Looking through each row of books in a library. Outer loop looks through each row and inner loop looks at each book in that row. More on reddit.com

Loops like you're 5: A loop is a way to tell the computer to do something specific repeatedly. For instance, if you were a (human) computer and I told you to walk to the end of the street and back until you have done so three times; this is a loop. It's a specific instruction done repeatedly. Loops don't always have an end. If I told you (as a computer) to walk to the end of the street and back until I tell you to stop; if suddenly I decide to move to France while you're walking back and forth, this is an example of a potential infinite loop. I may come back to visit and tell you to stop. You may keep going until you keel over. Nested loops like you're 5: If a loop is a set of specific instructions done repeatedly, what happens if you put a loop in a loop? Well, one of those loops has to finish entirely before the other can continue again. If I told you to walk down to the end of the street 3 times and back, but when you get halfway back, stop and touch your nose 5 times before you start again; this is a nested loop. You may say to yourself "but wait, I can touch my nose while I'm walking, why do I need to stop?" and you would be right, but what we need to realize about computers is that because of how they work, they can only do one thing at a time. (see below before any objections) Computers work in steps. Do A, then do B, then do C. Loops allow us to say "but do X over and over again." You may then say "but how come I can browse reddit, listen to music, and download stuff at the same time on my computer?" and this moves into the concepts of concurrency, or in simpler terms, making the computer do lots of different things at the same time. Things like this exist because we may want a loop to be indefinite, or to end when we tell it to end. Lots of things in the computer world depend on infinite loops, including browsing Reddit (for server side web services, some web browsing features) and video games (rendering and game logic loops). This is where concurrency and asynchronous operations come into play, but this is beginning to get out of the scope of the question, so I'll stop it here (unless you're interested). tl;dr More on reddit.com

There is nothing interesting about nested for loops. Nothing. NO thing. If you have a loop, you repeat the stuff in the loop body. It doesn't matter what that thing is, you repeat it. If that thing is a function call, you repeat the function call. If it's variable assignment, you repeat the variable assignment. If it's a loop, you repeat the loop for(i = 0; i <= 3; i++){ for(j = 0; j < i; j++){ System.out.print(" "); } System.out.println(i); } This is just: for(i = 0; i <= 3; i++){ doopitydo } What does this do? It repeats doopitydo 3 times (edit: 4 times. Stupid <=). That's it. What is doopitydo? Glad you asked. It's this: for(j = 0; j < i; j++){ System.out.print(" "); } System.out.println(i); So what does this do? Obviously it prints i spaces followed by the number i and then a carriage return, right? It doesn't matter that it's in a loop. It does exactly the same thing if it isn't. If all you had was: int i = 27; for(j = 0; j < i; j++){ System.out.print(" "); } System.out.println(i); Then the code would print 27 spaces and then the number 27. You can analyze the body of the loop (in almost all cases) without reference to the fact that it's in a loop, because it behaves the same way regardless of whether it's in a loop or not. Your problem is in thinking that there is something special about "loop within loop" vs "loop not within a loop". Nope. They are exactly the same. The loop repeats what's in the body of the loop. Period. It repeats it exactly the same way regardless of what is in the body. Variable assignment, function calls, other loops, loops within loops within loops within loops that call functions? None of that matters. More on reddit.com

when you want to do a thing repeatedly for every time that you're also doing some other thing repeatedly? More on reddit.com