C printf中的奇怪行为

Question

I've the following piece of code.

    char str[MAXS];
    gets(str);
    N = strlen(str);

    for (i = N / 2 - 1; i >= 0; i--) {
        printf("%c", str[i]);
    }

    for (i = N - 1; i > N / 2 - 1; i--) {
        printf("%c", str[i]);
    }
    printf("\n");

For and input string, for example "I ENIL SIHTHSIREBBIG S", it should print simply "THIS LINE IS GIBBERISH". But it only prints the content of the second loop, but, if I insert a simple "\\n" in the middle of the two loops, the content, the printf inside the first loop works. Whats's going on here, here comes the 'working' code (at least, I don't want that '\\n' in the middle')

#include <iostream>
#include <string>
#include <sstream>
#include <vector>
#include <set>
#include <map>
#include <list>
#include <queue>
#include <stack>
#include <memory>
#include <iomanip>
#include <numeric>
#include <functional>
#include <new>
#include <algorithm>
#include <cmath>
#include <cstring>
#include <cstdlib>
#include <cstdio>
#include <climits>
#include <cctype>
#include <ctime>

#define REP(i, n) for(int (i) = 0; i < n; i++)
#define FOR(i, a, n) for(int (i) = a; i < n; i++)
#define FORR(i, a, n) for(int (i) = a; i <= n; i++)
#define for_each(q, s) for(typeof(s.begin()) q=s.begin(); q!=s.end(); q++)
#define sz(n) n.size()
#define pb(n) push_back(n)
#define all(n) n.begin(), n.end()

template<typename T> T gcd(T a, T b) {
    if(!b) return a;
    return gcd(b, a % b);
}
template<typename T> T lcm(T a, T b) {
    return a * b / gcd(a, b);
}

template<typename T> void chmin(T& a, T b) { a = (a > b) ? b : a; }
template<typename T> void chmax(T& a, T b) { a = (a < b) ? b : a; }
int in() { int x; scanf("%d", &x); return x; }

using namespace std;

typedef long long Int;
typedef unsigned uint;

const int MAXS = 107;

int N, T;
char str[MAXS];

int main(void) {
    scanf("%d ", &T);

    int i;

    for ( ; T--; ) {
        gets(str);
        N = strlen(str);

        for (i = N / 2 - 1; i >= 0; i--) {
            printf("%c", str[i]);
        }

        for (i = N - 1; i > N / 2 - 1; i--) {
            printf("%c", str[i]);
        }
        printf("\n");
    }
    return 0;
}

Answer 1

There are 73 lines of code in your program in the question, 50 of which are irrelevant to the workings of the program. Please study how to create an SSCCE ( Short, Self-Contained, Correct Example ) so that people don't have to wade through 3 times as much code as necessary.

Here's a 22 line program that works:

#include <cstring>
#include <cstdio>

const int MAXS = 107;

int main(void)
{
    char str[MAXS];

    while (fgets(str, sizeof(str), stdin) != 0)
    {
        int N = strlen(str);
        if (str[N-1] == '\n')
            str[--N] = '\0';
        for (int i = N / 2 - 1; i >= 0; i--)
            printf("%c", str[i]);
        for (int i = N - 1; i > N / 2 - 1; i--)
            printf("%c", str[i]);
        printf("\n");
    }
    return 0;
}

It is C++ in name only; it uses the <cstring> and <cstdio> headers, but if you changed those to <string.h> and <stdio.h> , it would be C code. Note that it checks for buffer overflow (using fgets() — never, ever use gets() !) and tests that data was read. It doesn't bother with a count of the lines of input; it can detect EOF reliably. It removes the trailing newline that fgets() leaves but gets() does not. It then prints the first half of the string backwards, and then the second half of the string backwards. I didn't make any substantive changes to the logic of the two printing loops. I did remove the global variables; you should avoid them too, though sometimes global variables are necessary (whereas gets() is just poison and should never be used).

Answer 2

Your code seems to work. At least as this :

#include <stdio.h>
#include <string.h>

#define MAXS 1024
int main() {
  char str[MAXS];
  int N;
  int i;

  gets(str);
  N = strlen(str);

  for (i = N / 2 - 1; i >= 0; i--) {
    printf("%c", str[i]);
  }

  for (i = N - 1; i > N / 2 - 1; i--) {
    printf("%c", str[i]);
  }
  printf("\n");
}

Check your MAXS define. No other clue, sorry !

Edit: Oh! you're doing C++, actually...