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{{refimprove|date=November 2023}}

In [[computer programming]], '''conditional loops''' or '''repetitive control structures''' are a way for [[computer program]]s to repeat one or more various steps depending on conditions set either by the [[programmer]] initially or real-time by the actual program.

A conditional loop has the potential to become an [[infinite loop]] when nothing in the loop's body can affect the outcome of the loop's [[Conditional (programming)|conditional statement]]. However, infinite loops can sometimes be used purposely, often with an exit from the loop built into the loop implementation for every [[computer language]], but many share the same basic structure and/or concept. The [[While loop]] and the [[For loop]] are the two most common types of conditional loops in most [[programming language]]s.

==Types==
The following types are written in [[C++]], but apply to multiple languages.

===While loop===
Checks condition for truthfulness before executing any of the code in the loop.<ref>{{Cite web |title=while loop - cppreference.com |url=https://en.cppreference.com/w/cpp/language/while |access-date=2023-11-07 |website=en.cppreference.com}}</ref> If condition is initially false, the code inside the loop will never be executed. In [[PL/I]] this is a <code>DO WHILE...</code> statement.

<syntaxhighlight lang="c++">
while (condition) {
// code
}
</syntaxhighlight>

===Do-While loop===
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 <code>DO UNTIL...</code> statement.

<syntaxhighlight lang="c++">
do {
// code
} while (condition);
</syntaxhighlight>

===For loop===
A simplified way to create a while loop.<ref>{{Cite web |title=for loop - cppreference.com |url=https://en.cppreference.com/w/cpp/language/for |access-date=2023-11-07 |website=en.cppreference.com}}</ref>

<syntaxhighlight lang="c++">
for (initialization; condition; statement) {
// code
}
</syntaxhighlight>

'''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.

So for example, the following while loop:

<syntaxhighlight lang="c++">
int i = 0;

while (i < 10) {
// code

i += 1;
}
</syntaxhighlight>

Could be written as the following for loop:

<syntaxhighlight lang="c++">
for (int i = 0; i < 10; ++i) {
// code
}
</syntaxhighlight>

===For-Each loop===
A for-each loop is essentially equivalent to an [[iterator]]. 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:

<syntaxhighlight lang="c++">
std::vector<std::string> range = { "apple", "banana", "orange" };

for (auto item: range) {
// code
}
</syntaxhighlight>

==Examples==
The following is a [[C (programming language)|C-style]] [[While loop]]. 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 ([[infinite loop]]).
<syntaxhighlight lang="c">
int x = 0;

while (x != 3) {
// code that doesn't change x
}
</syntaxhighlight>
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.
<syntaxhighlight lang="c">
int x = 0;

while (x != 5) {
// code

x += 1;
}
</syntaxhighlight>

==Frequent bugs==
Conditional loops are often the source of an [[Off by one error]].

==References==
{{Reflist}}

{{DEFAULTSORT:Conditional Loop}}
[[Category:Control flow]]

Conditional loop - Revision history

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