How to take reverse of string

Question

I have string

 string s1="5 2 13 * +" //as prefix expression.

I want to obtain reversed of above matrix like this.

"+ * 13 2 5"

I have tried stringstream but it make "+ * 31 2 5" and I lost my "13" and I gets "31". This not good my computation.

How can I do? For helping thanks.

@BitFiddlingCodeMonkey silly me, I keep thinking the question titles mean something. — juanchopanza, Nov 28 '13 at 21:38
Looks like you do not want to reverse the "string". You have a two dimensional array like [["5"],["2"], ["13"], ["*"], ["+"]] and you want to reverse the outer array. Hope that answers your question too :-) — rnk, Nov 28 '13 at 21:40
Note that this is probably not the answer to the question you really wanted to ask, which is how to change postfix to prefix. The correct answer is `+ 5 * 2 13`. Of course, `+` and `*` are commutative so the end result is the same, but there are operators which are not. Try, for example, `5 2 13 / -` which is certainly not the same as the prefix expression `- / 13 2 5`. — rici, Nov 28 '13 at 22:14

yasen · Accepted Answer · 2013-11-28T21:52:34.537

4

Assuming that what you want to reverse is in str and the delimiters between values are whitespace characters:

stringstream ss(str);
string answer, current;
answer.clear();
while (ss >> current) answer = current + " " + answer;
answer.erase(answer.size()-1, 1);     // eliminate space in the end of answer

edited Nov 28 '13 at 21:52

answered Nov 28 '13 at 21:46

yasen

1,250
7
13

it return same string. not reversed. ? – burakim Nov 28 '13 at 21:54
@burakim Sorry, I had a mistake. I edited my answer with new code, which you can try. – yasen Nov 28 '13 at 21:56
Thank you last edited is correct and very simple. :) – burakim Nov 28 '13 at 21:59

Vlad from Moscow · Answer 2 · 2013-11-28T22:00:38.387

Another way

    std::string s = "5 2 13 * +";
    std::forward_list<std::string> lst;
    std::istringstream is( s );

    for ( std::string t; std::getline( is, t, ' ' ); ) lst.push_front( t );
    s.clear();

    for ( std::string t : lst ) s += t + ' ';

    std::cout << "s = " << s << std::endl;

Or without std::getline

    std::string s = "5 2 13 * +";
    std::forward_list<std::string> lst;
    std::istringstream is( s );

    while ( is >> s ) lst.push_front( s );

    s.clear();
    for ( std::string t : lst ) s += t + ' ';

    std::cout << "s = " << s << std::endl;

How to take reverse of string

2 Answers2