How to convert a string to uppercase or lowercase in C ?
In C , strings can be converted to uppercase or lowercase by processing each character using std::toupper or std::tolower from
In C , converting a string to uppercase or lowercase isn't something you can do directly with a built-in function for the entire string, but it's straightforward once you know how to work with character-by-character conversion using standard library functions.
Use std::toupper and std::tolower from <cctype></cctype>
C provides std::toupper and std::tolower in the <cctype></cctype> header. These functions operate on individual characters, so to convert an entire string, you'll need to loop through each character and apply the transformation.
Here’s a simple example:
#include <iostream>
#include <string>
#include <cctype>
int main() {
std::string input = "Hello World!";
// Convert to uppercase
for (char &c : input) {
c = std::toupper(static_cast<unsigned char>(c));
}
std::cout << input << std::endl;
// Convert back to lowercase
for (char &c : input) {
c = std::tolower(static_cast<unsigned char>(c));
}
std::cout << input << std::endl;
return 0;
}A few things to note:
- Always cast the character to
unsigned charbefore passing it tostd::toupperorstd::tolower. This avoids undefined behavior for negative values. - These functions modify characters in place. If you want to preserve the original string, make a copy first.
Consider using Boost for convenience
If you're working on a project where third-party libraries are allowed, the Boost library offers convenient utilities like boost::algorithm::to_upper_copy and boost::algorithm::to_lower_copy.
Example:
#include <boost/algorithm/string.hpp>
#include <string>
#include <iostream>
int main() {
std::string input = "Hello World!";
std::string upper = boost::algorithm::to_upper_copy(input);
std::string lower = boost::algorithm::to_lower_copy(input);
std::cout << upper << std::endl;
std::cout << lower << std::endl;
return 0;
}This method is cleaner and handles edge cases well, but it adds a dependency on Boost, which may not be ideal for small projects or embedded systems.
Watch out for localization issues
When dealing with non-English characters (like accented letters or characters from other alphabets), be aware that std::toupper and std::tolower behave according to the current locale settings. By default, they only handle ASCII characters correctly.
If your application needs to support Unicode properly:
- Stick to ASCII unless you have a specific need for internationalization.
- Otherwise, consider using libraries like ICU (International Components for Unicode) for robust handling of multilingual text.
Also, keep in mind:
- Some characters don’t have uppercase or lowercase equivalents.
- Case conversions might change the number of characters in some languages (e.g., German ß → SS when uppercased).
So, whether you're just tweaking basic strings or preparing for more complex text processing, knowing how to use std::toupper and std::tolower gives you solid control. For simpler code or larger applications, Boost or ICU can help clean things up. Either way, it's not too hard once you get used to looping through characters.
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