prefix to infix on stack

Question

I'm trying to implement prefix to infix in c++, that's what i've got so far. The input should be for example something like this:

/7+23

And the ouput:

7/(2+3) or (7/(2+3))

But instead I get:

(/)

That's the code I wrote so far:

void pre_to_in(stack<char> eq) { if(nowe.empty() != true) { char test; test = eq.top(); eq.pop(); if(test == '+' || test == '-' || test == '/' || test == '*') { cout << "("; pre_to_in(eq); cout << test; pre_to_in(eq); cout << ")"; } else { cout << test; } } } // somewhere in main() char arr[30]; stack<char> stosik; int i = 0; cout << "write formula in prefix notation\n"; cin >> arr; while(i < strlen(arr)) { stosik.push(arr[i]); i++; } pre_to_in(stc);

Thank you for trying the problem first, then showing us all the information you have. Thank you. — GManNickG
– GManNickG, Commented Dec 9, 2009 at 22:39
Do you mean to copy by value, or did you forget a &, ie: pre_to_in(stack<char>& eq) ? — Mic
– Mic, Commented Dec 9, 2009 at 22:41

Jason Orendorff · Accepted Answer · 2009-12-09 22:48:18Z

1

cin >> arr;

only reads one "word" of input, not a whole line. Here it's only getting the first slash character.

answered Dec 9, 2009 at 22:48

Jason Orendorff

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1 Comment

blid Over a year ago

Blame on me, i added extra spaces to clarify input & output. Question fixed. That's not a solution, unfortunately.

rysson · Accepted Answer · 2009-12-09 23:15:03Z

This is a stack. First in, last out. You need reverse input string "32+7/".
You use many stacks. In every enter to pre_to_in() stack is copied. Use reference or pointer, ex: void pre_to_in(stack<char> &eq);

Thats all.

P.S. Unify names (s/nowe/eq/g && s/stc/stosik/g)

Ad.1. See pl.wikipedia.org/wiki/Odwrotna_notacja_polska Ad.2. See pl.wikibooks.org/wiki/C%2B%2B/Referencje Both sites in polish.

Dominic.wig · Accepted Answer · 2009-12-10 20:39:20Z

not sure if you are looking for such solution, anyway for the input you've mentioned it gives the output from you post

it reads tokens from std input

I've built it now under Visual Studio 2005 - to terminate input press Enter, Ctrl+Z, Enter

but on other compilers termination may work in another way

#include <algorithm> #include <deque> #include <iostream> #include <string> typedef std::deque< std::string > tokens_t; void pre_to_in( tokens_t* eq ) { if ( !eq->empty() ) { const std::string token = eq->front(); eq->pop_front(); if ( ( token == "+" ) || ( token == "-" ) || ( token == "/" ) || ( token == "*" ) ) { std::cout << "("; pre_to_in( eq ); std::cout << token; pre_to_in( eq ); std::cout << ")"; } else { std::cout << token; } } } int main() { std::cout << "write formula in prefix notation" << std::endl; tokens_t tokens; std::copy( std::istream_iterator< std::string >( std::cin ), std::istream_iterator< std::string >(), std::back_inserter( tokens ) ); pre_to_in( &tokens ); }

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