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C++
- While Loops
The
while loop is almost exactly the same as the do loop except that its condition
is tested at the start of the loop intead of at the end... and the format
is slightly different. The easiest way to see this is simply to re-write the
above example with a while loop.
#include
<iostream.h>
int
main(void) {
int x = 0;
int y = 0;
cout << "Please enter an integer
between 1 and 10: ";
cin >> x;
cout << "You entered: " <<
x << endl << endl;
while ((x
< 1) || (x > 10)) {
cout << "Please enter an
integer between 1 and 10: ";
cin >> x;
cout << "You entered: "
<< x << endl << endl;
if
((x < 1) || (x > 10)) {
cout << "Your value
for x is not between 1 and 10!"
<< endl;
cout << "Please re-enter
the number!" << endl << endl;
}
}
cout << "Thank you for entering
a valid number!" << endl;
return 0;
}
Note
that since the condition is tested at the start, we have to prompt the user
for a number before we start. Depending on the circumstance, we could arrange
this loop in a more intelligent way. For instance, if we set the value of
x to one that would make the loop execute as false the first time through,
then it would work without gathering input before the loop. This is generally
not a good idea though since we might have this code in a function and the
variable in question might be an argument (more later on that if you don't
understand it). A more accepted way of accomplishing our goal of laziness
would be to use a sentinel. Usually a boolean (true or false) variable that
we would set to false when the condition was not satisfied and set to true
when it was. Then we can loop so long as our sentinel is false. Check it out.
#include
<iostream.h>
int
main(void) {
int x = 0;
int y = 0;
bool validNumber
= false;
while (validNumber
== false) {
cout << "Please enter an
integer between 1 and 10: ";
cin >> x;
cout << "You entered: "
<< x << endl << endl;
if
((x < 1) || (x > 10)) {
cout << "Your value
for x is not between 1 and 10!"
<< endl;
cout << "Please re-enter
the number!" << endl << endl;
}
else
validNumber = true;
}
cout << "Thank you for entering
a valid number!" << endl;
return 0;
}
So
there you go, a sentinel lets us accomplish the same thing in less code (always
a bonus). One last thing to note is that do and while loops can be nested
just like for loops in the same way. Sometimes it's not apparent so I just
thought I'd let you know.
In
conclusion, loops are an easy way to execute the same block of code more than
once. Very good for those nitty gritty repetative tasks that make programming
so much fun. You might want to fool around with loops a little just to get
a feel for them, but they are pretty simple and after seeing a few more applications
of them, you'll have it down pat if you don't already.
Written For: Mike of http://www.bluesfear.com
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