Lecture 10: Modular Programming (functions)

B Burlingame

13 April 2015

Learning Objectives Introduce more C Discuss the importance of style in code Explain the concept of modular program design Explain the concept of a function in C Explain why functions are important in programming Explain the structure of a function

Return data type Parameters

Apply the concept of a function to a practical problem

Announcements Homework #5 due in two weeks Midterm in lab this week

Compound assignments

C has many assignment operators which perform an operation and then store the results

value += 5; Same as value = value + 5;

value *= 10; value = value * 5;

All compound assignments += (addition) -= (subtraction) *= (multiplication) /= (division) %= (modulo division) &= (bitwise and) |= (bitwise or) ^= (bitwise xor) <<= (left shift) >>= (right shift)

Increment and decrement

value++; similar to value = value + 1, increments after returning value

++value; similar to value = value + 1, increments before returning value

value--; similar to value = value – 1; --value;

Programming Style

The layout of program directives using white space, syntax, & punctuation to visually highlight a programmer’s intention

Good Style is Consistent – A given structure always looks

the same Illustrative – The intent of the code is shown

by the grammar Clear – Any reasonable programmer should

be able to follow the intent

Programming Style

Why does it matter? Reduces bugs – the human eye is excellent

at detecting patterns Reduces development time Allows others to understand how to approach

your code Style is a fundamental part of code

documentation

#include <stdio.h> // Programmer: B. Burlingame // Program: Good_form.cdouble square( double x ); // Version: 1.0 // int main( void ) // Prints the area of a square{ double side = 0.0; // Stores the length of the side double area = 0.0; // Stores the area of the square char buffer[100] = ""; // Input buffer printf( "Enter the side of a square: " ); fgets( buffer, sizeof(buffer), stdin ); sscanf( buffer, "%lf", &side ); // Obtain the length of a side area = square( side ); // Call square function printf( "The area is %.3lf.\n", area ); // Display the area to the screen return 0; // Return 0 to the OS}

//////////////////////////////////////////////////////////////////////////////double square( double x ) // Return x squared{ return( x * x );}

#include <stdio.h>double square( double x );int main( void ) {double side = 0.0, area = 0.0; char buffer[100] = "";printf( "Enter the side of a square: " );fgets( buffer, sizeof(buffer), stdin );sscanf( buffer, "%lf", &side ); area = square( side );printf( "The area is %.3lf.\n", area );return 0; }double square( double x ) { return( x * x ); }

Modular Programming

Break a large problem into smaller pieces Smaller pieces sometimes called ‘modules’ or

‘subroutines’ or ‘procedures’ or functions Why?

Helps manage complexity Smaller blocks of code Easier to read

Encourages re-use of code Within a particular program or across different programs

Allows independent development of code Provides a layer of ‘abstraction’

a = sqrt(9.0);

Functions

The ‘building blocks’ of a C program You’ve used predefined functions already:

main() printf(), sscanf(), pow()

User-defined functions Your own code In combination with predefined functions

Functions - Mathematical View

32)( 2

xxxf

11 is )2(

113443)2(2)2()2( 2

f

f

f(2)? isWhat

)(xf2 11X Function

Returnedvalue

Flowchart notation

Function NameStart Function Name

Functions - Definition Structure Function 'header'

Return data type (if any)

Name Descriptive

Arguments (or parameter list)

Notice: data type and name

Statements Variable declaration Operations Return value (if any)

type function_name (type arg1, type arg2 )

{statements;}

double product(double x, double y)

{ double result;

result = x * y; return result; }

A function that calculates the product of two numbers

Functions - Example Function prototype

Like a variable declaration Tells compiler that the function will

be defined later Helps detect program errors Note semicolon!!

Function definition See previous slide Note, NO semicolon

Function return return statement terminates

execution of the current function Control returns to the calling

function if return expression;

then value of expression is returned as the value of the function call

Only one value can be returned this way

Function call main() is the 'calling function' product() is the 'called function' Control transferred to the function

code Code in function definition is

executed

#include <stdio.h>

/* function prototype */

double product(double x, double y);

int main(){ double var1 = 3.0, var2 = 5.0; double ans;

ans = product(var1, var2); printf("var1 = %.2f\n" "var2 = %.2f\n",var1,var2); printf("var1*var2 = %g\n", ans);}

/* function definition */ double product(double x, double y)

{ double result;

result = x * y; return result; }

Function - Practice 1

Write a function named 'sum' sums two

integers returns the sum

2 min. on your own

Share with neighbor

Steps1. Function header

• return data type• function name• argument list with data types

2. Statements in function definition• variable declaration• operations• return value

Function - sum()

int sum_int(int x, int y)

{ int result;

result = x + y; return result; }

Passing Arguments into Functions How are the arguments

passed into functions? 'Pass by value' function arguments are

expressions

In the function call: Expressions are evaluated

and copies of their values are put into temporary memory locations

The names of the corresponding parameters in the function definition are made to be the names of the copies

The values of the expressions in the function call are not changed

#include <stdio.h>double product(double x, double y);

int main(){ int a = 10; double var1 = 3.0, var2 = 5.0; double ans;

ans = product(var1, var2); printf("var1 = %.2f\n" "var2 = %.2f\n",var1,var2); printf("var1*var2 = %g\n", ans);}

/* function definition */ double product(double A, double B)

{ double result;

result = A * B; return result; }

Identifiers and Scope

Identifier The name of a variable, function, label, etc.

int my_var1; /* a variable */ pow_table(); /* a function */ start: /* a label */

Question: Does it make a difference where in a

program an identifier is declared?

YES! --> concept of ‘scope’

Scope of Identifiers

Scope of a declaration of an identifier The region of the program that the declaration

is active (i.e., can access the variable, function, label, etc.)

Five types of scope: Program (global scope) File Function prototype Function Block (“between the { } scope”)

Scope of Identifiers - Program (Global) Scope Program (global) scope

if declared outside of all functions

"Visible" to all functions from point of declaration

Visible to functions in other source files

Use only when necessary and then very carefully!!

ex. from Ch var_scope.c

#include <stdio.h>int a = 10;double product(double x, double y);

int main(){ double var1 = 3.0, var2 = 5.0; double ans;

ans = product(var1, var2); printf("var1 = %.2f\n" "var2 = %.2f\n",var1,var2); printf("var1*var2 = %g\n", ans);}

/* function definition */ double product(double x, double y)

{ double result;

result = x * y; return result; }

Function scope Applies only to

labelsstart:

***

goto start; Active from the

beginning to the end of a function

Ex. Statement labels in a switch selection structure

Scope of Identifiers - Function Scope#include <stdio.h>

int main(){ int user_sel;

/* prompt user for entry */ /* get user entry */ switch( user_sel ) { case 1: printf("\n message..."); /* call game function1 here */ break; case 2: printf("\n message..."); /* call game function2 here */ break; default: printf("Error"); break;}

Scope of Identifiers - Block Scope Block (local) scope

A block is a series of statements enclosed in braces { }

The identifier scope is active from the point of declaration to the end of the block ( } )

Nested blocks canboth declare the same variable name and not interfere

ex. from Ch var_scope_block.c scope_nested_blocks.c

#include <stdio.h>double product(double x, double y);

int main(){ int a = 10; double var1 = 3.0, var2 = 5.0; double ans;

ans = product(var1, var2); printf("var1 = %.2f\n" "var2 = %.2f\n",var1,var2); printf("var1*var2 = %g\n", ans);}

/* function definition */ double product(double x, double y)

{ double result;

result = x * y; return result; }

Storage Duration How long the

identifier exists in memory

Static storage class Identifier exists when

program execution begins For variables:

Storage allocated and variable is initialized once

Retains their values throughout the execution of the program

#include <stdio.h>

void just_count(void); /* proto */int main(){ int i; for(i=0;i<10;i++) { just_count(); } return 0;}void just_count(void){ static int count_a; int count_b; count_a = count_a + 1; count_b = count_b + 1; printf("count_a== %d\n", count_a); printf("count_b== %d\n", count_b);}

just_count.c

For functions: function name

exists when execution begins

For variables with global scope:

i.e., declared outside of all functions and uses static keyword

"Visible" to all functions from point of declaration in this source file only

Keeps data ‘private’ to this file only

Storage Duration, cont.#include <stdio.h>static int a = 10;double product(double x, double y);

int main(){ double var1 = 3.0, var2 = 5.0; double ans;

ans = product(var1, var2); printf("var1 = %.2f\n" "var2 = %.2f\n",var1,var2); printf("var1*var2 = %g\n", ans);}

/* function definition */ double product(double x, double y)

{ double result;

result = x * y; return result; }

References

Modular Programming in C http://www.icosaedro.it/c-modules.html

math.h http://www.opengroup.org/onlinepubs/007908799/xsh/math.h.html

http://www.cprogramming.com/tutorial/style.html