strlen 在函数中改变输出

Question

Okay, I tried to figure it out, but couldn't;

So there is a function in my source code with two arguments passed (9 and 1, as Strings)

char * plus( char *a, char *b ){
   char sa[101] = {0};
   char sb[101] = {0};
   static char _s[101] = {0};
   int i;
   int st = 0;
   int sto;
   for( i = strlen( a ) - 1; i >= 0; i-- ){
       sa[strlen( a ) - 1 - i] = a[i]; 
   }
   for( i = strlen( b ) - 1; i >= 0; i-- ){
       sb[strlen( b ) - 1 - i] = b[i];
   }
   for( i = 0; i < strlen( sa ); i++ ){
       sto = 0;
       if(( sa[i] + sb[i] + st - 2 * '0' ) > 9 ) sto = 1;
       sa[i] = ( sa[i] + sb[i] + st - 2 * '0' ) %10 + '0';
       st = sto;
       if( st && sa[i + 1] == 0) sa[i + 1] = '1';
   }
   for( i = strlen( sa ) - 1; i >= 0; i-- ){
       _s[strlen( sa ) - 1 - i] = sa[i];
   }
   return _s;
}

It should return "10" but instead "*0", but constantly, (so not "undefined behaviour") What's wrong?

Edit:

if I pass strlen( a ) instead of strlen( sa ) in line for( i = 0; i < strlen( sa ); i++ ){ then it works correctly

Answer 1

sa[i] + sb[i] + st - 2 * '0'

is negative. Because you do sa[i + 1] = '1' then strlen(sa) get's incremented in the loop. Because of that, sb[i] is equal 0 because it's the zero terminating character on the last run when i == strlen(sa) == 2 . Then the calculation becomes:

sa[i] + sb[i] + st - 2 * '0' = 
  '1' +     0 +  1 - 2 * '0' = 
   49 +     0 +  1 - 2 * 48  =
                50 - 96      =
                -46 

Because that's minus 46, sa[i] becomaes:

a[i] = ( sa[i] + sb[i] + st - 2 * '0' ) %10 + '0' =
                                    -46 %10 + '0' =
                                         -6 + '0' =
                                              '*'

Most probably in the loop:

 for( i = 0; i < strlen( sa ); i++ ){

You have to take care what happens when string lengths are not equal. You do sa[i] = '1' , but sb stays the same. I guess for now you can fix it by replacing

  sa[i] + sb[i] + st - 2 * '0'

with something along:

  sa[i] - '0' + (sb[i] ? sb[i] - '0' : '0') + st

Notes: I do not enjoy your indentation. Please make your code as readable as possible. Use newlines to denote new expressions. Use { } to denote where you enter loop or if bodies.

Answer 2

This loop

   for( i = 0; i < strlen( sa ); i++ ){
       sto = 0;
       if(( sa[i] + sb[i] + st - 2 * '0' ) > 9 ) sto = 1;
       sa[i] = ( sa[i] + sb[i] + st - 2 * '0' ) %10 + '0';
       st = sto;
       if( st && sa[i + 1] == 0) sa[i + 1] = '1';
   }

is incorrect because within the loop the array sa is being changed. So also the value returned by strlen( sa ) that is calculated in each iteration is also changed.

When you are using strlen( a ) instead of strlen( sa ) then the array s is not changed in the loop . So the value returned by the expression is fixed.