CODEWITHSUNDEEP
c++vectorstl
Animesh Pandey
Animesh Pandey

Reputation: 6018

Error while using 2d Vectors in c++

vector <vector <int>> frameVecs(4);
vector <int> t1 = getPeaks (vec1);
vector <int> t2 = getPeaks (vec2);
vector <int> t3 = getPeaks (vec3);
vector <int> t4 = getPeaks (vec4);

frameVecs[0].reserve(t1.size());
frameVecs[1].reserve(t2.size());
frameVecs[2].reserve(t3.size());
frameVecs[3].reserve(t4.size());

frameVecs.push_back (t1);
frameVecs.push_back (t2);
frameVecs.push_back (t3);
frameVecs.push_back (t4);

I always get subscript out of range error during the pushback part of the snippet.

Just two days back I was with this code that does the same thing:

vector <vector <int>> frameVecs;

frameVecs.push_back (getPeaks (vec1));
frameVecs.push_back (getPeaks (vec2));
frameVecs.push_back (getPeaks (vec3));
frameVecs.push_back (getPeaks (vec4));

And this was running just fine for the last 2 months .. and suddenly I started getting subscript out of range error in the pushback part and hence I had to change thsi particular part into the one above it, and still I am getting this error.

I mean, are vectors so uncertain in their behavior ?

PS: getPeaks() returns an int vector!

Upvotes: 0

Views: 103

Answers (2)

john
john

Reputation: 8027

Excelcius's answer is good, but here's another way to do what I think you are trying to do.

vector <vector <int>> frameVecs(4);
frameVecs[0] = getPeaks (vec1);
frameVecs[1] = getPeaks (vec2);
frameVecs[2] = getPeaks (vec3);
frameVecs[3] = getPeaks (vec4);

The main thing here is that this code is simpler than your code. When simple things get as complicated as your code it's a pretty clear indication that something is wrong.

Main misunderstanding seems to be that either you set the size of a vector in the constructor or you dynamically increase the size using push_back, rarely do you want to do both.

Upvotes: 0

Excelcius
Excelcius

Reputation: 1690

I think push_back doesn't do what you think it does. 

vector <vector <int>> frameVecs(4);

Passing 4 to the vector on construction already creates 4 empty int-vectors. If you push_back the 4 vectors t1, t2, t3 and t4 you add another 4 vectors to frameVecs, so you actually have 8 entries in frameVecs. Maybe removing (4) and just writing:

vector <vector <int>> frameVecs;

will solve the problem.

If you now try to access frameVecs[0][0] for example (although not shown in your example) you get an error.

After the 4th push_back frameVecs looks like this:

frameVecs[0] // empty
frameVecs[1] // empty
frameVecs[2] // empty
frameVecs[3] // empty
frameVecs[4] // contains t1
frameVecs[5] // contains t2
frameVecs[6] // contains t3
frameVecs[7] // contains t4

Calling reserve is also unnecessary in this example.

Upvotes: 4

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