incorrect character recognition in string array C++ - c++

Question

i have to write a program that asks user to input characters, store them in a string array, then read the last character of the string and then determine whether it's an integer or a letter or neither of those.
#include<iostream>
using namespace std;
int main()
{
string str;
cout << "Enter a line of string: ";
getline(cin, str);
char i = str.length()-1;
cout<<str[i];
cout<<endl;
if ((str[i]>='a' && str[i]<='z') || (str[i]>='A' && str[i]<='Z')) cout << "it's a letter";
else if (str[i] >=0) cout << "it's a number";
else cout << "it is something else";
return 0;
}
i came up with this code, it recognizes both letters and integers, but i have 2 problems with it:
1) if the last character of string is a symbol, for example an '*', then the program says it's an integer, although it's not.
how could i correct this?
2) also, if i type in an additional condition for the following statement in order to recognize integers from 0 to 9, the code fails:
else if ((str[i] >=0) && (str[i] <=9)) cout << "it's a number";
it works only if it's stated as (str[i] >=0)
why is it so?

Answer 1

Because numbers in string are not integer numbers, but font symbols/characters. Think about what is 'a', if the computer operates only with 1 and 0 values, it doesn't have any "a" symbol in HW. So the letter "a" must be encoded into some 01100001 pattern (for example 0x61 is 'a' in ASCII and UTF8 encoding). In similar way the digits are also encoded as particular bit patterns, for example 0x30 to 0x39 in ASCII and UTF8 for digits 0 to 9.
So your test has to be if ((str[i] >= '0') && ... (notice the digit zero is in apostrophes, telling compiler to use the character encoding, and will compile to 0x30 or in binary 00110000).
You can also write if ((str[i] >= 48) && .. and it will compile to identical machine code, because for compiler there is no difference between 48, 0x30 or '0'. But for humans reading your source later and trying understand what you were trying to achieve, the '0' is probably best fit to explain your intent)
Also notice the encoding which is used (as defined by your compiler and source) may use different values to encode particular symbol, causing the string processing to be sometimes quite tricky business, for example in UTF8 the common English characters are encoded identically with the 7 bit ASCII encoding (which is trivial old encoding, and you should start by learning a bit about it) and each letter fits into single byte (8 bits), but the accented and extra characters are encoded as series of bytes (two to five), so for example trivial things like "count how many characters are in string" turn into complex task requiring rich tables with definition of unicode characters and their category/function, to correctly count characters as in human way (simple Arabic text consisting of 4 letters may require for example 12 bytes of memory, so simple strlen will return 12 then). ... if you need these things for unicode encoding, you better use some library, than implementing it yourself.
Your code will work well for simple ASCII-like strings only, which is enough to practice stuff like this, just remember everything in the computer is encoded as 1 and 0 (sounds, characters, pixel/vector graphics, ... everything, at some point, down below, under many layers of abstraction and utilities, there is some point where the information is encoded as series of bit values 1 or 0, and nothing else), so when you are trying to process some data, it may be helpful to learn about how the thing is actually encoded in the HW and how much of that is abstracted from you by your programming language/library+OS API.
You can also check in debugger in memory view, what values are used to encode your string, and how it looks in terms of byte values.

Answer 2

Your condition in 'else if' doesn't seem to satisfy your requirement.
Try something like:
if ((str[i]>='a' && str[i]<='z') || (str[i]>='A' && str[i]<='Z'))
cout << "it's a letter";
else if (str[i] >= '0' && str[i] <= '9')
cout << "it's a number";
else
cout << "it is something else";