What is a nested loop used for?

Together the two loops visit every cell of the table. For each row, the inner loop touches every column, so no cell is missed. If the table has 3 rows and 4 columns, the inner steps happen 12 times, once for each cell. This is exactly why nested loops are used: to reach every combination of row and column. 10

This row-and-column pattern is called two-dimensional data. Two-dimensional data has two directions, like a grid or a table. A nested loop is a natural fit whenever data has two directions. The outer loop moves along one direction, and the inner loop moves along the other. So nested loops are used to process tables, grids, and similar structures. 11

The repetition counts multiply. A 10 by 10 table means the inner steps happen 100 times, not 20. You might guess 10 plus 10, but it is 10 times 10. This multiplication is why nested loops can do much more work than they appear to. Always multiply the outer count by the inner count to find the total. 12

With large data the amount of processing can grow very fast. Doubling both sizes makes the work four times bigger. For example, a 20 by 20 table needs 400 inner steps, while a 10 by 10 table needs only 100. So nested loops are powerful but can become slow when the data is large. Keep this in mind when you use them. 13

A common mistake is thinking the inner loop runs only once. In fact it runs fully again at every step of the outer loop. If you forget this, you will misjudge how many times the inner steps happen. Always remember that the inner loop restarts from the beginning for each outer step. This is the heart of how a nested loop works. 14

Another common mistake is forgetting that the counts multiply. Beginners expect a small number of steps, but nested loops can do far more work than they guess. For example, 100 rows and 100 columns means 10,000 inner steps, not 200. This surprise can make a program slow. Always multiply the counts to see the real total. 15

A nested loop is a loop inside a loop, where the inner loop repeats fully at each outer step. It is used for two-dimensional data like rows and columns, because it visits every combination. But remember the repetition counts multiply, so processing grows quickly with large data. Use nested loops when you need to cover two directions, and check the total count before you run them. 16