Concept of functions

📖 Theory

The program that repeats itself

Look honestly at the marks program by now: the percentage formula appears when printing a marksheet, again inside the grade ladder, and again in the class summary. Three copies.

Last week the formula changed (out of 400, not 300). You fixed two copies and forgot the third. Two screens now disagree, and nobody knows which is right.

Repeated code is not extra typing; it is repeated bugs. The cure is the biggest structuring idea in C: the function.

📖 Theory

The tiffin service

A hostel does not cook in every room. There is ONE kitchen: raw ingredients go in, a finished tiffin comes out, and nobody eating needs to know the recipe's internals. A function is that kitchen: inputs in (arguments), one job done inside, one dish out (return value). Improve the recipe once, and every meal improves, no room-by-room fixing.

📖 Theory

Functions, formally

A function is a named block that performs one job:

float percentage(int obtained, int total) {

return (obtained * 100.0) / total;

}

Read the header: return type (float comes back), name, parameter list (what must be supplied). Using it is a call:

p = percentage(marks, 300);

You have been calling functions since day one: printf, scanf, getch, kitchens someone else built. Even main is a function, the one the OS itself calls.

⌨️ Practical

One kitchen, used twice

#include <stdio.h>

float percentage(int obtained, int total) {
    return (obtained * 100.0) / total;
}

int main() {
    printf("Riya : %.2f%%\n", percentage(234, 300));
    printf("Aman : %.2f%%\n", percentage(198, 300));
    return 0;
}

This example runs in Gri-Learn on the web, where you can edit it and see the output.

💭 Think first

Follow the jump

When main reaches percentage(234, 300), what happens step by step? Where does control GO, what travels with it, and how does main resume? Trace it before revealing.

🧩 Quiz

The percentage formula changes again (out of 500). In the function version of the program, how many places need editing?

1. One: inside the percentage() function
2. One per call: every line that calls percentage()
3. Three, same as the copy-paste version
4. Zero: functions update themselves

🧩 Quiz

Which statement about main() is TRUE?

1. main is itself a function, the one the operating system calls
2. main is a keyword, not a function
3. main cannot call other functions
4. A C program may have several main functions

⚠️ Watch out

Where marks leak

Definition vs call: defining a function (writing the kitchen) runs NOTHING; code executes only when CALLED, exams show a defined-but-never-called function and ask what prints (answer: nothing from it). Multi-job functions: "computeAndPrintAndSave" is a design smell; one function, one job, say it in theory answers. Missing return: a function promising float must return one on every path.

📖 Theory

main() as a table of contents

Professionally refactored, the marks program's main() will read like an index: readMarks(...); computeResults(...); printMarksheet(...), a story anyone can follow, each chapter testable alone. The rest of this unit builds that skill: ready-made function libraries (string and math), writing your own properly, two ways of passing arguments, and a function that calls itself.

✅ Summary

Key takeaways

• A function: named block, ONE job, inputs (parameters), one return value.
• Calls jump in with argument values and resume exactly where they left, via a call stack.
• Fix once, correct everywhere: repeated code is repeated bugs (DRY).
• printf and scanf are functions; main is a function the OS calls.
• Definition runs nothing; only calls execute the body.
• Benefits for exams: reuse, readability, testability, teamwork.
• Memory hook: the tiffin kitchen, one recipe, every meal improves.