Queries on a string [closed] - c++

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It's a B problem in codeforces
http://www.codeforces.com/problemset/problem/598/B
and i submit this code, but i get Wrong Answer.
It's brute force nothing special, but iam new at problem solving.
#include <iostream>
#include<string>
using namespace std;
int main()
{
char x[10000];
scanf_s("%s",x);
int num;
scanf_s("%d",&num);
int *l = new int[num];
int *r = new int[num];
int *k = new int[num];
for(int i =0;i<num;i++)
{
scanf_s("%d,%d,$d",&l[i],&r[i],&k[i]);
}
char temp;
for(int i =0; i<num;i++)
{
for (int j =0;j<k[i];i++)
{
temp= x[l[i]-1];
x[l[i]-1]=x[r[i]-1];
x[r[i]-1]=temp;
}
}
printf("%s",x);
return 0;
}
Any idea what is wrong or does it need to be optimized ?Is there better way to handle with case of many queries entered ?

Answer 1

scanf is a C function. string is a C++ data type. C++ can often use C datatypes, but it is very rare that C can use C++ datatypes. scanf was written about 20 years before C++ existed and has no clue what a string is.
Next, scanf takes a variable arguments list. It has no clue if the parameter types are correct and cannot easily check. It assumes that the programmer knows what they are doing.
End result, it tries to place char data as specified by the %s format option into a string. The string is written over with incompatible data and undefined behaviour occurs.
Replace the scanf with
cin >> x;
and go all C++. Alternative is to eschew C++ and go C style:
char x[appropriate size goes here];
scanf("%s",x);
Don't know the appropriate size? That's going to be a problem. string resizes to fit. The char array expected by scanf cannot. If you read more data than you can fit, Undefined Behaviour.
In
scanf("%d",num);
%d says the programmer passed in a pointer to an integer, in this case it would be the location of num so that scanf can update the value stored at num with whatever was read. The value of num was passed in. scanf assumes this is a pointer and Undefined Behaviour results. Most likely whatever uninitialized garbage value that is in num is used as a memory location and some unsuspecting block of memory gets overwritten. This will cause problems at some point in the future when you actually need the data that was at that memory.
scanf("%d",&num);
or in C++
cin >> num;
The remaining problems are variations on the preceding two problems.
scanf("%d,%d,%d",l[i],r[i],k[i]);
needs pointers
scanf("%d,%d,%d",&l[i],&r[i],&k[i]);
and
printf("%s",x);
wants a char array, not a string.
printf("%s",x.c_str());
gets the char array equivalent to the string.
Recommendation: Compile with a higher level of intolerance to errors that the compiler can survive. In g++ I use at least -pedantic -pedantic-errors -Wall -Wextra -Werror.
On a logical front, your input is all unchecked. A user could type in "rutabaga" for num with possibly comical results as your program tries to deal with non-numeric input. Again Undefined Behaviour. The program could crash. It could lock up. It could impregnate a male Llama.
In C++
if (cin >> num)
will catch some but not all forms of bad input. If this test fails, the contents of num are undefined and should not be used. Further, the stream will be in an error state and unreadable until the error is acknowledged and cleared. In C the equivalent is
if (scanf("%d",&num) == 1)
if scanf read exactly one value, num, all is good. Any other number of values read means scanf did not succeed and the contents of num are undefined.