#include<iostream>
using namespace std;
main()
{
int test =3;
while(test--)
{
cout<<test;
}
}
The above code shows output 210.My question is that why it stop at 0??Why it can't go beyond 0?
In c and c++ any non zero is evaluated to true and zero is evaluated to false. Thats why its stop at 0. The fact is true for for loop too.
For this reason may sometimes found -
while(1){
//do something
if(some_condition) break;
}
It means its a infinite loop - runs always.
"The above code shows output 210.My question is that why it stop at 0??Why it can't go beyond 0?"
Because a int value of 0 evaluates to false in a boolean expression, and the while() loop stops at a false in the condition.
you set the condition in while loop to test--, and it will be executed 3 times. When test is 1, it's equal to while(1) or while(true) and it continue while loop.
When test is 0, it's equal to while(0) or while(false) and it stops while loop ..
Why 210? Because you print it whit no "new line" between prints.
Related
Sorry in advance if the question sounds naive. I am writing a simple bool function to check if all digits of a number are same. The following works, however the one after, gives me an error of
17:1: warning: control reaches end of non-void function [-Wreturn-type]
What I am doing wrong with the second one?
Working:
# include <iostream>
# include <string>
using namespace std;
bool samedigits (int number){
bool result = true;
std::string snumber = to_string (number);
for (int i=0; i < snumber.size(); i++){
if (snumber[i] != snumber[0]){
result = result and false;
break;
}
}
return result;
}
int main()
{
cout << samedigits (666);
return 0;
}
Non working:
# include <iostream>
# include <string>
using namespace std;
bool samedigits (int number){
std::string snumber = to_string (number);
for (int i=0; i < snumber.size(); i++){
if (snumber[i] != snumber[0]){
return false;
break;
}
return true;
}
}
int main()
{
cout << samedigits (666);
return 0;
}
Your algorithm is incorrect, you are only checking if the first character is equal to itself, and returning true for every input. Instead, you should move the return true outside the loop, so that you check all the characters first.
Unfortunately, the warning is a false positive. The compiler fails to realize that std::to_string is guaranteed to return a non-empty string when passed an int. This means that the for loop body will be entered, and the function will return a value.
The compiler is right. There is a code path in your second snippet that won't return.
for (int i=0; i < snumber.size(); i++){
Here, the std::string size function can return 0 according to its contract. When it does happen, then your function won't enter the loop. After that, you exit the function without returning.
The second version of your function (combined with some information obtained via comments) indicates a misunderstanding of what return does. The second version would work (here is the misunderstanding) if a return statement were to simply store the indicated value for use when the function eventually ends. However, this is not how return works. A return statement stores the indicated value for use when the function ends and immediately ends the function. In the second version of your function, the for statement might as well be an if and the break is never executed as it comes right after a return.
To demonstrate, let's do a code walk-through for a call to samedigits(123).
bool samedigits (int number){
As we enter the function, number is set to 123.
std::string snumber = to_string (number);
The string snumber is set to "123".
for (int i=0; i < snumber.size(); i++){
The loop initializes by setting i to 0 then checks if 0 < 3 (the size of snumber is 3). This is true, so we enter the loop. Note that the result of entering the loop depends only on snumber not being empty.
if (snumber[i] != snumber[0]){
We check to see if snumber[0] does not equal snumber[0]. This is a bit trivial, but the computer is willing to do it. The result, of course, is false (independent of what the input was – this part might be more interesting if the loop started at 1 instead of 0). So skip to the statement after the if.
return true;
The function immediately ends, returning true.
And that's it. There is no second iteration of the loop. No other code is executed during this function call. Since to_string never returns an empty string, the second version of your function is functionally equivalent to the following:
bool samedigits (int /*number*/){
return true;
// Execution ends with the return statement, so nothing after
// this comment ever executes.
std::cout << "This is dead code. It will never execute.\n";
std::cout << "You will never see this output.\n";
}
To fix the second version, you need to return inside the loop only when the if condition is true. Move return true; to be after the loop so that the loop can iterate multiple times. You do not want to end the function and tell the caller that all the digits are the same (which is what return true; does) until after you've checked all the digits. (If your loop finds a mismatch, execution will reach the return false; inside the loop. At that point, the function ends, so code after the loop has no effect on that function call.)
A smaller (cosmetic) fix is to get rid of the break. Suppose the loop did iterate enough times to find a mismatch. Execution would go into the if statement body and encounter return false. At that point, not only is the loop stopped, but the function as a whole ends, before the break statement is seen. The break statement is dead code, meaning code that can never be executed. In order to get to the break, execution has to go through a return. Execution may arrive at a return statement, but it never goes through one.
Also, be sure to thank your compiler for finding this error, as it does point to a bug in your code. It's not the exact bug the compiler reported, but then again, compilers are not exactly the best thinkers in the world. :)
Can someone explain the purpose of having two different types of while loops? I am new to programming. Also supply example situations with the proper while loop if possible.
I understand how to use a while loop. This is what I made:
bool myBool = true;
int i = 0;
while (myBool) {
if (i > 10) {
myBool = false;
}
i = i + 1;
}
A while loop will only execute when the boolean condition is true.
while (true) {
// INSERT CODE HERE
std::cout << "boolean condition is true - inside my while loop";
}
A do while whill check the boolean condition after the loop executes once.
do {
// INSERT CODE HERE
std::cout << "inside my while loop regardless of boolean condition";
} while (true);
Explicitly: the do while loop is guaranteed to execute at least once, whereas the while loop is not guaranteed to execute at all.
Similarly,
while (false) {
// INSERT CODE HERE
std::cout << "this will never execute";
}
will never execute and
do {
// INSERT CODE HERE
std::cout << "this will execute only once";
} while (false);
will execute once.
The do while loops are control flow statements, they execute a block of code at least once and then the iteration of loops depends on the condition which is checked at the bottom of the loop, They are best to use when you want at least once the loop to be executed, for ex
#include <stdio.h>
int main () {
int c = 50;
/* The do will be executed */
do {
printf("value of c: %d\n", c);
c = c + 1;
}while( c < 20 );//It will depend on the condition
printf("any string");
return 0;
}
Here is a Flow diagram of do while loop
Simple answer is while loop will execute only if condition inside of while statement is true.
do while loop will execute once regardless of the while statement condition.
#include <iostream>
using namespace std;
int main(int argc, char *argv[]){
int i = 1;
while( i < 1){ // this loop will never execute as 1 is not smaller then 1
i++; // if the loop was working we would get print 2 here
cout << i << endl;
}
cout << i << endl; // this one will print outside of loop value 1
do{
i++; // increase i to 2
cout << i << endl; // this will print 2
} while (i < 1); // This loop will execute at least once and as the condition of 2 is not smaller then 1 it will exit after one execution
return 0;
}
The difference between while and do-while is that in
while (<condition>)
{
//statements
}
we can control whether to enter the loop by using the test condition.
Whereas in
do
{
//statements
} while (<condition>);
the code has to enter the loop at least once before it can exit by using the condition.
So if we want to enter the loop at least once we should use do-while whereas if we want to test and decide whether to enter the loop or not, we have to use while.
To explicitly answer your first question:
Why does C++ have different kinds of loops? -> Legacy. Other languages (in particular, C) before C++ had this feature, so C++ chose to have it.
Why did other languages have it? -> This gets muddy, but a good explanation is that early languages often did not have optimizing compilers, so your code mapped quite directly to machine code. Providing various loop syntaxes allowed programmers to write structured code that still generates good machine code for their particular case.
In practice, it is rare to see a true do {} while () loop. This may be because for (or range-based for) and while () {} have strictly greater capabilities than do {} while (): An unconditional first loop iteration is possible with every loop, but the reverse is not true. In the very common case of iterating over a (possibly empty) sequence, having a guaranteed loop body execution like do {} while () is actually wrong.
There are plenty of answers with examples and explanations about the loops, so I won't bother repeating this here. I will add though that the most that I personally have seen do {} while () used is, ironically, not for looping but for this.
do-while loop performs the task before the condition in while(). It is for situations when you are trying to prompt the same thing for every wrong action (i.e., user authentication, wrong menu entry). On the other hand, a simple while loop performs till a condition is true (Note: In most cases people use for loops instead of while to define counter, initialize, set condition and increment/decrement - all in the same line).
I tried testing the following code and found that the loop is never executed :
int i=0;
for(;i++;cout<<i)
{
if(i==5)
break;
}
I read the following post about the value returned by cout from the following post :
What's the difference between cout<<cout and cout<<&cout in c++?
But, I am unable to figure out why. Can someone help me with this.
int i = 0;
for (; i++; cout << i)
At the 1st loop, i++ is evaluated as 0 before increment happens and thus terminates the loop.
The first time the loop exit condition (i++) is checked, i's value is 0 (i.e. false). Hence it never enters the loop.
i++ is post increment. So i becomes 1 but the value which is checked in the loop exit condition is the value before increment - i.e. 0.
So I am currently learning how to code using C++. I came across the following code below.
// =======================
// Lesson 2.4.3 - Do while
// =======================
#include<iostream>
using namespace std;
int main()
{
bool condition = false;
do
{
cout << "Enter a 0 to quit or 1 to continue: ";
cin >> condition;
}
while (condition);
}
Why is it that C++ automatically knows that 0 breaks the loop and that 1 continues the loop? Is it to do with a command knowing that 0 = false and that anything above is true?
Thanks to those who can help.
That's just how boolean logic works. 0 is false, anything non-0 is true.
The variable condition has type bool, so it's values can be true or false. When it's false the loop terminates. On input and output for a bool, 0 is false and 1 is true.
It's because while (0) evaluates to false therefore terminating the loop.
A bool variable can have one two conditions i.e true or false. 0 is considered as false and any value other than 0 is considered as true. So the condition you have written is (when condition=true.
while(!0) // body opf while will execute
and when the condition=false it, c++ will interpret it like this
while(0) // and body of do while will not execute.
because after the input is read the condition is checked while (condition);
while (condition = true);
the first code gets set to the above by default
This means the code in the body of the do while loop will loop while the condition value is true (1)
thanks for reading this.
I am writing a code to read a big data file. And I try to use a while loop to read it one piece at a time.
But when I write
while(TimeStep++)
it will exit at the first loop.
if I write,
while(TimeStep+=1)
it will be just fine.
Also, if I initialize
int TimeStep=-1;
it will exit at the first loop. But if I initialize
int TimeStep=0;
it will be fine. The magic of while() confuse me. Please help me understand while loop.
Here is all my code.
//get diffusion of all the particles in the 256Coordinate.txt file and diffusion of a single particle.
using namespace std;
typedef vector<double> vec;
int ReadStructure(vec & Coordinate,int size,ifstream & TrajectoryFile){
double a;
for(int i=0;i<size*3;i++){
if(!(TrajectoryFile.eof())){
TrajectoryFile>>a;
Coordinate[i]=a;
}
}
//cout<<Coordinate[1]<<endl;
if(TrajectoryFile.eof()){
return 1;
} else {
return 0;
}
}
int main(){
int ContinueFlag=0,i,j,k;
double a,b,c;
vec Coordinate;
string filename= ("../256Coordinate.txt"); // a file that contains 256*5000*3 numbers
int size=256;
Coordinate.resize(size*3);
int TimeStep=0;
ifstream TrajectoryFile(filename.c_str());//open the .txt file and begin the read data
//TrajectoryFile>>a;
//cout<<a<<endl;
while(TimeStep+=1){//keep looping untils breaks.
ContinueFlag=ReadStructure(Coordinate,size,TrajectoryFile);//read the .txt file and store the values in the vector Coordinate[3*256]. Read 3
*256 numbers at a time.
// cout<<"ContinueFlag= "<<ContinueFlag<<endl;
if(ContinueFlag==1) break;//if we reach the end of the file, exit.
// cout<<Coordinate[1]<<endl;
}
cout<<"total number of timesteps= "<<TimeStep-1<<endl;
}
the body of while loop will execute when the loop condition under
while(loop condition)
is true.
So if you set TimeStep =0 to start with. It will test whether TimeStep ==0 before executing the while loop. Any non-zero value is treated as True. If it is 0, loop body will not execute.
If you initialize as int TimeStep=-1;, TimeStep+=1 will set TimeStep =0, which is equivalent to false, so loop body will not execute.
If you do not know the loop termination condition beforehand, simply use
while (true)
is better than using such a TimeStep variable.
Try:
while(true){
ContinueFlag=ReadStructure(Coordinate,size,TrajectoryFile);
if(ContinueFlag==1)
break;
}
In C++ the integer value 0 is False, any other value including negative integer is True. While loop exits when false.
I think your main problem is not understanding the while loop, it's understanding the increment operator ++.
Let's work with an example:
int x = 5;
int y = x++;
Here, x will have a value of 6 (because you made ++), but which value will y have? Actually, it will be 5. This is a so-called 'postincrement' operator: see, you assign first, and increment later.
If you wrote this
int x = 5;
int y = (x += 1);
Then you would have x = 6 as before, but this time y = 6 also, so you first increment x and only then assign it to y.
This should make your while loop misunderstanding go away:
int TimeStep = 0;
while(TimeStep++)
Here, TimeStep will get the value of 1, but only after it was used by while to test for exit, but while will see the old value (as y in the example above), and the old value is 0, so while exits immediately.
int TimeStep = 0;
while(TimeStep+=1)
In this case the loop goes on because you first increment the TimeStep and then let while test if it's nonzero.
I would really suggest you write a simple loop, why are you testing if TimeStep is nonzero anyway? Just do it like this:
while(true) { // Infinite cycle, until brake is encountered
TimeStep++;
}
The while loop expects a true/false value, according to that, TimeStep++ if TimeStep = -1 is false, because TimeStep++add 1 to TimeStep , so == 0, if TimeStep = 0and you add 1 then is ALWAYS true, because true is every value != 0...
I think you may need to get a better understanding of boolean algebra.
Here's a link to a tutorial http://www.electronics-tutorials.ws/boolean/bool_7.html.
A while loop is based around a boolean expression. If the expression within the while loop parentheses is true it will enter the loop and stop until that expression evaluates to false.
It works when the integer that you are using is set to 0 or 1 because 0 represents false and 1 represents true. You can't use an integer as a boolean expression if it is not 0 or 1.
It looks like you want the loop to break when ContinueFlag==1. So just use that as the while loop parameter. An alternative way would be to just change that code to while (true).
Since you want ContinueFlag to be set at least once (so you know when to break) I would suggest using a do while loop which executes at least once and then repeats if the expression is true.
USE THIS:
do {
ContinueFlag=ReadStructure(Coordinate,size,TrajectoryFile);
TimeStep++; //This allows for TimeStep to increment
} while (ContinueFlag!=1); //It will loop while ContinueFlag!=1 which will cause
//the loop to end when ContinueFlag==1
This is a better way of writing your code (as opposed to while (true)). This allows you to easily see what the purpose of the loop is.