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ESc101: Fundamentals of Com utin

2011-12-Monsoon Semester

Lecture #13, August 25, 2011

Please switch off your mobile phones.

Announcements

Monday lab scheduled on 22nd August will instead be held on

Saturday, 27th August from 2:00 PM to 5:00 PM.

Wednesday lab scheduled on 31st August will instead be held

on Saturday, 3rd September from 2:00 PM to 5:00 PM.

Lec-13 Dheeraj Sanghi, CSE Dept., IIT Kanpur

ESc101, 2011-12-Monsoon

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Quiz on lab days from 5th to 9th September in Lab at 2:00 PM.

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Recap

Arrays

Two dimensional arrays



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Functions

,

well-defined task.

A function has some inputs, better known as arguments and

an output, better known as result or return value A program may consist of any number of functions.

Any function can call (or use) any other function.

We have been writing only one function so far (called, main).

We have used functions, such as, printf, scanf, sqrt, etc.



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Why Use Functions

.

It is best to put such code in one place and re-use (call) it

as many times from as many places as is needed.

Will ensure that if there is an error or a change is needed in

the code, then that change needs to be done only at one place.

Hence, improves reliability of the program.

Modular code

Easy to understand

It is easy to understand 5 programs of 20 lines each rather

than one program of 100 lines.



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Function - Syntax

Function-name (type1 arg1, type2 arg2, , typen argn)

Function-name is the name of the function.

This name is needed to use or call this piece of program.

We have to follow the same naming convention as for any variable.

Function can have any number of arguments

Each argument is a variable (memory area) whose value is set

during the function call.

, , , .

Function carries out some activity and has a result, which is

returned to the caller of this function.

The type of this result has to be specified.



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Function (contd.)

Functions are called from other functions as:

var a e = unc on_name argumen s

Example:

int sum (int a, int b) // Function definition

{

return (a + b); // Return the result

}

z = sum (x, y); // Calling the function



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Functions (contd.)

.

In such cases, the return type can be void

Any type of argument can be used

Even arrays can be arguments

Function body may contain any valid C code



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Function to add two integers: Complete Program

#include // Include standard I/O functions

int main () // main function

{

n x, y, z;

scanf(%d, &x); // Read an integer and store it in x

scanf(%d, &y); // Read an integer and store it in y

z = sum (x, y); // Call sum function with parameters x, y

printf(%d\n, z); // Print z, which stores the return value of sum

}

int sum (int a, int b) // Function declaration two integer arguments

eturn value s also an ntegerint c; // Need some memory area in this function

c = a + b; // Compute the sum

return c; // Return the sum

}



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Parameters and Arguments

Function is called with parameters.

.

These expressions are evaluated before calling the function

The values of these parameters are stored in memory areas

called arguments.

Parameters can be variables, but need not be variables.

Even if parameters are variables, their values are not changed

when value of arguments is changed inside the function.

Arguments are local memory areas of the function.



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Swap: A Function that will not work

#include

int main ()

{

int x = 10, y = 43;

printf(x = %d, y = %d\n, x, y); // Print initial values of x and y

swap (x, y); // Expectation is that x and y will be swapped

printf(x = %d, y = %d\n, x, y); // If x and y are swapped, opposite values

} // should be printed, but does not happen.

void swap (int u, int v)

{

int temp;temp = u; u = v; v = temp; // Swap u and v.

return; // Will not swap x and y.

}



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Parameter Passing

Only values are passed to the arguments

Changes to arguments in the function do not affect the callee

We will, later, find a way to ensure that changes in the

function arguments are reflected in the parameters at callee.



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