Q:

I am difficulty with printing a newline

I am new to problem solving. And I was solving a problem in UVA called Expression. I think I have solved the problem because my code gives the correct output for every possible test case. But still I am getting WA. It seems that somewhere I have print a newline which I am not doing properly. The problem says "The output file will have each postfix expression all on one line. Print a blank line between different expressions." Can someone please explain this to me a little bit clearly. I have asked the question in the group but I am not getting an answer. And the previous discussions ain't helping either. Thanks in advance.
#include<iostream>
#include<map>
#include<stack>
#include<vector>
#include<cstdio>

using namespace std;

void push_into_stack(char c, vector< char > &ans, stack< char > &st);
void work_with_stack(vector< char > &ans, stack< char > &st);

int main(void)
{
    freopen("input.txt", "r", stdin);
int t;
char dummy;
cin >> t;

for(int i=1; i<=t; i++)
{
    vector< char > exp, ans;
    stack< char > st;
    char c;

    while(cin >> c)
        exp.push_back(c);

    for(int i=0; i<exp.size(); i++)
        if(exp[i]=='+' || exp[i]=='-' || exp[i]=='*' || exp[i]=='/') push_into_stack(exp[i], ans, st);
        else if(exp[i]=='(') st.push(exp[i]);
        else if(exp[i]==')') work_with_stack(ans, st);
        else ans.push_back(exp[i]);

    while(!st.empty())
    {
        ans.push_back(st.top());
        st.pop();
    }

    for(int i=0; i<ans.size(); i++)
        cout << ans[i];
    cout << endl;
}
return 0;

}
void push_into_stack(char c, vector< char > &ans, stack< char > &st)
{
    map< char, int > mp;
    mp['/']=2;
    mp['*']=2;
    mp['+']=1;
    mp['-']=1;

while(true)
{
    if(!st.empty() && mp[c]<=mp[st.top()])
    {
        ans.push_back(st.top());
        st.pop();
    }
    else
    {
        st.push(c);
        break;
    }
}
return;

}
void work_with_stack(vector< char > &ans, stack< char > &st)
{
    while(true)
    {
        if(st.top()=='(') break;
        ans.push_back(st.top());
        st.pop();
    }
    st.pop();
    return;
}

A:

Um... I guess the quality of the answer can only reflect the quality of the question... but how about:
int main(void) {
   char postfixone[] = "4 5 7 2 + - *            -16";
   char postfixtwo[] = "3 4 + 2  * 7 /             2";
   char postfixthree[] = "5 7 + 6 2 -  *            48";
   printf("%s\n\n",postfixone);
   printf("%s\n\n",postfixtwo);
   printf("%s\n\n",postfixthree);
}

mike@linux-4puc:~> ./a.out 
4 5 7 2 + - *            -16

3 4 + 2  * 7 /             2

5 7 + 6 2 -  *            48

Each one is on a line with a new line in between...
EDIT:
I guess you're using C++ and printing the lines here:
for(int i=0; i<ans.size(); i++)
     cout << ans[i];
 cout << endl; 

You're printing one new line per postfix with the endl, try:
 cout << endl << endl;

instead to insert the extra blank inbetween lines. 

