Lowercasing Capital Letters in char array[] in C++ through Pointers - c++

Question

I am trying to use pointers to recursively lowercase all capital letters
using the C++ programming language. Below is the code snippet:
// Example program
#include <iostream>
#include <string>
using namespace std;
void all_lower(char* input) {
if ( *input ) {
cout << input << endl;
return;
}
if ( *input >= 'A' && *input <= 'Z') {
*input += 32; // convert capital letter to lowercase
}
cout << *input << endl;
all_lower(++input); // simply move to next char in array
}
int main() {
char test[] = "Test";
all_lower(test);
return 0;
}
The output ends up being:
"Test"
even though I tried to increase the ASCII code value of the element by 32.

Answer 1

You are exiting the function on the first non-null character detected, which is 'T', and then you output the entire array before exiting, so you are seeing the original unmodified input. You are not recursing through the array at all. You need to recurse through the array until you reach the null terminator.
You need to change this:
if ( *input ) {
cout << input << endl;
return;
}
To this instead:
if ( *input == 0 ) {
return;
}
Then the function will work as expected.
That being said, I suggest you remove the cout statements from the function, and do a single cout in main() after the function has exited. This will speed up the function, and prove that the content of the test[] array is actually being modified:
#include <iostream>
using namespace std;
void all_lower(char* input)
{
if ( *input == 0 ) {
return;
}
if ( *input >= 'A' && *input <= 'Z') {
*input += 32; // convert capital letter to lowercase
}
all_lower(++input); // simply move to next char in array
}
int main()
{
char test[] = "TEST";
cout << "Before: " << test << endl;
all_lower(test);
cout << "After: " << test << endl;
return 0;
}
Live Demo
And, since you are using C++, consider removing all_lower() altogether and use the STL std::transform() algorithm instead:
#include <iostream>
#include <algorithm>
using namespace std;
int main()
{
char test[] = "TEST";
cout << "Before: " << test << endl;
transform(test, test+4, test, [](char ch){ return tolower(ch); });
cout << "After: " << test << endl;
return 0;
}
Live Demo

Answer 2

Something short and easy:
#include <iostream>
#include <string>
using namespace std;
void all_lower(const char* input) {
if (!*input) {
std::cout << std::endl;
return;
}
std::cout << (char)(std::isalpha(*input) ? tolower(*input) : *input);
all_lower(++input); // simply move to next char in array
}
int main() {
all_lower("Test");
return 0;
}