From Code to Running Program

#Programming languages

Computers only execute machine code — binary instructions specific to a CPU. Programmers write in high-level languages like C, Python and Java that are readable by humans, and a translator converts them.

#Compilers vs interpreters

Java and Python actually do both: they compile to an intermediate bytecode, which a virtual machine then runs.

#The C build pipeline

gcc -E hello.c -o hello.i     # 1. preprocess: expand #include and #define
gcc -S hello.i -o hello.s     # 2. compile: C to assembly
gcc -c hello.s -o hello.o     # 3. assemble: assembly to machine code (object file)
gcc hello.o -o hello          # 4. link: add library code (printf) into an executable
./hello                       # 5. load and run

Normally one command does it all: gcc hello.c -o hello. See C Introduction for the diagram.

#Editors and IDEs

• A text editor (VS Code, Sublime, Vim) edits code; extensions add colours, suggestions and error checks.
• An IDE (integrated development environment) bundles an editor, compiler, debugger and project tools: CLion and Visual Studio for C/C++, PyCharm for Python, IntelliJ for Java.
• Online compilers (like this site's Compiler) need nothing installed.

#The three kinds of errors

#1. Syntax errors

The code breaks the language's grammar, so it will not compile (or start). The compiler tells you the line.

int main(void) {
    printf("Hello")      // missing semicolon -> error: expected ';'
    return 0;
}

#2. Runtime errors

The program compiles but crashes or misbehaves while running: dividing by zero, reading past an array, opening a missing file, running out of memory.

marks = [78, 85, 92]
total = 0
for i in range(4):          # one too many
    total += marks[i]
print(total)

Traceback (most recent call last):
  File "main.py", line 4, in <module>
    total += marks[i]
             ~~~~~^^^
IndexError: list index out of range

#3. Logical errors

The program runs and finishes, but gives the wrong answer. No error message appears — these are the hardest to find.

marks = [78, 85, 92]
average = sum(marks) / 2       # bug: should divide by len(marks)
print("Average:", average)

Average: 127.5

#Debugging strategies

1. Read the error message from the top — it names the file, line and problem.
2. Reproduce the bug with the smallest input that triggers it.
3. Print values at key points (printf / print debugging).
4. Use a debugger: set breakpoints, step line by line, watch variables (gdb, VS Code debugger).
5. Rubber-duck: explain the code line by line to someone (or a rubber duck). Often you spot the bug while explaining.
6. Check edge cases: 0, 1, negative numbers, empty input, the largest input.