A loop in programming is used to iterate over items. It allows you to repeat instruction lines several times. For instance, if we want to double each item in an array, it can be possible with just one line of code. Let's see several loops in Python, JavaScript, C, and C++.
Simple for loop
The for loop has 4 parts :
- initialization
- condition
- increment
- statement
There are all expressions (any unit of code that can be evaluated to a value).
The initialization determines the start of the counter. It also allows the creation of variables. The counter(s) generally starts at zero, but it can be 1, 2, etc.
The condition is the step where the expression is evaluated. It allows the execution as long as the condition is true. This means that the loop stops when the condition is false.
The increment determines how the loop control is incremented each time the loop repeats successfully. It could be just one step ( + 1) or two steps, divided, and so on. There are shortcuts (i++ for instance).
The statement (in the body) executes (instruction to perform a specific action).
Two times more
The number of each fruit column is multiplied by two. There are three columns of fruits, first column one fruit, second column two fruits, and third column three fruits. A machine with x2 appears below each column to indicate that the machine will multiply each column by two, and this for three times because there are three columns. In the last comic strip, we find the columns of fruits doubled.
Code
python1fruits = [1, 2, 3]
2
3for x in fruits:
4 print(x * 2)Time complexity : linear
Data
Before
- 1
- 2
- 3
Result
- 2
- 4
- 6
Formula
parameters
As said earlier the loop can start at 0 but also 1 and so on. So now the loop starts at the second index (1) and as a result we get 2 items instead of 3.
Code
javascript1const fruits = [1, 2, 3];
2
3for (let i = 1; i < fruits.length; i++) {
4 console.log(fruits[i] * 2);
5}Data
Before
- 1
- 2
- 3
Result
- 4
- 6
decrementation
We can also decrement the counter (- 1). For that, we need to be careful about the condition expression, even the initialization part. For example, we could swap items. Here we will also stop the loop at a specific index (2). For the initialization, the number has to be bigger than the condition.
Let's switch and leave!
We find a lineage of different fruits. Two steps are performed, the change of direction and the removal of a fruit. The change of direction corresponds to the decrement, and we stop the inversion just before reaching the pineapple, so it is deleted.
Code
javascript1const fruits = [1, 2, 3, 4, 5];
2
3for (let i = fruits.length; i >= 2; i--) {
4 console.log(i);
5}Data
Before
- 1
- 2
- 3
- 4
- 5
Result
- 5
- 4
- 3
- 2
nested
A nested loop can be handy to compute two nested arrays. First, we must think in terms of the parent-child relationship. The parent's loop is executed first, then the child. The child restarts every time, but the parent continues when the child's condition is false.
The "j < i" is important, "j" is the child, "i" the parent. The "j" always restarts, the "i" continues its path. As a result, we have several groups that correspond to the parent.
We can see that the groups are getting bigger as we go along.
Finally, in the second box of the result, which corresponds to the children, we can observe that the child runs from zero each time and that the end of its cycle depends on its parent.
Boarding
There are several cars that must accommodate passengers. But there are specific conditions. At each departure, we add one more passenger. The wagons correspond to the parents and the passengers (the fruits), to the children. At each departure the number of cars increases. So for the first departure, we have one wagon. For the second departure, we have two wagons, and so on. So there are two departures in this cartoon.
Code
javascript1const cars = [1, 2, 3, 4, 5];
2
3for (let i = 0; i < cars.length; i++) {
4 for (let j = 0; j < i; j++) {
5 console.log(i, j);
6 }
7 console.log('');
8}
9Data
Before
- 1
- 2
- 3
- 4
- 5
Result
- 1
- 0
- 2
- 0
- 2
- 1
- 3
- 0
- 3
- 1
- 3
- 2
- 4
- 0
- 4
- 1
- 4
- 2
- 4
- 3
While
This is a smaller loop that runs as long as the condition is true. Always put a condition to avoid an infinite loop.
Code
python1fruits = [1, 2, 3]
2i = 0
3
4while i < len(fruits):
5 print(fruits[i] * 2)
6 i += 1Data
Before
- 1
- 2
- 3
Result
- 2
- 4
- 6
Do while
Same as while but we execute first, the test is done at the exit of the loop.
Code
javascript1const fruits = [1, 2, 3];
2let i = 0;
3
4do {
5 console.log(fruits[i] * 2);
6 i++;
7}
8while (i < fruits.length);Data
Before
- 1
- 2
- 3
Result
- 2
- 4
- 6
Break
The break statement is used to stop the loop. However, it is not recommended to use it.
For example, we can stop the loop if the second element of the array is equal to 2.
The silver medal
Code
python1fruits = [1, 2, 3]
2i = 0
3
4while i < len(fruits):
5 if i == 2:
6 break
7 i += 1
8print(i)Data
Before
- 1
- 2
- 3
Result
- 2
As we have seen, loops are very useful in programming. They are built-in loops, but it's possible to use loops that are easier to use but slower. Depending on the type of data, you will have to be careful to avoid infinite loops.
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