Debug assertion failed: C++ vector subscript out of range

Question

I am trying to make a simple application based on stack and my code is giving an error when I am trying to run it. The code is here:

// Stacks.cpp : Defines the entry point for the console application.
    //

    #include "stdafx.h"
    #include<iostream>
    #include<vector>
    using namespace std;

    class stackx{
    private:
        int top;
        vector<double> stackVect;
        int maxSize;

public:
    stackx(int s): maxSize(s),top(-1){
        stackVect.reserve(maxSize);
    }

    void push(double a){
        stackVect[++top]=a;
    }

    double pop(){
        return stackVect[top--];
    }

    double peek(){
        return stackVect[top];
    }

    bool isEmpty(){
        return (top==-1);
    }

    bool isFull(){
        return (top == maxSize-1);
    }

};


int main(){

    stackx stackvect(6);
    stackvect.push(20);
    stackvect.push(22);
    stackvect.push(13);
    stackvect.push(69);
    stackvect.push(123);

    while(!stackvect.isEmpty()){
        double value = stackvect.pop();
        cout<<value<<" ";
    }
    cout<<endl;
    return 0;
}

Where am I making the mistake ?

Vectors don't resize when you access them out of bounds (well, they could due to the undefined behaviour, but it's not guaranteed in any way). — chris, Jul 23 '14 at 12:17
I am not getting what are you trying to say. I am a beginner Help me. — shashankgaurav, Jul 23 '14 at 12:19
The `reserve` member function of a vector does not change the number of elements the vector contains. It merely changes the amount of memory the vector manages so that it can add new elements faster. The `resize` member function on the other hand *does* change the number of elements. — dyp, Jul 23 '14 at 12:20
To be honest, I didn't even notice the `reserve` call, or I would have focused on that. — chris, Jul 23 '14 at 12:21
see http://stackoverflow.com/questions/13029299/stdvectorresize-vs-stdvectorreserve — SleuthEye, Jul 23 '14 at 12:22
Go up the call stack to your own code. Did you mean resize instead of reserve? — Neil Kirk, Jul 23 '14 at 12:22
I found similar code in Sam's teach yourself data structure and algorithm in 24 hrs. — shashankgaurav, Jul 23 '14 at 12:22
*"It's saying line 1140. Although there is no line 1140"* It's probably referring to line 1140 in a different file. — dyp, Jul 23 '14 at 12:23
Anything which says you can learn C++ in 24 hours is suspect. — Neil Kirk, Jul 23 '14 at 12:23
Omg it's truly all the book's fault. The code in there uses `reserve` and `operator[]`. How utterly terrible. — chris, Jul 23 '14 at 12:26

score 0 · Answer 1 · answered Jul 23 '14 at 12:21

I guess you're not using correctly std::vector.

You should use push_back instead of stackVect[++p], pop_back instead of stackvect[p--] and back for peek.

As for the answer to why, it's simply because the vector is empty/not of size "top", and accessing a value won't work. stackVect[10000] does not create up to 10000 items, simply access it. If it does not exists, result is unpredictable.

And in that case, you should use at which does the same, but throws an exception upon invalid offset.

score 0 · Accepted Answer · answered Jul 23 '14 at 12:22

0

Vectors do not resize automatically when you access them with [], so the code

stackVect[++top]=a;

which you expected to add an element to the vector is out-of-bounds access. You need to use methods like push_back to add elements to vectors. However, in your code top is not needed at all. Use the size of the vector instead. So e.g. isEmpty would be defined as

return stackVect.empty();

Then push_back, pop_back, and back methods of the vector is all you need. Or even better, use std::stack, which has all of it builtin (in slightly different, exception-safe way to implement pop).

answered Jul 23 '14 at 12:22

Wojtek Surowka

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Thanks. and by default what is the size of the vector which I defined? – shashankgaurav Jul 23 '14 at 12:26
Vectors are initially empty. – Wojtek Surowka Jul 23 '14 at 12:27
Ok thanks. I get it I was using vector without defining its size. – shashankgaurav Jul 23 '14 at 12:28

Debug assertion failed: C++ vector subscript out of range

2 Answers2