## Issue

I’ve been trying to do this problem on Project Euler

This is what I’ve done so far –

```
#include<stdio.h>
#include<stdlib.h>
#include<string.h>
int main(){
char nums[] = "7316717653133062491922511967442657474235534919493496983520312774506326239578318016984801869478851843858615607891129494954595017379583319528532088055111254069874715852386305071569329096329522744304355766896648950445244523161731856403098711121722383113622298934233803081353362766142828064444866452387493035890729629049156044077239071381051585930796086670172427121883998797908792274921901699720888093776657273330010533678812202354218097512545405947522435258490771167055601360483958644670632441572215539753697817977846174064955149290862569321978468622482839722413756570560574902614079729686524145351004748216637048440319989000889524345065854122758866688116427171479924442928230863465674813919123162824586178664583591245665294765456828489128831426076900422421902267105562632111110937054421750694165896040807198403850962455444362981230987879927244284909188845801561660979191338754992005240636899125607176060588611646710940507754100225698315520005593572972571636269561882670428252483600823257530420752963450"
int64_t sum = 1, new_sum = 0;
int num;
for(int i = 0; i < 988; i+= 13){
sum = 1
for(int j = 0; j < 13; ++j){
//Converting nums[i] to int and storing it to num here
sum *= num;
}
if(sum > new_sum){
new_sum = sum;
}
}
printf("%lld", new_sum);
}
```

I don’t know how to convert each charNum in the String to an integer, I’ve tried atoi and sscanf and both of them ask for a char pointer.

I get these errors respectively

```
passing argument to parameter here
int atoi(const char *); //sum *= atoi(nums[i])
```

```
format specifies type 'int *' but the argument has type 'int' [-Wformat]
sscanf(nums[i], "%d", num);
```

Any help is appreciated.

## Solution

Just converting a character representing a digit to the digit itself is simply done by

```
int digit = nums[position] - '0';
```

C language guarantees for *any* character set that the digits 0-9 are succeeding one another in exactly that order (which is not necessarily the case for alphabetic characters, see e.g. (in-?) famous EBCDIC).

As I read the problem the maximum can be at *any* location, not only multiples of 13. For this speaks as well that the maximum of four subsequent digits is found at a location where the number of digits preceding is *not* a multiple of four.

Apart from, unless the number if digits is a multiple of 13, your loops would exceed array bounds of `nums`

, so undefined behaviour (if not a multiple of 13, then for last iteration of `i`

there are less than 13 digits left, but `j`

still tries to iterate over all 13 digits).

Fixing both:

```
size_t sequenceLength = 13; // could be a function parameter
size_t len = strlen(nums); // sizeof would include the terminating 0 character!
if(len < sequenceLength)
{
return 0; // if within a function, else whatever appropriate error handling
}
uint64_t max = 1; // can't have negative values anyway...
for(size_t i = 0; i < sequenceLength; ++i)
{
max *= nums[i] - '0'; // see above
}
// now we found first possible maximum: the product of first 13 digits
uint64_t num = max;
for(size_t i = sequenceLength; i < len; ++i)
{
// for next product, we have already included the first 12 digits
// within the previous product!
// but there is one surplus digit contained we need to eliminate:
num /= nums[i - sequenceLength] - '0';
// now we can include the yet missing one:
num *= nums[i] - '0';
// or as a one-liner:
num = num / (nums[i - sequenceLength] - '0') * (nums[i] - '0');
// TODO: update maximum, if need be, just as in your code
}
```

Answered By – Aconcagua

Answer Checked By – Gilberto Lyons (BugsFixing Admin)