# 4.4. Nested loops#

Just like how if-statements can be nested, loops can be nested too. Nested loops would be necessary if we decompose a problem into multiple parts, and each part has its own repetition.

For example, in this next exercise, we print multiple lines where each line has different repetition pattern.

## 4.4.2. Let’s tweak the pattern a little!#

Exercise

Write a C program that prints the following pattern with $$n$$ rows. $$n$$ would be taken as input from the user. In the following pattern, $$n$$ is set to $$5$$.

    *
**
***
****
*****


Step 1: Toy example. The expected output is shown above. We can use it as a toy example to develop our solution.

Step 2: Think of a solution! Again, here we need an outer loop for the number of rows to print, and an inner loop for the number of spaces and number of stars to print in each row.

In the following figure, we can see a pattern in the number of spaces and number of stars in each line. The number of spaces is equal to $$n$$ - row number and the number of stars is equal to row number.

We will use a loop to print out each row. For each row, we will use another loop to print out the number of spaces and number of stars. In every row, we can print out the number of spaces that is equal to $$n$$ - row number. Then, we can print the number of stars that is equal to row number.

Step 3: Decompose into steps. We can decompose the problem into two parts. The first part is to print out the number of rows. The second part is to print out the number of spaces and number of stars in each line. The second part can be decomposed into the following steps:

1. Initialize a variable row to $$1$$.

2. Print out the number of spaces that is equal to $$n$$ - row number and stars that is equal to row number.

3. Increment the variable row by $$1$$.

4. Repeat 2 and 3 until the variable row is greater than $$n$$ for the toy example.

The second step “2. Print out the number of spaces that is equal to $$n$$ - row number and stars that is equal to row number” can be decomposed into the following steps:

1. Initialize a variable column to $$1$$.

2. Print a space if the value of column is less than or equal to $$n$$ - row number.

3. Otherwise, print out a *.

4. Increment the variable column by $$1$$.

5. Repeat 2 to 4 until the variable column is greater than $$n$$.

Step 4: Write the code. The code is shown below. Download reverse-stars-pattern.c if you want to run the program yourself.

#include <stdio.h>

int main(void) {
int n = 0;
printf("Enter the number of rows: ");
scanf("%d", &n);
for (int row = 1; row <= n; row += 1) {
for (int col = 1; col <= n; col += 1) {
if (col <= n - row) {
printf(" ");
} else {
printf("*");
}
}
printf("\n");
}
return 0;
}


Output1

Enter the number of rows: 5
*
**
***
****
*****


Step 5: Test the code. Test the code with corner numbers that may break your code. For example, try changing $$5$$ to $$1$$ and see what happens. It should print only one *. Try changing $$5$$ to $$0$$ and see what happens. It should print nothing.

Step 6: Debug the code. It is possible that you do not get the expected output. Some common mistakes in printing patterns were discussed in the previous exercise.

1

Inputs to programs are in bold.