Now we will learn how to work with flags.
The flag is a special variable that can only
take two values: true and false.
Using flags, you can solve problems that check
for the absence of something: for example, you
can check that there is no element with a certain
value in an array. Let's get started.
Let's solve the following problem: given an array
of numbers, you need to check if it has an element
with the value 3 or not. If there is,
we print '+++', if not,
we print '---'.
First, let's try to print '+++'. To do this,
let's go through all the elements of our array
and with if we ask if the current element is
equal to the value 3. If equal - we'll
output '+++':
let arr = [1, 2, 3, 4, 5];
for (let elem of arr) {
if (elem == 3) {
console.log('+++');
}
}
But our solution is not very good: after all,
if the array contains more than one value
3, but several, then '+++' will
be displayed several times. Let's remake our
array (make two elements with value 3)
and make sure of it:
let arr = [1, 2, 3, 4, 3, 5];
for (let elem of arr) {
if (elem == 3) {
console.log('+++'); // outputs several times
}
}
Let's fix the problem: end the loop with
break if the element
is already found:
let arr = [1, 2, 3, 4, 3, 5];
for (let elem of arr) {
if (elem == 3) {
console.log('+++');
break; // terminates the loop
}
}
Let's now try to make it so that if there
are no elements with the value 3 in
the array at all, '---' is displayed.
A common mistake is to add else to
our if - in this case, '---' will be
displayed for all elements that are not 3:
let arr = [1, 2, 3, 4, 5];
for (let elem of arr) {
if (elem == 3) {
console.log('+++'); // outputs for element 3
} else {
console.log('---'); // outputs for elements 1, 2, 4, 5
}
}
So the idea of adding else is a bad
idea, not a working one. To solve problems
of this type (such problems are quite common)
they use the so-called flags.
As mentioned above, a flag is a variable
that can take two values: true or
false.
So, let's make the variable flag with
the following value: if it is equal to
true, then there is an element 3
in the array, and if false⁅ /c⁆, then
there is no such element.
Initially, we set the variable flag
to the value false - that is, we will
assume that there is no element 3
in the array:
let arr = [1, 2, 3, 4, 5];
let flag = false; // consider that element 3 is not in the array
Then we'll run the loop with if just like
we did before. If the loop detects that
the array has element 3, then set the
variable flag to true and
exit the loop with break:
let arr = [1, 2, 3, 4, 5];
let flag = false; // consider that element 3 is not in the array
for (let elem of arr) {
if (elem == 3) {
flag = true; // the element exists - redefine the flag variable
break; // get out of the loop
}
}
The answer to the question, if the array
has 3 or not, we can only give after
the loop. And we already have this answer:
after the loop, the variable flag
could remain false or could change
its value to true if the loop found
3 in the array:
let arr = [1, 2, 3, 4, 5];
let flag = false;
for (let elem of arr) {
if (elem == 3) {
flag = true;
break;
}
}
// here the flag variable is either true or false
Now, after the loop, we can make an if that
looks at the variable flag and
displays '+++' or '---':
let arr = [1, 2, 3, 4, 5];
let flag = false;
for (let elem of arr) {
if (elem == 3) {
flag = true;
break;
}
}
if (flag === true) {
console.log('+++');
} else {
console.log('---');
}
Given an array:
let arr = ['a', 'b', 'c', 'd', 'e'];
Check that this array contains the element
'c'. If yes, print '+++', and
if not, print '---'.
Write code to check if a number is prime or not. A prime number is only divisible by 1 and itself, and is not divisible by other numbers.