Segmentation fault in base number program? - c

Question

I keep trying to test this code but I keep getting a segmentation fault in my power() function. The code is supposed to take a word made up of lowercase letters and change the word to a number of base 10. The word is supposed to take on the form of a number of base 20, where 'a' = 0, 'b' = 1,...., 't' = 19;
int power(int i){
if(i==1){
return 20;
}else{
return 20*power(i--);
}
}
int main(){
int len;
char mayan[6];
int n;
int val;
while(scanf("%s", mayan)){
val = 0;
n = 0;
for(len = 0; mayan[len] != '\0'; len++){
mayan[len] = tolower(mayan[len]);
mayan[len] = mayan[len] - 'a';
}
for(i = 0; len >= 0; len--, i++){
if(mayan[len] <= 19){
n = n + mayan[len] * power(i);
}else{
fprintf(stderr, "Error, not a base 20 input \n");
val = 1;
break;
}
}
if(val==0){
printf("%d \n", n);
}
}
return val;
}

Answer 1

There were three mistakes in your code.
Case for i==0 not added in the power function, which basically translates to any number to the power of zero is one i.e. x^0 = 1;.
Instead of using return 20*power(i--); for your recursive call, use return 20*power(i-1);. i-- is post decrement operator, which means that, it will return the value of i as it is and will the decrement it for further use, which is not what you want. Also, you altogether don't even want to change the value of i for this iteration too; what you want to do is use a value one less than i for the next iteration, which is what, passing i-1, will do.
Add a len-- in the initialization of the for(i = 0; len >= 0; len--, i++) loop, because len is now over the last index of the input because of the previous loop.
Correcting these mistakes the final code is:
#include<stdio.h>
int power(int i)
{
if(i==0)
{
return 1;
}
if(i==1)
{
return 20;
}
else
{
return 20*power(i-1);
}
}
int main()
{
int len,i;
char mayan[6];
int n;
int val;
while(scanf("%s", mayan))
{
val = 0;
n = 0;
for(len = 0; mayan[len] != '\0'; len++)
{
mayan[len] = tolower(mayan[len]);
mayan[len] = mayan[len] - 'a';
}
for(i = 0, len--; len >= 0; len--, i++)
{
if(mayan[len] <= 19)
{
n = n + mayan[len] * power(i);
}
else
{
fprintf(stderr, "Error, not a base 20 input \n");
val = 1;
break;
}
}
if(val==0)
{
printf("%d \n", n);
}
}
return val;
}
Note that, your code would essentially only work for at most a five digit base 20 number, because, the array mayan that you are using to store it has size 6, of which, one character will be spent for storing the terminating character \0. I recommend that you increase the size of the array mayan unless you want to support only five digit base 20 numbers.