Source code
this is the code that the programmer writes in a high-level language like Java, BASIC, or, in this case, C. A high-level language is one that humans can easily read and understand.

Stack
In programming, a stack is a special type of data structure in which items are removed in the reverse order from that in which they are added, so the most recently added item is the first one removed. 
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Subroutine
Good programmers break up their programs into smaller steps called subroutines. A program to make coffee consists of subroutines of preparing the filter, putting water in the coffee maker, and turning on the coffee maker. Each of these subroutines can be split into more subroutines. Putting water in the coffee maker could be broken down further: get a cup, turn on the water, put the cup under the faucet, turn off the faucet when the cup is full...