Lessons learned from a Simple C++ Capitalisation Function

Original Code

#include <iostream>
#include <string>

using namespace std;

string capitalise(string raw){
    if(raw[0] >= 'a' && raw[0] <= "z"){
        raw[0] += ('A' - 'a');
    }
    string capitalised = raw;
    return capitalised;
}

int main(){
    // Ask for character from user
    cout << "Please input string: " << endl;
    string rawString = "";
    rawString += cin;
    cout << endl;
    rawString = capitalise(rawString);
    cout << rawString << endl;
    return 0;
}

Character vs. String Literals ('z' vs "z")

Error:

if (raw[0] >= 'a' && raw[0] <= "z")  // Wrong: "z" is a string, not a char

Lesson:

• 'z' is a character literal (type char).
• "z" is a string literal (type const char*, a null-terminated array).
• I can’t compare a char (raw[0]) with a const char* ("z").
• Fix: Always use single quotes (' ') for characters and double quotes (" ") for strings.

Correct Way:

if (raw[0] >= 'a' && raw[0] <= 'z')  // Correct: compares chars

---

Reading Input Properly (cin vs getline)

Error:

rawString += cin;  // Invalid: cannot append `cin` to a string

Lesson:

• cin is an input stream (istream), not a string.
• Operator += does not work with streams.
• Ways to read input:
  • cin >> str (reads until whitespace, not full lines).
  • getline(cin, str) (reads entire line, including spaces).

Correct Way:

string rawString;
getline(cin, rawString);  // Reads entire line

---

Handling Empty Strings Safely

Error:

Original code did not check if raw was empty before accessing raw[0].

Lesson:

• Accessing raw[0] on an empty string is undefined behavior.
• Always check !str.empty() before accessing str[0].

Correct Way:

if (!raw.empty() && raw[0] >= 'a' && raw[0] <= 'z') {
    raw[0] += ('A' - 'a');  // Safely capitalize first char
}

---

Avoiding Unnecessary Copies

Original Code:

string capitalised = raw;  // Unnecessary copy
return capitalised;

Lesson:

• Copying strings is inefficient (especially for long strings).
• Return the modified string directly (C++ optimizes this via Return Value Optimization (RVO)).

Optimized Way:

return raw;  // No extra copy

---

Understanding C++ Streams (cin, cout)

Error:

Trying to use cin as if it were a string.

Lesson:

• cin is an input stream (istream), not a string.

• Use >> for word-by-word input or getline() for full-line input.

• Example:

  int num;
  cin >> num;  // Reads an integer
  
  string line;
  getline(cin, line);  // Reads a full line

---

Compiler Warnings

What the Compiler Told Me:

• “Comparison between pointer and integer” → I compared char with const char*.
• “No viable overloaded ‘+=’” → I tried to add cin to a string.

---

Final Corrected Code (Recap)

#include <iostream>
#include <string>

using namespace std;

string capitalise(string raw) {
    if (!raw.empty() && raw[0] >= 'a' && raw[0] <= 'z') {
        raw[0] += ('A' - 'a');  // Convert lowercase to uppercase
    }
    return raw;
}

int main() {
    cout << "Please input string: " << endl;
    string rawString;
    getline(cin, rawString);  // Read entire line
    rawString = capitalise(rawString);
    cout << rawString << endl;
    return 0;
}

Key Takeaways for Future Code:

Mistake	Lesson
raw[0] <= "z"	Use 'z' for chars, "z" for strings.
rawString += cin	Use getline(cin, str) to read input.
No empty check	Always check !str.empty() before str[0].
Unnecessary copy	Return the string directly (RVO optimises it).
Misusing cin	cin is a stream, not a string. Use >> or getline().

• Be careful with string literals and char literals. "s" vs 's'
• Don’t treat cin as if it was a std::string type variable.
• Always check for empty strings before accessing elements.
• Avoid unnecessary copies (let C++ optimise).
• Learn more about getline() function and enhance the understanding stream.