Level Up - Dev: Integer Overflow Not Overflowing

An integer overflow (or wraparound) occurs when a number larger than the max integer value or smaller than the min integer value is tried to be stored in the integer's memory. You could imagine a circular number line.

Ex: If you have an integer at the max positive value and add one, then the integer will wraparound and be set to the most negative value (if using a signed int. An unsigned int will wraparound to 0). And, vise-versa.

Today, I experienced a bug where I wanted to get a (signed) integer overflow, but unfortunately the int value was just going to zero instead of going to the negative numbers. Thankfully, I knew the memory/bit representation of int and was able to figure out what was going on.

Here is some simplified code, with only one small change:

    // Test #1 - has wraparound
    int wrap = 1;
    for (int i = 0; i < 34; i++) {
        print(i + ": wrap: " + wrap);
        wrap = wrap * 3;
    }

    // Test #2 - no wraparound
    int wrap = 1;
    for (int i = 0; i < 34; i++) {
        print(i + ": wrap: " + wrap);
        wrap = wrap * 2;
    }

Here is the relevant output from running the code above:

// Test #1 - has wraparound
...
13: wrap: 1594323
14: wrap: 4782969
15: wrap: 14348907
16: wrap: 43046721
17: wrap: 129140163
18: wrap: 387420489
19: wrap: 1162261467
20: wrap: -808182895
21: wrap: 1870418611
22: wrap: 1316288537
23: wrap: -346101685
24: wrap: -1038305055
25: wrap: 1180052131
26: wrap: -754810903
27: wrap: 2030534587
28: wrap: 1796636465
29: wrap: 1094942099
30: wrap: -1010140999
31: wrap: 1264544299
32: wrap: -501334399
33: wrap: -1504003197

// Test #2 - no wraparound
...
20: wrap: 1048576
21: wrap: 2097152
22: wrap: 4194304
23: wrap: 8388608
24: wrap: 16777216
25: wrap: 33554432
26: wrap: 67108864
27: wrap: 134217728
28: wrap: 268435456
29: wrap: 536870912
30: wrap: 1073741824
31: wrap: -2147483648
32: wrap: 0

33: wrap: 0

If test #1 were to continue, then the values would keep wrapping. If test #2 were to continue, then the values would stay the same at 0.

The easiest way for me to make sense of this experiment was to know that multiplying by a power of 2 (2, 4, 8, 16, 32..) is the exact same as a bit shift in the binary computer. And, the bits that made up the int were all just getting pushed out of memory in test #2 until there were no more set bits to manipulate. In test #1, more int memory bits are being flipped in order to continue the wraparound.

I would explain this at a lower level, but there are already many great resources for "bit representation of integer".


~ Danial Goodwin ~

ps - This post was mainly for myself and written quickly. If there are any questions about this, then I'd be happy to elaborate.

---

My Android Apps | My Windows Phone Apps | My Developer Blog

Arthur C. Clarke's Three Laws of Prediction

Arthur C. Clarke was a British writer and wrote three "laws" about predictions. They are as follows:

1. When a distinguished but elderly scientist states that something is possible, he is almost certainly right. When he states that something is impossible, he is very probably wrong.
2. The only way of discovering the limits of the possible is to venture a little way past them into the impossible.
3. Any sufficiently advanced technology is indistinguishable from magic.

My summary of the first point is, "the future is possible", "don't discount the future", or perhaps "".

My summary of the second point is, "Don't give up until you try. Then try harder."

My summary of the third point is, "technology is magic."

Source (Wikipedia)

Bonus Quote: "Perhaps it is better to be un-sane and happy, than sane and un-happy." - Arthur C. Clarke

Out of the ~100 more quotes I read by him, here are a few more that stood out:

• Interesting: "One of the biggest roles of science fiction is to prepare people to accept the future without pain and to encourage a flexibility of mind. Politicians should read science fiction, not westerns and detective stories."
• Enjoyable: Clarke's Law of Revolutionary Ideas: Every revolutionary idea — in science, politics, art, or whatever — seems to evoke three stages of reaction. They may be summed up by the phrases:
  • (1) "It's completely impossible — don't waste my time";
  • (2) "It's possible, but it's not worth doing";
  • (3) "I said it was a good idea all along."
• Sounds Good: "I have great faith in optimism as a guiding principle, if only because it offers us the opportunity of creating a self-fulfilling prophecy."

=]
~ Simply Advanced ~

ps - My own quote is "Don't be stuck in a paradigm. Facts change." =b

---

My Android Apps | My Windows Phone Apps | My Computer/Programming Blog