Transform uppercase letters to lowercase and vice-versa using single parameter function (C++)

Question

I have the trans function which uses a single parameter, has to be void, and returns through c the opposite case of a letter from a word input in main.

Example:
input: dOgdoG
output: DoGDOg

The function does change the case, but i cant figure out a way to build the new word / replace the old one because i keep getting compiling errors regarding "const char" or "invalid conversions".

The following program gives error "invalid conversion from char to const char*

I only changed the type of the function for example purposes.

#include <iostream>
#include <cstring>
#include <fstream>
using namespace std;

char trans(char c)
{
    if(c >= 'a' && c <= 'z')
        return c-32;
    else
        if(c >= 'A' && c <= 'Z')
            return c+32;
}

int main()
{
    char s[101], s2[101] = "";
    cin >> s;
    int i;
    for(i=0; i<strlen(s); i++)
    {
        strncat(s2, trans(s[i]), 1);
    }
    cout<<s2;
    return 0;
}

EDIT: I changed from the char function to a void function and removed the body of the for.

void trans(char c)
{
    if(c >= 'a' && c <= 'z')
        c-=32;
    else
        if(c >= 'A' && c <= 'Z')
            c+=32;
}

int main()
{
    char s[101], s2[101] = "";
    cin >> s;
    int i;
    for(i=0; i<strlen(s); i++)
    {
        /// i dont know what to put here
    }
    cout<<s2;
    return 0;
}

Don't mess around with `strncat`; you **know** that `s2` is large enough to hold the result, because you wrote it that way. So just do it: `s2[i] = trans(s[i]);`. And don't forget the nul terminator; that comes after the last character, so looping up to `strlen(s)` won't reach it. — Pete Becker, Dec 01 '21 at 14:07
@ScottHunter -- maybe, but the question asks about "build the new word / replace the old one", and the code in the quesiton builds the new word. — Pete Becker, Dec 01 '21 at 14:10

Pete Becker · Accepted Answer · 2021-12-01T14:34:32.677

Don't reinvent the wheel. The standard library has functions to identify uppercase and lowercase letter, and to change case. Use them.

char trans(char ch) {
    unsigned char uch = ch; // unfortunately, character classification function require unsigned char
    if (std::isupper(uch))
        return std::tolower(uch);
    else
        return std::toupper(uch);
}

You might be inclined to change that else branch to else if (std::islower(uch) return std::toupper(uch); else return uch;, but that's not necessary; std::toupper only changes lowercase letters to uppercase, so it won't affect characters that aren't lowercase.

Then, when you call it, just copy the result:

int i = 0;
for ( ; i < strlen(s); ++i)
    s2[i] = tran(s[i]);
s2[i] = '\0';

EDIT:

Since there seems to be a requirement to do things the hard way, let's change trans to match:

void trans(char& ch) {
    unsigned char uch = ch; // unfortunately, character classification function require unsigned char
    if (std::isupper(uch))
        ch = std::tolower(uch);
    else
        ch = std::toupper(uch);
}

And now, you can just apply it in place:

for (int i = 0; i < strlen(s); ++i)
    trans(s[i]);

I called this "the hard way" because with the original version of trans you can use it directly to modify the original string:

for (int i = 0; i < strlen(s); ++i)
    s[i] = trans(s[i]);

and you can use it to copy the string:

for (int i = 0; i < strlen(s); ++i)
    s2[i] = trans(s[i]);
// don't forget the terminating nul

With pass by reference, you can only modify in place; copying requires an additional step:

strcpy(s2, s1);
for (int i = 0; i < strlen(s); ++i)
    trans(s2[i]);

not trying to reinvent anything, i know those functions, but i need to do it the way i did, plus, i did mention the char function was for example purposes only, as i need to use a void function which is what i dont understand how to do I will edit the question to make it more obvious — MickeyMoise, Dec 01 '21 at 14:22

score 1 · Answer 2 · answered Dec 01 '21 at 14:07

1

strncat takes 2 strings and a number as parameters; your second argument is a char, not a string.

answered Dec 01 '21 at 14:07

Scott Hunter

48,888
12
60
101

Ch3steR · Answer 3 · 2021-12-01T14:28:56.420

You can use std::transform with string utility functions like std::isupper, std::toupper and similarly for lowercase. Since, the question is tagged c++ std::string is preferred over const char*¹ for strings.

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

char trans(unsigned char c){

   if (std::isupper(c)) 
      return std::tolower(c);
   else if (std::islower(c)) 
      return std::toupper(c);
   else
      return c;
}

int main(){
  std::string s = "dOgdoG12";
  std::string out;
  out.resize(s.length());
  
  std::transform(s.begin(), s.end(), out.begin(), trans);
  std::cout << out; // DoGDOg12
}

Demo

_{1. SO post on char* vs std::string}

Transform uppercase letters to lowercase and vice-versa using single parameter function (C++)

3 Answers3