Substrating each element of character .
for ( i = 0 ; i < l1 ; i ++ ) { d = ( v1 [ i ] - '0' - c ) - ( v2 [ i ] - '0' ) ; if ( d < 0 ) { d += 10 ; c = 1 ; } else c = 0 ; v1 [ i ] = '0' + d ; } v2 [ l2 ] = ' \0' ; i = l1 - 1 ; while ( i > 0 && v1 [ i ] == '0' ) i -- ; v1 [ i + 1 ] = ' \0' ; }