C++ “add explicit braces to avoid dangling else” - c++

Question

bool isPalindromeIterative(const char *s1){
int len=strlen(s1)-1;
if(len>0)
if(s1[0]==s1[len]){
len-=2;
isPalindromeIterative(s1+1);
}
else
return false;
return true;
}
I am writing about Palindrome.
And when I run it, it appears warning like this:
":79:13: warning: add explicit braces to avoid dangling else
[-Wdangling-else]"
Please help me ! Thanks!

Answer 1

There is formally nothing wrong with the code, unless you wanted the else to match the outer if. A common mistake.
If you add braces everywhere, it will be clear what you intended:
if(len>0)
{
if(s1[0]==s1[len])
{
len-=2;
isPalindromeIterative(s1+1);
}
else
{
return false;
}
}

Answer 2

When you write,
if(len>0)
if(s1[0]==s1[len]){
// This has no effect on the recursive call.
len-=2;
// You missed the return in your post.
return isPalindromeIterative(s1+1);
}
else
return false;
it's most likely that you meant to associate the else with the second if.
if(len>0)
{
if(s1[0]==s1[len])
{
return isPalindromeIterative(s1+1);
}
else
return false;
}
However, the compiler does not use the indent to figure that out. It's possible that, from the compiler writer's point of view, you meant to associate the else with the first if:
if(len>0)
{
if(s1[0]==s1[len])
{
return isPalindromeIterative(s1+1);
}
}
else
{
return false;
}
Since this is common mistake made by developers, the compiler warns you and hopes that you will update the code so that it's less likely to turn into a runtime error.
I want to point out that the recursion logic you are using to detect whether a string is palindrome is wrong.
Let's say your string is "abxba".
In the first iteration, you compare 'a' with 'a'.
In the next iteration, you compare 'b' with 'a', which is incorrect. You end up with the wrong answer.
You have to change your strategy a bit. Use:
bool isPalindromeIterative(const char *s1, int start, int end)
{
if ( start >= end )
{
return true;
}
if ( s1[start] == s1[end] )
{
return isPalindromeIterative(s1, start+1, end-1)
}
return false;
}
The start of the iterative call has to be made as:
isPalindromeIterative(s1, 0, strlen(s1)-1);

Answer 3

It is a style warning against reading the if clauses wrong.
if(len>0) {
if(s1[0]==s1[len]){
len-=2;
isPalindromeIterative(s1+1);
}
} else {
return false;
}
is better to read and less error-prone.
We have similar coding guidelines in our company; for an if, which has bracket clauses, all else branches and all other ifs within the if of the highest order have to have brackets.
Else your example could too easily be misread as e.g.
if(len>0)
if(s1[0]==s1[len]){
len-=2;
isPalindromeIterative(s1+1);
}
else
return false;

Answer 4

Your original post headline mentioned iterative, and 'Iterative' is still part of your function name (even though it is recursive).
You marked this post as c++, but did not use classes.
The other answers addressed your specific questions about your error messages.
For your consideration, and because you have already selected a recursive answer, here is a possible C++ iterative solution and tail-recursive solution using std::string&.
#include <iostream>
#include <string>
class T589_t
{
public:
int exec(int argc, char* argv[])
{
if (argc < 2)
{
std::cerr << "\n please provide one or more string(s) to test"
<< std::endl;
return -1;
}
for (int i = 0; i < argc; ++i)
{
std::string s = argv[i];
{
std::string yesno = (isPalindromeIterative(s) ? " is" : " is not");
std::cout << "\n '" << s << "'" << yesno << " a palindrome (iterative)" << std::flush;
}
std::cout << std::endl;
{
std::string yesno = (isPalindromeRecursive(s) ? " is" : " is not");
std::cout << " '" << s << "'" << yesno << " a palindrome (recursive)" << std::endl;
}
} // for 0..argc
return 0;
}
private: // methods
bool isPalindromeIterative(const std::string& s1)
{ // ^^^^^^^^^
bool retVal = false; // guess s1 is not palindrome
int left = 0; // index of left most char
int right = static_cast<int>(s1.size()) - 1; // index of right most char
do { // iterative loop
if (s1[left] != s1[right]) break; // when not equal, is not palindrome
left += 1; right -= 1; // iterate!
if (left >= right) // all chars tested?
{
retVal = true; // confirm palindrome
break; // exit
}
} while (true);
return retVal;
}
// Notes './dumy589' // odd length 9
// ^-------^ [0] vs [8]
// ^-----^ [1] vs [7]
// ^---^ [2] vs [6]
// ^-^ [3] vs [5]
// ^ [4] == [4] // left >= right, break
// Notes 'abccba' // even length 6
// ^----^ [0] vs [5]
// ^--^ [1] vs [4]
// ^^ [2] vs [3]
// [3] vs [2] // left >= right, break
// Notes 'abcba' // odd length 5
// ^---^ [0] vs [4]
// ^-^ [1] vs [3]
// ^ [2] vs [2] // left >= right, break
// and bonus: tail recursion based on above iterative
// vvvvvvvvv
bool isPalindromeRecursive(const std::string& s1)
{
// index of left most char
int left = 0;
// index of right most char
int right = static_cast<int>(s1.size()) - 1;
return (isPalindromeRecursive(s1, left, right));
}
bool isPalindromeRecursive(const std::string& s1, int left, int right)
{
if (s1[left] != s1[right]) return false;
left += 1; right -= 1;
if ( left >= right ) return true;
return (isPalindromeRecursive(s1, left, right));
}
}; // class T589_t
int main(int argc, char* argv[])
{
T589_t t589;
return t589.exec(argc, argv);
}
On Linux, argv[0] is the executable name.
environment:
Lubuntu 17.10,
g++ (Ubuntu 7.2.0-8ubuntu3.2) 7.2.0
With invocation:
./dumy589 aba abccba tx s
This code reports:
'./dumy589' is not a palindrome (iterative)
'./dumy589' is not a palindrome (recursive)
'aba' is a palindrome (iterative)
'aba' is a palindrome (recursive)
'abccba' is a palindrome (iterative)
'abccba' is a palindrome (recursive)
'tx' is not a palindrome (iterative)
'tx' is not a palindrome (recursive)
's' is a palindrome (iterative)
's' is a palindrome (recursive)