Lặp lồng

A loop inside a loop

• Nested iteration 嵌套迭代 puts one loop inside another's body.
• The inner loop runs completely for each pass of the outer loop.
• Outer runs m times, inner runs n times each → the inner body runs m × n times.
• Nesting handles grids, tables, and all-pairs comparisons.

Rows and columns

• A common use: outer loop for rows, inner loop for columns.
• for (int r = 0; r < rows; r++) { for (int c = 0; c < cols; c++) { ... } }
• For each row, the inner loop sweeps all columns before the outer advances.
• This visits every cell of a 2-D grid in order.

The inner loop runs completely for each pass of the outer loop

Tracing the order

• The inner loop finishes all its passes, then the outer takes one step.
• Outer r=0: inner c=0,1,2; outer r=1: inner c=0,1,2; and so on.
• Reset any inner counter at the start of each outer pass.
• Trace slowly — mixing up which loop advances is a common mistake.

Watch the inner variable

• Declare the inner loop's counter inside or reset it each outer pass.
• If you reuse an outer variable in the inner condition, watch for interference.
• The inner and outer counters should be independent unless you intend otherwise.
• A triangle pattern (c from 0 to r) makes the inner count depend on the outer.