Nested Loops and Pattern Programs

What are nested loops?

A nested loop is simply a loop inside another loop. For every single pass of the outer loop, the inner loop runs all the way through. This two-level counting is exactly what you need to print patterns.

Thinking in rows and columns

The trick to every pattern is this: the outer loop is the row, and the inner loop is the column (what to print across that row). Decide how many rows you want, then decide what each row should contain.

A square of stars

Each of 3 rows prints 3 stars:

for (int i = 1; i <= 3; i++) {       // rows
    for (int j = 1; j <= 3; j++) {   // columns
        cout << "* ";
    }
    cout << "\n";                     // new line after each row
}

A right triangle

Make the inner loop depend on the row number, so row i prints i stars:

for (int i = 1; i <= 4; i++) {
    for (int j = 1; j <= i; j++) {   // note: j <= i
        cout << "* ";
    }
    cout << "\n";
}

A number triangle

Print the column number instead of a star:

for (int i = 1; i <= 4; i++) {
    for (int j = 1; j <= i; j++) {
        cout << j << " ";
    }
    cout << "\n";
}

A pyramid

A pyramid adds a spaces loop before the stars — spaces shrink as stars grow:

int rows = 4;
for (int i = 1; i <= rows; i++) {
    for (int s = 1; s <= rows - i; s++) cout << " ";   // spaces
    for (int j = 1; j <= 2 * i - 1; j++) cout << "*";  // stars
    cout << "\n";
}

Common mistakes

• Forgetting the cout << "\n"; after each row, so everything prints on one line.
• Using j <= 3 (fixed) when you meant j <= i (grows with the row).
• Getting the spaces count wrong in a pyramid — it is rows - i.
• Reusing the same variable name for both loops, breaking the counting.

Summary

• A nested loop runs the inner loop fully for each outer pass.
• For patterns: outer loop = rows, inner loop = what fills each row.
• Make the inner bound depend on i to get triangles.
• Pyramids add a spaces loop before the stars.
• Always print a newline at the end of each row.