enamodeka - 5 months ago 39

C Question

I'm trying to figure out maximum value for type

`long`

`2`

Unfortunately the calculation overflows at step 61 and I don't understand why.

`long exponential(int base, int exponent)`

{

long result = (long)base;

for (int i = 0; i < exponent; i++) {

result *= base;

}

return result;

}

unsigned int sLong = sizeof(long);

long lResult = exponential(2, (sLong * 8) - 1);

`lResult`

`0`

What's odd is that when I do this for

`char`

`short`

`int`

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Answer Source

The code here has an off-by-one error.

Consider the following: what is the result of `exponential(10, 2)`

? Empirical debugging (use a `printf`

statement) shows that it's 1000. So `exponential`

calculates the mathematical expression b^{e+1}.

The `long`

type usually has 64 bits. This seems to be your case (seeing that the overflow happens around step 64). Seeing that it's a signed type, its range is (typically) from -2^{63} to 2^{63}-1. That is, the maximal power of 2 that the data type can represent is 2^{62}; if the code tries to calculate 2^{63}, then overflow happens (you probably want to avoid it).

So, because of the off-by-one error, the code will cause an overflow for `exponent`

greater or equal to 62.

To fix the off-by-one error, start multiplying from 1:

```
long power_of(int base, int exponent)
{
long result = (long)1; // 0th power of base is 1
for (int i=0; i<exponent;i++) {
result*=base;
}
return result;
}
```

However, this will not get rid of the overflow, because the `long`

data type cannot represent the number 2^{63}. Fortunately, you can use `unsigned long long`

, which is guaranteed to be big enough for the type of calculations you are doing:

```
unsigned long long power_of(int base, int exponent)
{
unsigned long long result = 1ULL; // 0th power of base is 1
for (int i=0; i<exponent;i++) {
result*=base;
}
return result;
}
```

Print it with the `llu`

format:

```
printf("%llu", power_of(2, 63));
```

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