Problems with array in for

Question

I'm pretty new to C++ and I've only used high level languages before.

Here is my question. It's only for trying some things out.

#include <iostream>
#include <vector>
using namespace std;
int main() {
    vector<int> ve(10);
    for (unsigned int i; i < ve.size(); i++) {
        ve[i] = i+1;
        cout << ve[i] << endl;
    }
    cout << "Done filling vector. Now showing" << endl;
    for (unsigned int y; y < ve.size(); y++) {
        cout << ve[y] << endl;
    }
    cout << "We're done" << endl;
}

With the first "for" I want to fill the vector/array with values and output those values.

The second one is supposed to output them all again. However this doesn't happen. the array seems to be empty after the first for is done.

In C++ there is no default values. i will contain garbage if you don't initialize it. (Whereas in Java it would have been 0). — Maroun
– Maroun, Commented Dec 14, 2013 at 11:13

Angew is no longer proud of SO · Accepted Answer · 2013-12-14 11:12:33Z

The problem is that you're not initialising the iteration variables, which means they get pretty much random values (whatever happened to lie in memory at their location). Do this instead:

for (unsigned int i = 0; i < ve.size(); i++) {
    ve[i] = i+1;
    cout << ve[i] << endl;
}

and

for (unsigned int y = 0; y < ve.size(); y++) {
    cout << ve[y] << endl;
}

(Note: the difference is the initialisation = 0)

A few unrelated tips:

Variables declared in the for loop initialisation clause are local to the loop, they are not accessible outside of it. Which means they could both be called i.
std::endl is a combination of "output a newline" and "flush the buffer." Unless you really want both operations, it's preferrable (performance-wise) to just output \n for the newline. E.g.: cout << "We're done.\n";
For test programs it's fine, but generally it's advisable not to do using namespace std; even in source files (and it's downright dangerous to do it in header files). It's largely a matter of style, but still sometimes you could be bitten by a name clash.

Vlad from Moscow · Accepted Answer · 2013-12-14 11:12:57Z

1

You did not initialize variables i and y in the for-loop statements

for (unsigned int i; i < ve.size(); i++)

Should be

for (unsigned int i = 0; i < ve.size(); i++)

answered Dec 14, 2013 at 11:12

Vlad from Moscow

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Algo · Accepted Answer · 2013-12-14 11:14:57Z

1

for (unsigned int i=0; i < ve.size(); i++)

Not

for (unsigned int i; i < ve.size(); i++)

and the same for the printing loop.

answered Dec 14, 2013 at 11:14

Algo

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