A nested loop is a (inner) loop that appears in the loop body of another (outer) loop. The inner or outer loop can be any type: while, do while, or for. For example, the inner loop can be a while loop while an outer loop can be a for loop. Of course, they can be the same kind of loops too.
Both, Java and Python allow to use one loop inside another loop. Following section shows few examples to illustrate the concept.
Nested While loops
The following program uses a nested for loop to find the prime numbers from 2 to 100:
Java nested loops // prime numbers from 2 to 100 int i = 2; while(i < 100){ int j = 2; while(j <= (i / j)){ if (i % j == 0) break; j = j + 1;// or j++; } if (j > i/j) System.out.println(i + " is prime"); i = i + 1; // or i++; } System.out.println("Good bye!"); | Python nested loops # prime numbers from 2 to 100 i = 2 while(i < 100): j = 2 while(j <= (i/j)): if not(i%j): break j = j + 1 if (j > i/j) : print i, " is prime" i = i + 1 print "Good bye!" |
Nested for loops
You can often use for-loops to make code easier to read. The following code with nested for-loops prints...
Java nested loops // Print the below statement 3 times for (int number = 0; number < 3; number++) { System.out.println("-------------------------------------------"); System.out.println("I am outer loop iteration " + number); // inner loop for (int another_number = 0; another_number < 5; another_number++) { System.out.println("****************************"); System.out.println("I am inner loop iteration " + another_number); } } | Python nested loops # Print the below statement 3 times for number in range(3) : print("-------------------------------------------") print("I am outer loop iteration "+str(number)) # Inner loop for another_number in range(5): print("****************************") print("I am inner loop iteration "+str(another_number)) break |
You will find out that the control enters the first for loop and the value of the variable number
is initialized as 0. The first print statement is printed, and then control enters the second for loop, where the value of the variable another_number
is initialized to 0
. The first print statement in the second for loop is printed once.
Now, the control returns to the inner for loop once again and the value of another_number
is again initialized to the next integer followed by printing the statement inside the println()
function.
The aforementioned process continues until the control has traversed until another_number reaches value 4, and then the control returns back to the outermost loop, initializes the variable number
to the next integer, prints the statement inside the println()
function, visits the inner loop and then repeats all of the above steps until the value of variable number reaches 2;
This journey of the control traveling from the outermost loop, traversing of the inner loop and then back again to the outer for loop continues until the control has covered the entire range, which is 3 times in your case.
Additional example
Consider the timetable that contains seven days, and each day contains 24 time slots. Each time slot is a string, which is empty if there is nothing scheduled for that slot. How can we iterate over all the time slots and print out all our scheduled events?
Java nested loops | Python nested loops # first let's define weekday names WEEKDAYS = ('Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday', 'Sunday') # now we iterate over each day in the timetable for day in timetable: # and over each timeslot in each day for i, event in enumerate(day): if event: # if the slot is not an empty string print("%s at %02d:00 -- %s" % (WEEKDAYS[day], i, event)) |
Note that we have two for
loops – the inner loop will be executed once for every step in the outer loop’s iteration. Also note that we are using the enumerate
function when iterating over the days – because we need both the index of each time slot (so that we can print the hour) and the contents of that slot.
You may have noticed that we look up the name of the weekday once for every iteration of the inner loop – but the name only changes once for every iteration of the outer loop. We can make our loop a little more efficient by moving this lookup out of the inner loop, so that we only perform it seven times and not 168 times!
Java nested loops | Python nested loops for day in timetable: day_name = WEEKDAYS[day] for i, event in enumerate(day): if event: print("%s at %02d:00 -- %s" % (day_name, i, event)) |
This doesn’t make much difference when you are looking up a value in a short tuple, but it could make a big difference if it were an expensive, time-consuming calculation and you were iterating over hundreds or thousands of values.