garbage values for vector push_back

Question

I'm trying to assign an array's values to a vector. It seems to be working fine for one vector, but when I do it for a second, I'm getting back garbage values. I cout the number and I know it's correct, but it's not assigning correctly. I don't understand because it's working fine for the first vector.

int sorted[] = {0,1,2,3,4,5,6,7,8,9,10};

// make two smaller arrays, do this untill they are a base case size; 
void split(int *dataIN, int dataSize){
    // new data will be broken up into two vectors with each half of the 
    // original array. These will be size firstHalfSize and secondHalfSize.     
    int firstHalfSize; 
    int secondHalfSize;     
    vector<int> firstHalf; 
    vector<int> secondHalf;     
    // test to see if array in is odd or even   
    bool isOdd; 
    if (dataSize%2 == 1){
        isOdd = true;   
    }else if (dataSize%2 == 0){
        isOdd = false; 
    }   
    // determine length of new vectors
    // second half is firstHalf + 1 if odd.     
    firstHalfSize = dataSize/2; 
    if (isOdd){
        secondHalfSize = firstHalfSize + 1; 
    }else if (!isOdd){
        secondHalfSize = firstHalfSize; 
    }           
    // assign first half of dataIn[] to firstHalf vector    
    cout << "firs: " << firstHalfSize << endl;
    for (int i = 0; i < firstHalfSize; i++){
        cout << "a: " << dataIN[i] << endl;// make sure i have the right number 
        firstHalf.push_back(dataIN[i]);// assign    
        cout << "v: " << firstHalf[i] << endl;// make sure assigned correctly   
    }   
    // do the same for second half  
    cout << "second: " << secondHalfSize << endl;   
    for (int i = firstHalfSize; i < (firstHalfSize+secondHalfSize); i++){
        cout << "a: " << dataIN[i] << endl; 
        secondHalf.push_back(dataIN[i]);    
        cout << "v: " << secondHalf[i] << endl; 
    }   

}


int main(void){
    split(sorted, sizeof(sorted)/sizeof(int));  
    return 0;
}

This is my result. As you can see the first vector push_back went fine and the array values (after "a: ") are also correct.

firs: 5
a: 0
v: 0
a: 1
v: 1
a: 2
v: 2
a: 3
v: 3
a: 4
v: 4
second: 6
a: 5
v: -805306368
a: 6
v: 2
a: 7
v: -805306368
a: 8
v: 0
a: 9
v: 0
a: 10
v: 0

score 4 · Accepted Answer · answered Dec 06 '14 at 06:58

4

In the second case, you are indexing from firstHalfSize.

You need to cout the values starting from index 0. For example:

cout << "v: " << secondHalf[i-firstHalfSize] << endl;

answered Dec 06 '14 at 06:58

Harry

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score 2 · Answer 2 · answered Dec 06 '14 at 06:59

2

You are iterating firstHalf from 0 to firstHalfSize with the variable i, so i will be within the range of firstHalf, when you use operator[] - in the second vector's case, i does not mean the same thing.

answered Dec 06 '14 at 06:59

fbrereto

35,429
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score 0 · Answer 3 · answered Dec 06 '14 at 08:12

The filling of the vector is working. It is just your debug output that is incorrect. When outputting values from secondHalf you need to use indexes from 0, not from firstHalfSize.

You can write your code more simply if you take advantage of the std::vector range constructor that takes a pair of iterators. Array pointers can be treated as iterators:

void print(const std::vector<int>& data){ 
  for(int value : data)
    std::cout << value << " ";
  std::cout << "\n";
}

void split(int *dataIN, int dataSize){
    auto firstHalfSize = (dataSize + 1) / 2;
    std::vector<int> firstHalf(dataIN, dataIN + firstHalfSize); 
    std::vector<int> secondHalf(dataIN + firstHalfSize, dataIN + dataSize);

    std::cout << "firstHalf: ";
    print(firstHalf);
    std::cout << "seconHalf: ";      
    print(secondHalf);
}

Live demo

garbage values for vector push_back

3 Answers3