Iterating through a string, and switch statements: C++ - c++

Question

I was writing some code, and came across some trouble. I wanted to write a function that checks if a string has any vowels, and tried to do it through a for loop with a switch statement inside. Apparently, it doesn't work and never returns true for some reason.
bool scanStr(string userInp) {
for (int i = 0; i < userInp.size(); i++) {
switch (userInp[i])
{
case 'a':
case 'A':
case 'e':
case 'E':
case 'i':
case 'I':
case 'o':
case 'O':
case 'u':
case 'U':
case 'y':
case 'Y':
return true;
break;
default:
return false;
}
}
}
I tried just testing if the program was actually iterating through the string, and it was, so I don't understand why in the function, it always returns false?
int main() {
string userInp;
string pigLatin;
cout << "Please enter a string to convert to pig Latin: " << endl;
cin >> userInp;
cout << endl;
// tests
for (int i = 0; i < userInp.size(); i++) { //checking if it actually iterates
cout << userInp[i];
}
cout << endl;
if (scanStr(userInp))
cout << "it has a vowel" << endl;
else
cout << "no vowel" << endl;
system("pause");
return 0;
}
At first I thought that it was because the loop kept on continuing even though there was a break statement after the last case, but I'm not entirely sure if that is the reason.
Any ideas?

Answer 1

I would suggest that you extract the logic of vowel testing into its own function:
bool is_vowel(char x)
{
switch (x)
{
case 'a':
case 'A':
case 'e':
case 'E':
case 'i':
case 'I':
case 'o':
case 'O':
case 'u':
case 'U':
case 'y':
case 'Y':
return true;
default:
return false;
}
}
Then you can use a standard algorithm instead of a loop:
#include <algorithm>
#include <string>
bool contains_vowel(const std::string& str)
{
return std::any_of(str.begin(), str.end(), is_vowel);
}
(I renamed scanStr to contains_vowel because that name is much more descriptive.)

Answer 2

Drop this lines from your function:
default:
return false;
They make your function return false on the first non-vowel it encounters.
You only want to return false if you reach the end of the loop and haven't returned true yet.
bool scanStr(string userInp)
{
for (int i = 0; i < userInp.size(); i++)
{
switch (userInp[i])
{
case 'a':
case 'A':
case 'e':
case 'E':
case 'i':
case 'I':
case 'o':
case 'O':
case 'u':
case 'U':
case 'y':
case 'Y':
return true;
}
}
return false;
}
A better approach in modern C++ would be:
bool scanStr(const std::string& userInp)
{
for (const auto c : userInp)
{
switch (c)
{
case 'a':
case 'A':
case 'e':
case 'E':
case 'i':
case 'I':
case 'o':
case 'O':
case 'u':
case 'U':
case 'y':
case 'Y':
return true;
}
}
return false;
}
But if you don't know what it means, don't worry about it now, your book or tutorial will explain soon enough.

Answer 3

Problem was, if any character wasn't vowel, that function imediatelly returned false. Also use const &. const allows you to pass const strings and reference saves some time, because C++ doesn't have to copy whole string.
bool scanStr(const string & userInp) {
for (int i = 0; i < userInp.size(); i++) {
switch (userInp[i])
{
case 'a':
case 'A':
case 'e':
case 'E':
case 'i':
case 'I':
case 'o':
case 'O':
case 'u':
case 'U':
case 'y':
case 'Y':
return true;
break;
}
}
return false;
}

Answer 4

First the compiler will turn the switch case into a lookup table, then auto will be decided as a data-type by the compiler based on the value assigned (in this case it's char).
Or just give it to your compiler, it knows how to do it.
bool scanStr(string userInp)
{
for(auto c : userInp)
{
switch (c)
{
case 'a': case 'A':
case 'e': case 'E':
case 'i': case 'I':
case 'o': case 'O':
case 'u': case 'U':
case 'y': case 'Y':
return true;
}
}
return false;
}