CODEWITHSUNDEEP
c++for-loopvectorcout
MinaThuma
MinaThuma

Reputation: 123

Why am I not printing out the vector when I use the for loop and cout command

I cannot seem to find the reason why the contents of my vector components are not supplied using the for loop and cout. Background to the code is that I want to know what happens when you resize a vector that already exists (if made smaller, which components fall away, if larger, which components are added,...)

#include <iostream>
#include <vector>
#include <iomanip> 
#include <cmath>

using namespace std;

int main()
{
vector<double> a;
for (int i = 1; i < 10; ++i)
a[i] = 5;
a.resize(15)
for (int i = 1; i < a.size(); ++i)
cout << a[i] << endl;
return 0;
}

Upvotes: 0

Views: 361

Answers (3)

Vlad from Moscow
Vlad from Moscow

Reputation: 311176

The program has undefined behavior because the declared vector is empty and you may not use the subscript operator for an empty vector.

vector<double> a;
for (int i = 1; i < 10; ++i)
a[i] = 5;

Moreover the indices of vectors start from 0.

You could either write

vector<double> a( 10 );
for (int i = 0; i < 10; ++i)
a[i] = 5;

or

vector<double> a;
for ( int i = 0; i < 10; ++i)
a.push_back( 5 );

Or to make the last code snippet more efficien you could write

vector<double> a;
a.reserve( 10 );
for ( int i = 0; i < 10; ++i)
a.push_back( 5 );

Again to output the full content of the vector you should start indices from 0. Or you could use the range based for loop like

for ( const auto &item : a ) std::cout << item << '\';

Upvotes: 2

Thomas
Thomas

Reputation: 5148

You did not change the size of the vector. To insert into a vector you can use the push_back or emplace_back methods. If you change the insertig code to 

for (int i = 1; i < 10; ++i)
  a.push_back(i);

or alternatively use the resize method before you assign 

a.resize(10);
for (int i = 1; i < a.size(); ++i)
   a[i] = i;

then the print loop should work fine.

Upvotes: 0

Valeriy Savchenko
Valeriy Savchenko

Reputation: 1614

You have an out-of-bounds access in your code. You need to resize your vector before filling it. Alternatively you can use push_back to insert elements. In either case, it will work as you expect.

Upvotes: 0

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