String initialization fails

Question

This is my program. The output should be a sequence of 'a' chars, but for some reason it's not. Why?

#include <iostream>

using namespace std;

const int NAME_LENGTH = 16;

struct Record {
    char hotel_name[NAME_LENGTH];
};

int main() {
    int amount = 5;
    for (int i = 0; i < amount; i++) {
        Record * elementToBeAdded = new Record;
        for (int j = 0; j < NAME_LENGTH; j++)     
            elementToBeAdded->hotel_name[i] = 'a';
        elementToBeAdded->hotel_name[NAME_LENGTH-1] = '\0';
        cout << "string-" << elementToBeAdded->hotel_name << "-\n\n";
    }
}

+1 for including a short, complete testcase. See sscce.org

Robᵩ
– Robᵩ

2012-03-29 20:15:20 +00:00
Commented Mar 29, 2012 at 20:15 — Robᵩ
– Robᵩ, Commented Mar 29, 2012 at 20:15

DRVic · Accepted Answer · 2012-03-29 19:54:03Z

6

Because you have a typo in

        elementToBeAdded->hotel_name[i] = 'a';

You mean j, not i.

answered Mar 29, 2012 at 19:54

DRVic

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Comments

Ernest Friedman-Hill · Accepted Answer · 2012-03-29 19:54:31Z

Your innermost loop is using i when it surely intends to use j:

elementToBeAdded->hotel_name[i] = 'a';

as a result, you'll never set the zeroth element of Record #1's char[], nor the one-th element of Record #2's, etc. That means that every time through the outermost loop except the first, the very first char in the hotel name will remain uninitialized, very possibly \0.

Also, the Record objects you're creating are never being deleted, so this is leaking memory at each iteration.

codetantra · Accepted Answer · 2012-03-29 19:57:33Z

1

elementToBeAdded->hotel_name[i] = 'a';

should be

elementToBeAdded->hotel_name[j] = 'a';

answered Mar 29, 2012 at 19:57

codetantra

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Comments

Guangchun · Accepted Answer · 2012-03-29 20:01:38Z

1

Because there is a mis-spelling in your source code above:

 for (int j = 0; j < NAME_LENGTH; j++)     
  elementToBeAdded->hotel_name[i] = 'a';

the index should be [j] here.

answered Mar 29, 2012 at 20:01

Guangchun

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Comments

Luchian Grigore · Accepted Answer · 2012-03-29 19:53:54Z

0

You got i and j mixed up:

for (int j = 0; j < NAME_LENGTH; j++)     
    elementToBeAdded->hotel_name[i] = 'a'; //<-- should be j here

You're also leaking memory, you should delete elementToBeAdded; at the end of the outer loop.

answered Mar 29, 2012 at 19:53

Luchian Grigore

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Comments

Mahesh · Accepted Answer · 2012-03-29 20:01:36Z

0

Now that you know the mistake, use std::fill_n instead which is in algorithm header.

#include <algorithm>

// .....

for (int i = 0; i < amount; i++) {         

   Record * elementToBeAdded = new Record;
   std::fill_n( elementToBeAdded->hotel_name, NAME_LENGTH-2, 'a' );
   elementToBeAdded->hotel_name[NAME_LENGTH-1] = '\0';         

   cout << "string-" << elementToBeAdded->hotel_name << "-\n\n";
   delete  elementToBeAdded;
}

answered Mar 29, 2012 at 20:01

Mahesh

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