Strange behaviour in nested For Loops - c++

Question

First of all, I'm pretty new to C++ so try not to be too harsh on me. I wrote this block of code:
int LargestProduct (string numStr, int groupSize) {
int numOfGroups = numStr.size() / groupSize;
int groupsRemaining = numStr.size() % groupSize;
int largestProduct = 0, thisProduct = 1;
for (int i = 1; i <= numOfGroups; i++) {
for (int j = i; j <= groupSize; j++)
thisProduct *= (numStr[j-1] - '0');
if (thisProduct > largestProduct)
largestProduct = thisProduct;
thisProduct = 1;
}
// .. A bit more irrelevant code here
return largestProduct;
}
The function call LargestProduct ("1234567890", 2) should yield 72, but it wrongly yields 6. So, for some reason, this code will work but not as expected (Note: this code I wrote should compute the largest product of groupsSize-adjacent numbers in a big, given number called numStr).
I did some debugging, and found a strange behaviour in the nested for-loop. I set up a breakpoint inside the second for-loop
thisProduct *= (numStr[j] - '0');
After some iterations (for example, 8 iterations), this is what I would expect i and j to be:
+--------+---------+
| i | j |
+--------+---------+
| 1 | 1 |
| 1 | 2 |
| 2 | 1 |
| 2 | 2 |
| 3 | 1 |
| 3 | 2 |
| 4 | 1 |
| 4 | 2 |
+--------+---------+
This is what really happens:
+--------+---------+
| i | j |
+--------+---------+
| 1 | 1 |
| 1 | 2 |
| 2 | 2 |
+--------+---------+
And suddenly the program spits out a wrong result (6, instead of 72)
But this seems counterintuitive, to say the least. The variable i goes from 0 to numOfGroups, which in the example above equals 5. On the other hand, j goes from i to groupSize, which happens to be 2.
There should be 5*2 = 10 iterations, but there are only 3 of them. Also, in the last iteration, j should be "re-initialized" to 0. This doesn't happen though.
Anyone please help this C++ newbie?
EDIT
The problem was that the j-for-loop ranged from a moving index (i) to a non-moving index(groupSize). This was causing that "shrinking" effect in the second for-loop, which is easily fixed by changing this line:
for (int j = i; j <= groupSize; j++)
To this other one:
for (int j = i; j <= i + groupSize - 1; j++)
And to make the full algorithm to work as expected, one should also replace these lines:
int numOfGroups = numStr.size() / groupSize;
int groupsRemaining = numStr.size() % groupSize;
with this single one:
int numOfGroups = numStr.size() - 1;
EDIT 2
Everything is OK now, thank you for your kindness guys! I appreciate it. The whole code is:
int LargestProduct (string numStr, int groupSize) {
int numOfGroups = numStr.size() - 1;
int largestProduct = 0, thisProduct = 1;
for (int i = 1; i <= numOfGroups; i++) {
for (int j = i; j <= i + groupSize - 1; j++)
thisProduct *= (numStr[j-1] - '0');
if (thisProduct > largestProduct)
largestProduct = thisProduct;
thisProduct = 1;
}
return largestProduct;
}

Answer 1

You said:
On the other hand, j goes from 0 to groupSize
But the code says:
for (int j = i; j <= groupSize; j++)
This means j is going from i to groupSize, not 0 to groupSize