Conditional loop

<h2 id="types">Types</h2>
<p>The following types are written in <a href="/facts/C%252B%252B/Et0F2qn9">C++</a>, but apply to multiple languages.
</p>
<h3>While loop</h3>
<p>Checks condition for truthfulness before executing any of the code in the loop.<a class="footnote-ref" id="fnref:1" href="#fn:1"><sup>1</sup></a> If condition is initially false, the code inside the loop will never be executed. In <a href="/facts/PL%2fI/469mVYgd">PL/I</a> this is a DO WHILE... statement.
</p>
while (condition) {
    // code
}

<h3>Do-While loop</h3>
<p>Checks condition for truthfulness after executing the code in the loop. Therefore, the code inside the loop will always be executed at least once. PL/I implements this as a DO UNTIL... statement.
</p>
do {
    // code
} while (condition);

<h3>For loop</h3>
<p>A simplified way to create a while loop.<a class="footnote-ref" id="fnref:2" href="#fn:2"><sup>2</sup></a>
</p>
for (initialization; condition; statement) {
    // code
}

<p>Initialization is executed just once before the loop. Condition evaluates the boolean expression of the loop. Statement is executed at the end of every loop.
</p><p>So for example, the following while loop:
</p>
int i = 0;

while (i < 10) {
    // code
    
    i += 1;
}

<p>Could be written as the following for loop:
</p>
for (int i = 0; i < 10; ++i) {
    // code
}

<h3>For-Each loop</h3>
<p>A for-each loop is essentially equivalent to an <a href="/facts/Iterator/f0HhTBWz">iterator</a>. It allows a program to iterate through a data structure without having to keep track of an index. It is especially useful in Sets which do not have indices. An example is as follows:
</p>
std::vector<std::string> range = { "apple", "banana", "orange" };

for (auto item: range) {
    // code
}

<h2 id="examples">Examples</h2>
<p>The following is a <a href="/facts/C_(programming_language)/Ky2No763">C-style</a> <a href="/facts/While_loop/hLrKdAFo">While loop</a>. It continues looping while x does not equal 3, or in other words it only stops looping when x equals 3. However, since x is initialized to 0 and the value of x is never changed in the loop, the loop will never end (<a href="/facts/Infinite_loop/NG4b7SFa">infinite loop</a>).
</p>
int x = 0;

while (x != 3) {
    // code that doesn't change x
}

<p>The while loop below will execute the code in the loop 5 times. x is initialized to 0, and each time in the loop the value of x is incremented. The while loop is set up to stop when x is equal to 5.
</p>
int x = 0;

while (x != 5) {
    // code
    
    x += 1;
}

<h2 id="frequent-bugs">Frequent bugs</h2>
<p>Conditional loops are often the source of an <a href="/facts/Off_by_one_error/2rMj4LYM">Off by one error</a>.
</p>

<h2 id="references">References</h2>

<ol>
<li id="fn:1"><p>"while loop - cppreference.com". en.cppreference.com. Retrieved 2023-11-07. <a href="https://en.cppreference.com/w/cpp/language/while" target="_blank">https://en.cppreference.com/w/cpp/language/while</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li>
<li id="fn:2"><p>"for loop - cppreference.com". en.cppreference.com. Retrieved 2023-11-07. <a href="https://en.cppreference.com/w/cpp/language/for" target="_blank">https://en.cppreference.com/w/cpp/language/for</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li>
</ol>

Conditional loop open-in-new

Conditional loop