isalpha(<mychar>) == true evaluates to false? - c++

string temp is equal to "ZERO:\t.WORD\t1" from my debugger. (the first line of my file)
string temp = RemoveWhiteSpace(data);
int i = 0;
if ( temp.length() > 0 && isalpha(temp[0]) )
cout << "without true worked" << endl;
if ( temp.length() > 0 && isalpha(temp[0]) == true )
cout << "with true worked" << endl;
This is my code to check if first character of temp is a a-z,A-Z. The first if statement will evaluate to true and the 2nd to false. WHY?!?!?! I have tried this even without the "temp.length() > 0 &&" and it still evaluates false. It just hates the "== true". The only thing I can think of is that isalpha() returns != 0 and true == 1. Then, you could get isalpha() == 2 != 1. But, I have no idea if C++ is that ... weird.
BTW, I dont need to know that the "== true" is logically pointless. I know.
output was
without true worked
Compiled with CodeBlock using GNU GCC on Ubuntu 9.10 (if this matters any)

The is* functions are only guaranteed to return a non-zero value if true, NOT necessarily a 1. A typical implementation is table based, with one entry in the table for each character value, and a set of bits defining which bit means what. The is* function will just AND the right bitmask with the table value, and return that, which will only be the value 1 for whichever type happens to have been given bit position 0.
E.g.:
#define __digit 1
#define __lower 2
#define __upper 4
extern int __type_table[];
int isdigit(int c) {
return __type_table[c+1] & __digit;
}
int isalpha(int c) {
return __type_table[c+1] & (__lower | __upper);
}
int islower(int c) {
return __type_table[c+1] & __lower;
}
int isupper(int c) {
return __type_table[c+1] & __upper;
}
Where __type_table is defined as something like int __type_table[UINT_MAX+1]; and would be initialized so (for example) __type_table['0'+1] == __digit and __type_table['A'+1] == __upper.
In case you care, the '+1' part is to leave a spot at the beginning of the table for EOF (which is typically defined as -1).

isalpha doesn't return true, it returns non-zero. This is quite common for API designed for C.
Note that in the expression isalpha(ch) == true, the subexpression true is promoted to type int with value 1.

Well, the documentation suggests that it returns either zero or non-zero, not necessarily just false or true. So you'd better check (isalpha(temp[0]) != 0).

Do not isalpha(ch) == true, but !!isalpha(ch) == true

Related

what does if(!(x%3)) mean?

I am attempting to solve a hw problem in which I need to write down what the program will output. I am stuck however, on the syntax "if ( !(i%3)). What does that really mean? Does it mean that the program is checking for any i that is divisible by three? aka, is the if statement only runs if i is divisible by three?
int main () {
for (int i=0; i<10; (i<3?i++;i+=2)) {
if (!(i%3)) {
continue;
}
else if (i%7 ==0) {
break;
}
cout << i<< endl;
}
Does it mean that the program is checking for any i that is divisible by three? aka, is the if statement only runs if i is divisible by three?
Correct. The longer version of that check would be
if (i % 3 == 0)
continue;
The most common use case for such branching is probably FizzBuzz.
İt means if i is not(!) divisible by 3 continue.
For example if i is 3,6,9 it won't continue otherwise it will continue.
if (x) where x is int implicitly compared with zero. I.e. if (x) equals to if (x != 0). ! is negation. So if (!x) equals to if (x == 0). And the last step is if (!(i%3)) equals to if ((i%3) == 0) what is the same with check, that i deivisible by 3
The if() statement is false only if the result inside the parentheses is 0 (false). Take a look at your program:
i%3 may return 0 (false), 1 (true), or 2 (true)
The negation operator ! changes the result of the operation (i%3). So, if the i is divisible with 3 the statement will return 0 (false). Being negate, ! it will result in True. Otherwise the result of (i%3) will be true and with the operator ! the result of the hole statement will be false. Basically this code is checking if the value of i is divisible with 3.
Other options will be:
if (0==i%3)
{
/*code*/
}
Your code can be simplified as below
int main() {
for (int i=0; i<10;)
{
if (i % 3 == 0) {
continue;
}
else if (i % 7 == 0) {
break;
}
cout << i << endl;
i = i<3 ? i+1 : i+2;
}
}
When you write a integer variable like i as a condition, what happens is that if i==0 then the result of the condition is false, otherwise it would be true.
Let's check it out in your program, if(!(x%3)), let's name condition= !(x%3), when this condition is true? when x%3 == 0, note that the negation operator ! is behind x%3, so in this case the condition would be equal to true, more formally the condition is equal to :
if(x%3==0)
these kinds of conditions are common, check this example out :
int t = 10;
while(t--){
cout<<t<<endl;
}
The above condition i.e if(!(i%3)) will true when " i is not disvisable by 3"
Hope this helps.
In java and other languages there is a special type to represent booleans and evaluate expressions; in c and its variants there is no such thing, instead an expression is considered "true" if -taken as a integer number- is equal to 0; false for every other value. (Fun fact: this is why you usually end the code by return 0)
So if(x%3) in c is equivalent to if(x%3==0) in other languages. That said, if(x%3) execute the if body when the remainder of x/3 is 0, that is when x is a multiple of 3.
In your code you have the ! before, that -as you may know- "inverts" the expression. That means that if(!(x%3)) can be read as "If the remainder of the integer division of x by 3 is not 0", or alternatively: "If x is not a multiple of 3".
So, basically, you saw it right.
Hope I helped!

C++ multiple operators: assign A or B not equal to C

I am currently learning from an SDL2/OpenGL2 example code, for ImGui. Then, I ran into a line (and a few more alike) as shown below. I believe this part binds SDL mouse events to IMGUI API. However, I do not quite understand how it works.
io.MouseDown[0] = g_MousePressed[0] || (mouseMask & SDL_BUTTON(SDL_BUTTON_LEFT)) != 0
where,
ImGuiIO& io = ImGui::GetIO();
bool mouseDown[5]; // a member of ImGuiIO struct
static bool g_MousePressed[3] = { false, false, false };
Uint32 mouseMask = SDL_GetMouseState(&mx, &my);
(I hope above is enough information...)
What makes me the most confused is the the last part, not equal to 0. I could understand it, if it was a simple assignment of the result of an And and an Or operations. However, there is not equal to zero following at the end and I have not seen this before. I would like to get some help to understand this code please.
expression != 0
is a boolean expression and thus evaluates to either true or false which can be converted to integer values of 0 or 1.
#include <iostream>
int main() {
constexpr size_t SDL_BUTTON = 5;
for (size_t mouseMask = 0; mouseMask < 16; ++mouseMask) {
std::cout << mouseMask << ' '
<< (mouseMask & SDL_BUTTON) << ' '
<< ((mouseMask & SDL_BUTTON) != 0) << '\n';
}
}
Live demo: http://ideone.com/jcmosg
Since || is the logical or operator, we are performing a logical comparison that tests whether io.MouseDown != 0 or (mouseMask & SDL_BUTTON(SDL_BUTTON_LEFT)) != 0 and yields a boolean value (true or false) promoted to whatever type io.mouseDown[0] is.
The code could actually have been written as:
io.MouseDown[0] = g_MousePressed[0] || (mouseMask & SDL_BUTTON(SDL_BUTTON_LEFT))
or
const bool wasPressed = g_mousePressed[0];
const bool newPress = mouseMask & SDL_BUTTON(SDL_BUTTON_LEFT);
const either = (wasPressed == true) || (newPress == true);
io.MouseDown[0] = either;
or
if (g_mousePressed[0])
io.MouseDown[0] = 1;
else if (mouseMask & SDL_BUTTON(SDL_BUTTON_LEFT))
io.MouseDown[0] = 1;
else
io.MouseDown[0] = 0;
See http://ideone.com/TAadn2
If you are intending to learn, find yourself a good sandbox (an empty project file or an online ide like ideone.com etc) and teach yourself to experiment with pieces of code you don't immediately understand.
An expression a = b || c!=0 means a = (b!=0) || (c!=0). In boolean logic, b and b!=0 are equivalent. That's what I think the most confusing part. Once this is understood, there should be no problem.
Note that this expression should not be confused with a=b|c!=0, where | is a binary operation called "bit-wise or", as opposed to the logical operation ||, which is the logical "or". When doing b|c!=0, c!=0 is calculated first to yield a logical value 0 or 1, then b|0 or b|1 is calculated to do nothing (first case) or reset the last bit of the binary code of b to 1 (second case). Finally, that result is assigned to a. In this case, b and b!=0 are not equivalent, because the bit-wise or | is used instead of the logical or ||.
Similarly, & is the "bit-wise and" operator, while && is the logical "and". These two should not be confused either.
Notice that != has a higher precedence than ||, so the whole expression is indeed a simple assignment of the result of an OR.
The !=0 part is a way to turn the result of applying a bitmask into bool, as #AlekDepler said. Funny thing is its pretty much redundant (if mouseMask is of built-in integral type) as implicit conversion from say int to bool works exactly like !=0.

Integer to Boolean strange syntax [duplicate]

This question already has an answer here:
The Definitive C++ Book Guide and List
(1 answer)
Closed 7 years ago.
I'm less than a year into C++ development (focused on other languages prior to this) and I'm looking at a guy's code who's been doing this for two decades. I've never seen this syntax before and hopefully someone can be of some help.
bool b; // There exists a Boolean variable.
int i; // There exists an integer variable.
sscanf(value, "%d", &i); // The int is assigned from a scan.
b = (i != 0); // I have never seen this syntax before.
I get that the boolean is being assigned from the int that was just scanned, but I don't get the (* != 0) aspects of what's going on. Could someone explain why this person who knows the language much better than I is doing syntax like this?
Have a read here:
http://en.cppreference.com/w/cpp/language/operator_comparison
The result of operator != is a bool. So the person is saying "compare the value in i with 0". If 'i' is not equal to 0, then the '!=' returns true.
So in effect the value in b is "true if 'i' is anything but zero"
EDIT: In response to the OP's comment on this, yes you could have a similar situation if you used any other operator which returns bool. Of course when used with an int type, the != means negative numbers evaluate to true. If > 0 were used then both 0 and negative numbers would evaluate to false.
The expression (i != 0) evaluates to a boolean value, true if the expression is true (i.e. if i is non-zero) and false otherwise.
This value is then assigned to b.
You'd get the same result from b = i;, if you prefer brevity to explicitness, due to the standard boolean conversion from numeric types which gives false for zero and true for non-zero.
Or b = (i != 0) ? true : false; if you like extraneous verbosity.
(i != 0) is an expression that evaluates to true or false. Hence, b gets the value of true/false depending on the value of i.
This is fairly fundamental syntax. The != operator performs a "not equal to" comparison.
You may be being confused by the shorthand of initialising a bool directly from the result of a comparison operator, but the syntax itself is not esoteric.
The program is essentially equivalent to:
bool b;
int i;
sscanf(value, "%d", &i);
if (i != 0)
b = true;
else
b = false;
The key is that i != 0 is itself an expression that evaluates to true or false, not some magic that may only be used in an if statement.
Basically, if the condition (i not_equal_to 0 ) is satisfied, b gets the value "true". Else b gets the value "false".
Here, "i != 0" is a boolean expression that will be true if "i" is non-zero and false if it is zero.
All that is happening here is the result of that expression is being assigned to a variable.
You could also do things like...
boolean canDrinkAlcohol = (person.age() >= 18 && person.country.equals("UK") || person.age() >= 21 && person.county.equals("US"));
...
if(canDrinkAlcohol) {
...
}
or something

c++ need help to understand

So I have this piece of code.
I understand all the things beside the fact that when does the loop actually take place again. I mean what is meant by the e(!valid) statement. Does it refer to its numeric value or what? Can somebody please explain this to me. Consider all required variable declared. And ignore uppercase.
The code is:
do
{
valid=1;
gotoxy(22,7);
gets(emailid);
int flag=0;
for (int i = 0; emailid[i] != '\0'; i++)
if (emailid[i] == '#')
flag++;
If(!flag)
{
valid = 0;
cout << "not a valid id. Try again";
getch();
}
} while(!valid);
So mainly I want to know that it is working, with emphasis on what does !valid and !fail mean.
From what I could get, it has to do with its numeric values but I am still confused.
To answer the question:
} while (!valid);
means: treat the integer behind valid as a boolean. (assuming it is an integer, as it was given a value of 1)
i == 0 -> false
i != 0 -> true
!valid:
valid == 0 -> true
valid != 0 -> false
In C++ the value 0 is considered "false" and any other integer is considered "true". In this case while valid is equal to 0 the loop runs.
Numeric value can be promoted to bool type this way:
Zero means false and other values mean true. So !valid will return true only if valid == 0.
What it means is your do while() loop will repeat itself until valid equals value other than 0.

c++ what is the correct syntax here?

what is the correct syntax for checking a varable value and then setting a varable in the same condition also checking that new set varables var, all in one if statement?
so basically something like
if(this->somevar > 0 && this->newvar = this->GetNewVar(this->somevar),this->newvar > 0)
i know that is not the correct syntax or at least its not working for me anyway, hence the topic, i am using that as an example, so if this->somevar is null or 0, i don't want it to execute the next condition which is && this->newvar = this->GetNewVar(this->somevar,this->newvar but instead skip the statement and ignore that part.
what is the correct syntax for something like this?
&& is an operator with short circuit evaluation, right part is not executed if left part is true.
But why don't you simply write:
if(this->somevar > 0)
{
this->newvar = this->GetNewVar(this->somevar);
if (this->newvar > 0)
{
...
This will certainly makes things clearer ...
the logical AND && operator is short-circuited if this->somevar evaluates to zero, meaning the rest of your if expression would not be evaluated in that situation
The expression after the comma is not necessary. Also, there is one thing missing, parentheses arround the assignment:
if(this->somevar > 0 && (this->newvar = this->GetNewVar(this->somevar)) > 0)
Without the parentheses you may end up setting this->newvar to the value of the boolean expression
this->GetNewVar(this->somevar),this->newvar > 0, which will be evaluated to a boolean result (true/false which, in turn, may be converted to 0 or 1 or -1 depending on the compiler, when cast to the type of this->newvar).
I think only the bit after the comma is evaluated for the if condition. The expression on the left of the comma is ignored.
int main() {
if( false, true) { cout << " got to if( false, true ) "; }
if ( true, false ) { cout << "got to if( true, false ) "; }
}
to answer your question, you can put anything on the left of the comma and do whatever you like, as long as the expression you want to evaluate is the last expression in the list.
so if ( exp1, exp2, exp3 , exp4 ) dowhatever(); only gets run if exp4 is true. You should really run exp1 to exp3 outside the if condition for readability.