Welcome to Programming and Data Structures

Like conditional statements, loops can be nested one inside the other. Nesting loops allow you to iterate over all the possible values for one variable for each value of the other variable. Among other things, this is useful for generating 2-dimensional tables. For example, a “multiplication table up to 8x8” needs to take each number from 1-8 and then multiply it by each value from 1-8:

In this example, the first loop (for i) is known as the “outer loop”, and the second loop (for j) is known as the “inner loop”. The outer loop is responsible for iterating over the rows. For each row, the inner loop repeats for each of the 8 columns, printing out the row number i multiplied by the column number j followed by a tab. After the inner loop finishes, a newline is printed to start a new row. Then the outer loop continues to the next row. This process can be seen in the Codelense example below. It has the same structure, but produces a smaller table and calls the counters row and col` for clarity:

The two loops create multiple areas to write code. Anything before the inner loop happens once per iteration of the outer loop at the start of the “row”. Anything in the inner loop happens at each “cell” (row/col). Anything after the inner loop happens once at the end of the “row”:

for (int i = 0; i < 8; i++) {
    // Anything that is done at the start of each "row"
    for (int j = 0; j < 8; j++) {
        // Anything that is done for each "cell"
    }
    // Anything that is done at the end of each "row"
}

This is especially important to keep in mind when nesting while loops. We need the inner loop’s counter to reset EVERY time the outer loop iterates. That means the inner loop counter needs to be declared inside the outer loop. Also, we need to make sure that our updates to the loop control variables happen at the end of the appropriate loop body:

int i = 0;
while (i < 8) {
    int j = 0;  // inner loop counter starts at 0 every time
    while (j < 8) {
        // do work 
        ++j;  // next step of inner loop
    }
    ++i;  // next step of outer loop
}

Console output always needs to happens line by line, hence our focus on the outer loop being responsible for the “row”s. If we were processing data in a spreadsheet, we could have the outer loop be responsible for columns and the inner loop handle each row in that column. We will see examples like that later on.