Triangle Pattern in C++

Right, inverted, and hollow triangles in C++. Hollow: star on j==1, j==i, and the last row.

IntermediateAdvanced Pattern ProgramsExample 2 of 6

triangle-pattern.cpp

1#include <iostream>
2using namespace std;
3
4int main() {
5    int rows;
6    
7    cout << "Enter number of rows: ";
8    cin >> rows;
9    
10    // Right Triangle
11    cout << "\nRight Triangle:" << endl;
12    for (int i = 1; i <= rows; i++) {
13        for (int j = 1; j <= i; j++) {
14            cout << "* ";
15        }
16        cout << endl;
17    }
18    
19    // Inverted Right Triangle
20    cout << "\nInverted Right Triangle:" << endl;
21    for (int i = rows; i >= 1; i--) {
22        for (int j = 1; j <= i; j++) {
23            cout << "* ";
24        }
25        cout << endl;
26    }
27    
28    // Hollow Triangle
29    cout << "\nHollow Triangle:" << endl;
30    for (int i = 1; i <= rows; i++) {
31        for (int j = 1; j <= i; j++) {
32            if (j == 1 || j == i || i == rows) {
33                cout << "* ";
34            } else {
35                cout << "  ";
36            }
37        }
38        cout << endl;
39    }
40    
41    return 0;
42}

Output

Enter number of rows: 5

Right Triangle:
*
* *
* * *
* * * *
* * * * *

Inverted Right Triangle:
* * * * *
* * * *
* * *
* *
*

Hollow Triangle:
*
* *
*   *
*     *
* * * * *

What's going on

Three triangles in one run. Right, inverted, hollow. Sample rows = 5.

Right triangle is the right-half pyramid: j <= i, "* ".

Inverted: outer i counts down rows to 1. Inner still j <= i. First line five stars, last line one. i++ instead of i-- reprints the right triangle.

Hollow is the new bit:

if (j == 1 || j == i || i == rows) cout << "* "; else cout << " ";

Star on the left edge (j == 1), the right edge (j == i), and the whole last row. Inside: two spaces so the column still lines up with "* ".

Walk 5, hollow:

row 1: one cell, always a star

row 2: star star (both edges)

row 3: star, gap, star

row 4: star, gap, gap, star

row 5: five stars

Use " " for the gap and the hollow leans. Skip i == rows and the base is two corners with a hole.

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