Problem Solving: Printing Triangular Pattern

Let's create different triangular patterns using nested loops.

We'll cover the following

Printing a triangular pattern

In this lesson, we’ll create different triangular shapes. We’ll learn to identify the patterns within different shapes by making tables and then writing the conditions of the for loops based on the observed patterns.

We’ll also be making functions to create each shape. So let’s begin!

Triangle shapes

Our goal is to print the following triangle shapes. Let’s look at each of these patterns one by one.

Height: 6
Symbol: *

*       ******  ******       *
**      *****    *****      **
***     ****      ****     ***
****    ***        ***    ****
*****   **          **   ***** 
******  *            *  ******
 (A)    (B)       (C)      (D)

1. Triangle A

To print the first triangle, we will take symbol and height from the user and use a nested for loop to print the following triangle.

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Height: 6
Symbol: *
* 
**
***
****
*****
******

Here, in the first line, there’s one asterisk (*) printed. In the second line, there are two asterisks (**) printed, and so on. 'Can you see a pattern here, in terms of ln (line)?

Look at the table below to see the pattern.

Triangle A Pattern

Line #

Symbol (No. of Times)

1

1

2

2

3

3

.

.

.

.

ln

ln

The above pattern is really straightforward. Although we did not need to make the table,we did it anyway because it’s good practice as we’ll see when we go through the lesson.

Let’s translate the pattern above into nested loops.

  • The outer for loop will be responsible for the total number of lines to be printed using ln as a counter, starting from 1 to height of the triangle.
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for(int ln = 1; ln<=height; ln++)
{
    // Add code to print pattern for each line.
   cout << endl;
}

  • The inner for loop will print the number of symbols for each line. The pattern suggests that we should print the symbol ln times for each line.

    • For example, in the first iteration (for ln=1) inner loop will print one asterisk (*),
    • In the second iteration (for ln = 2), the inner loop will print two asterisks (**)

Therefore in general:

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...
   //for all lines till the height
      for(int c = 1; c<=ln; c++) // print the pattern for each line
         cout << symbol; 
...

Look at the complete code below.

#include <iostream>
using namespace std;
void shapeA(char symbol, int height);
int main()
{
    int height;
    char symbol;
    cout << "Enter height: ";
    cin >> height;
    cout << "Enter symbol: ";
    cin >> symbol;
    cout<<'\n';
    shapeA(symbol,height);
    return 0;
}
void shapeA(char symbol, int height)
{
    for(int ln = 1; ln <= height; ln++) // line number 1 till height
    {
        for(int c = 1; c <= ln; c++) //In each line print symbol number of time 
        {
            cout << symbol; // print * or any character
        }
        cout << endl; // moving to next line
    }
}
Printing triangle (A)
  • In line 13, we call the function shapeA() in which we pass the inputs symbol and height.
  • In line 18, we have an outer for loop that ranges 1 till the height of the pattern. In the outer loop, we have endl in line 24, which adds a new line.
  • In line 20, we have an inner for loop that prints symbol in each iteration ln times.

2. Triangle B

Let’s look at our second pattern.

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Height: 6
Symbol: *
******
*****
****
***
**
*

  • Again, we have to print as many lines as height( using the outer for loop). That should be easy.

  • For each line, let’s try to analyze how the asterisk pattern is made.

    • IIn line 1, it should print number of symbols equal to the height variable (In the above example, it should print six asterisks ****** ).

    • In line 2, height-1 symbols (five asterisks *****) should be printed, and so on.

Let us look at our table below for this pattern.

Triangle B Pattern

Line #

Symbol (No. of Times)

1

height - 0

2

height - 1

3

height - 2

.

.

.

.

ln

height - (ln-1)

Based on the pattern above, try and write the code for this problem in the editor below.

#include <iostream>
using namespace std;
void shapeB(char symbol, int height);
int main()
{
    int height;
    char symbol;
    cout << "Enter height: ";
    cin >> height;
    cout << "Enter symbol: ";
    cin >> symbol;
    cout<<'\n';
    shapeB(symbol,height);
    return 0;
}
void shapeB(char symbol, int height)
{
    // Write your code here
}
Printing triangle B

3. Triangle C

Let’s look at our third triangular pattern now.

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Height: 6
Symbol: *
******
 *****
  ****
   ***
    **
     *

How can we create the above triangle?

Think for a while before you proceed.

Hint: In each line, are we printing just one symbol?

We are actually printing spaces followed by asterisks in each line. In the above example:

  • In the first line, we print zero spaces followed by six asterisks.
  • In the second line, we print one space followed by five asterisks.

Can you see a pattern yet?

We’ve created a table to help understand the pattern.

Triangle C Pattern

Line #

Space

Symbol (No. of Times)

1

0

height - 0

2

1

height - 1

3

2

height - 2

.

.

.

.

.

.

ln

ln-1

height - (ln-1)

So our code’s structure would be as follows:

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for(int ln=0; ln<=height; ln++)
{
    // 1. Printing spaces
    // Add code here to print spaces
    // 2. Printing symbols
    // Add code here to print symbols
    cout << endl;  // The end of line
}

Printing the spaces should follow the given “Space” column pattern in the following way:

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for(int sp = 1; sp < ln;  sp++) 
     cout<<" ";

Printing the symbol should follow the given ”Symbol” column’s pattern in the following way:

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for(int s = 1; s<= height-(ln-1); s++) 
    cout<<symbol;

Let us write the complete code below:

#include <iostream>
using namespace std;
void shapeC(char sym, int H);
int main()
{
    char symbol;
    int lines;
    cout << "Enter height: ";
    cin >> height;
    cout << "Enter symbol: ";
    cin >> symbol;
    shapeC(symbol, lines);
    return 0;
}
void shapeC(char symbol, int height)
{
    for(int ln=1; ln<=height; ln++)// line number 1 till height
    {
        // 1. Printing spaces
        for(int sp = 1; sp <= ln-1;  sp++) 
        {
            cout<<" ";
        }
        // 2. Printing symbols
        for(int a=1; a<=height-(ln-1); a++)// asterisks printing in each line
        {
           cout<<symbol;
        }
        
        cout << endl; // moving cursor to the next line
    }
}
Printing shape (C)
  • In line 10, we call the function shapeC() in which we pass input symbol and height as parameters.
  • In line 15, we have an outer for loop that ranges from 1 till the height of the shape. In the outer loop, we have endl in line 25 that is used to move the cursor to the next line.
  • In lines 18–21, we have an inner for loop that prints spaces in each iteration.
  • In lines 23–26, we have an inner for loop that prints a symbol in each iteration.

4. Triangle D

Let’s look at another example of a triangular pattern.

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Height: 6
Symbol: *
     *
    **
   ***
  ****
 *****
******

Again, we have to print symbols with spaces in each line.

  • In the first line, we print five spaces with one asterisk.
  • In the second line, we print four spaces with two asterisks.

It should be fairly straightforward by now.

Instruction: Think and figure out the pattern by making your own table on paper before looking at the table below.

Triangle D Pattern

Line #

Space

Symbol (No. of Times)

1

_________

_________

2

_________

_________

3

_________

_________

.

.

.

.

.

.

ln

_________

_________

Then, based on the pattern above, try and write the code for this one in the editor below.

#include<iostream>
using namespace std;
void shapeD(char symbol,int height);
int main()
{
    int height;
    char symbol;
    cout << "enter height: ";
    cin >> height;
    cout << "enter symbol: ";
    cin >> symbol;
    cout<<'\n';
    shapeD(symbol,height);
    return 0;
}
void shapeD(char symbol,int height)
{
    // write your code here
}
Printing triangle (D)