Computers don't really understand or execute C programs directly. Before you can have the computer run a C program, you first need to use a compiler to translate that program into another language that the computer does understand. That other language is called ``machine language.''
A compiler is a special program that translates programs written in C (or some other programming language) into programs written in machine language. There is a different compiler for each kind of programming language.
You might be wondering why people don't write programs in ``machine language.''
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