How to pass a cstring as a function parameter/argument

Question

I have a small program that prints out the capital form of each letter of a word, but I get the error signed/unsigned mismatch when I compile it because I'm passing a cstring as a normal string in this program. How do I pass it correctly so that I can still use text.length()? Here is the error that I get "Tester.cpp(22,23): warning C4018: '<': signed/unsigned mismatch". It's at for (int i = 0; i < text.length(); i++)

#include <iostream>
using namespace std;

string capitalizeFirstLetter(string text);

int main() {
    char sentence[100];
    for ( ; ; )
    {
        cin.getline(sentence, 100);
        if (sentence != "0")
            capitalizeFirstLetter(sentence);
    }

    return 0;
}

string capitalizeFirstLetter(string text) {

    for (int i = 0; i < text.length(); i++)
    {
        if (i == 0)
        {
            text[i] = toupper(text[i]);
        }
        if (text[i] == ' ')
        {
            ++i;
            text[i] = toupper(text[i]);
        }
    }
    cout << text;

    return text;
}

Answer 1

The simplest way to handle passing sentence as a string is to enclose it in a braced set, to provide direct initialization to the parameter std::string text eg..

    for ( ; ; )
    {
        std::cin.getline(sentence, 100);
        if (*sentence)
            capitalizeFirstLetter({sentence});
    }

This allows the character string sentence to be used as the Direct initialization to initialize std::string text in your capitalizeFirstLetter() function:

std::string capitalizeFirstLetter (std::string text) {

    for (size_t i = 0; i < text.length(); i++)
    {
        if (i == 0)
        {
            text[i] = toupper(text[i]);
        }
        if (text[i] == ' ')
        {
            ++i;
            text[i] = toupper(text[i]);
        }
    }
    std::cout << text;

    return text;
}

Your complete code, after reading Why is “using namespace std;” considered bad practice?, would then be:

#include <iostream>

std::string capitalizeFirstLetter (std::string text) {

    for (size_t i = 0; i < text.length(); i++)
    {
        if (i == 0)
        {
            text[i] = toupper(text[i]);
        }
        if (text[i] == ' ')
        {
            ++i;
            text[i] = toupper(text[i]);
        }
    }
    std::cout << text;

    return text;
}

int main (void) {
    
    char sentence[100];
    
    for ( ; ; )
    {
        std::cin.getline(sentence, 100);
        if (*sentence)
            capitalizeFirstLetter({sentence});
    }

    return 0;
}

(note: dereferencing sentence provides the first character which is then confirmed as something other than the nul-terminating character (ASCII 0))

A Better CapitalizeFirstLetter()

A slightly easier way to approach capitalization is to include <cctype> and an int to hold the last character read. Then the logic simply loops over each character and if the first character is an alpha-character, then capitalize it, otherwise only capitalize the letter when the current character is an alpha-character and the last character was whitespace, e.g.

std::string capitalizeFirstLetter (std::string text)
{
    int last = 0
    
    for (auto& c : text)
    {
        if (isalpha(c))
        {
            if (!i || isspace (last))
                c = toupper(c);
        }
        last = c;
    }
    std::cout << text;

    return text;
}

(note: the use of a range-based for loop above)

Either way works.

Answer 2

The error is not generating because of you passing a cstring as a normal string to the function but it is due to the fact that you are trying to compare c style string using != operator in the statement

if (sentence != "0")
    capitalizeFirstLetter(sentence);

try using strcmp() for that

Answer 3

Problems with your code :

1. if (sentence != "0") : illegal comparison. If you want to break on getting 0 as input then try using strcmp (include <cstring>) as if (strcmp(sentence, "0"). (Note that strcmp returns 0 when two strings are equal.) Or simply do if (!(sentence[0] == '0' and sentence[1] == 0)). Moreover this condition should be accompanied with else break; to prevent the for loop from running forever.

2. for (int i = 0; i < text.length(); i++) : generates warning because of comparison between signed and unsigned types. Change data-type of i to string::size_type to prevent the warning.

3. <string> (for std::string) and <cctype> (for std::toupper) were not included.

4. Thanks to @john for pointing this out. Your code has undefined behaviour if last character of a string is a space. Add a check if i is still less than text.length() or not before using text[i].

5. Another case of error is when an space is there after 0. Move getline to condition of for to fix this. Now there will be no need to input a 0 to terminate program. Moreover, I recommend using while loop for this instead of for.

6. You may also need to print a newline to separate sentences. Moreover, I would prefer printing the modified sentence in the main() function using the returned string from capitalizeFirstLetter.

7. It doesn't matter much in short (beginner-level) codes, but avoid acquiring the habit of putting using namespace std; on the top of every code you write. Refer this.

Fixed code :

#include <cctype>
#include <cstring>
#include <iostream>
#include <string>
using namespace std;

string capitalizeFirstLetter(string text);

int main() {
  char sentence[100];
  while (cin.getline(sentence, 100))
    cout << capitalizeFirstLetter(sentence) << '\n';
}

string capitalizeFirstLetter(string text) {
  for (string::size_type i = 0; i < text.length(); i++) {
    if (i == 0)
      text[i] = toupper(text[i]);
    if (text[i] == ' ')
      if (++i < text.length())
        text[i] = toupper(text[i]);
  }
  return text;
}

Sample Run :

Input :

hello world
foo bar

Output :

Hello World
Foo Bar

---

My Version (Requires C++20) :

#include <cctype>
#include <iostream>
#include <string>

auto capitalizeFirstLetter(std::string text) {
  for (bool newWord = true; auto &&i : text) {
    i = newWord ? std::toupper(i) : i;
    newWord = std::isspace(i);
  }
  return text;
}

int main() {
  std::string sentence;
  while (std::getline(std::cin, sentence))
    std::cout << capitalizeFirstLetter(sentence) << std::endl;
}

Sample Run

Answer 4

Several things bugging me here.

First off, don't use using namespace std, it's "ok" in this case, but don't get used to it, it can cause quite some trouble. See Why is “using namespace std;” considered bad practice?

Next thing is, just use std::string instead of cstrings here, it's easier to write and to read and doesn't produce any measurable performance loss or something. And it's harder to produce bugs this way. So just use

std::string sentence;

and

getline(std::cin, sentence);

And why do you handle the output inside the function that transforms your string? Just let the main print the transformed string. So your main could look like this:

int main() {
    std::string sentence;
    
    while(true)
    {
        getline(std::cin, sentence);
        auto capitalized = capitalizeFirstLetter(sentence);
        std::cout << capitalized;
    }

    return 0;
}

PS: the 'error' you get is a warning, because you compare int i with text.length() which is of type size_t aka unsigned int or unsigned long int.