TalG TalG - 3 months ago 8
C++ Question

Generating random numbers went wrong

I have this code. I'm generating 2 arrays with random numbers and then creating 2 strings out of those arrays using the

arrayToString
function, but my output is strange.

class job1Instance : public pp::Instance {
public:
explicit job1Instance(PP_Instance instance): pp::Instance(instance) {}
virtual ~job1Instance() {}

virtual void HandleMessage(const pp::Var& message) {
// declare all the zises
int32_t minNum = 1;
int32_t maxNum = 100;
int32_t arrayElements = maxNum;

// the arrays
int32_t unsorted1[arrayElements/2];
int32_t unsorted2[arrayElements/2];

// fill the arrays with random numbers
unsortedArrays(unsorted1, unsorted2, arrayElements, minNum, maxNum);
std::string outRes1, outRes2, jsonStr;
arrayToString(unsorted1, arrayElements/2, outRes1);
arrayToString(unsorted2, arrayElements/2, outRes2);
PostMessage(pp::Var(outRes2));
}



private:
// function to create a random number between min and max
int32_t rangeRandomAlg (int32_t min, int32_t max) {
int32_t num = max - min + 1;
int32_t remainder = RAND_MAX % num;
int32_t x;
do {
x = rand();
} while (x >= RAND_MAX - remainder);
return min + x % num;
}

// function to create arrays with random numbers
void unsortedArrays (int32_t unsorted1[], int32_t unsorted2[], int32_t arrayElements, int32_t &minNum, int32_t &maxNum) {
for(int32_t i = 0; i < arrayElements; i++) {
if (i < arrayElements/2) {
//unsorted1[i] = rangeRandomAlg(minNum, maxNum);
unsorted1[i] = rand() % maxNum + minNum;
} else {
//unsorted2[i] = rangeRandomAlg(minNum, maxNum);
unsorted2[i] = rand() % maxNum + minNum;
}
}
}


// convert the arrays to string
void arrayToString (int32_t array[], int32_t arraySize, std::string& arrayString) {
for (int i = 0; i <= arraySize; ++i){
arrayString+= std::to_string(array[i]);
if (i != arraySize) {
arrayString+= ',';
}
}
}


Can someone tell me why my
outRes2
output has those numbers?


-18700984,-18701112,8,0,2,0,-66124,0,-66124,0,267757568,0,-65608,0,1,0,-65608,0,266960448,0,-66124,0,1,0,-18699984,0,-66124,0,-18699984,0,266959840,0,7,-66124,-18699984,0,-66124,0,-68200,0,-18699984,0,266959360,0,1,0,536870911,0,-18700016,0,91


They are clearly not between 1 and 100 as my
minNum
and
maxNum
define, I can't find the problem.

Answer

You declared two arrays, each of size arrayElements/2:

    int32_t unsorted1[arrayElements/2];
    int32_t unsorted2[arrayElements/2];

Your loop initializes them as follows:

        if (i < arrayElements/2) {
            //unsorted1[i] = rangeRandomAlg(minNum, maxNum);
            unsorted1[i] = rand() % maxNum + minNum;
        } else {
            //unsorted2[i] = rangeRandomAlg(minNum, maxNum);
            unsorted2[i] = rand() % maxNum + minNum;
        }

Therefore, when, for example, the value of i reaches arrayElements/2, the else part of the if statement will execute:

            unsorted2[arrayElements/2] = rand() % maxNum + minNum;

Since the size of unsorted2 is arrayElements/2, this array contains only values unsorted2[0] through unsorted2[arrayElements/2-1], and this assignment runs off the end of the array, resulting in undefined behavior.