>>sentence;
    getline(cin,sentence);
    cout<<sentence.length()-1<<endl;
    /*reverse the whole string*/
    reverse(sentence,0,sentence.length()-1);
    cout<<sentence<<endl;

    /*reverse the substrings*/
    int start=0, end;
    for (int i=0; i<sentence.length(); i++){
        if (sentence[i]==' '){
            end=i-1;
            reverse(sentence,start,end);
            start=i+1;
        }
        if (i==sentence.length()-1){
            end=i;
            reverse(sentence,start,end);
        }
    }
    cout<<sentence<<endl;
}