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B. RAMAMURTHYUNIVERSITY AT BUFFALO

BINA@BUFFALO.EDU

The C Language

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Introduction

About C language: “.. features economy of expression..”; written for Unix Operating system (1978)

The C Language since then has taken a life of it own and has become the foundation for many modern languages.

It has also become a language of choice for RTOS.Reference: The C Programming Language by Kernighan

& Ritchie (available online)We will learn C by repeated spiral mode hands-on

exposure to various elements of the languageWe will also try to work on the Linux system and another

system called Nexos (Next generation embedded operating system) at the CSE department.

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Program Structure

A C program is a collection of functions with at least one function called “main”

Here is the classical example that has become a metaphor for a first program in any language.

Hello World: lets compile it and see what happens.#include <stdio.h>int main (){ printf(“Hello World \n”); return 0;}

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Processing the C program

Save the program in a file called “hello.c”Compile it using an appropriate compilerCC or cc or gcc or g++ (where G/g stands for

“gnu” organization)Compiler parses the input, checks for syntax

correctness and if syntax is correct generates code;

This code is further linked and loaded to generate the executable.

Source code Compile loader/linker executable code Object code

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C program structure

Program

Directives (#include libraries)functions

statements

Different types of statements

Variables/constants

Sequential, assignment, selection, iterative, input/output

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Variables and arithmetic expressions

#include <stdio.h>/* print Fahrenheit-Celsius tablefor fahr = 0, 20, ..., 300 */main(){

int fahr, celsius;int lower, upper, step;lower = 0; /* lower limit of temperature scale */upper = 300; /* upper limit */step = 20; /* step size */fahr = lower;while (fahr <= upper) {

celsius = 5 * (fahr-32) / 9;printf("%d\t%d\n", fahr, celsius);fahr = fahr + step;}

}

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Lets analyze the program

1. #include directive2. Comment // single line comment/* multiple line comment */3. Main function4. Variable declarations {variable type, variable name} int step;5. Initialization: step= 20;6. Statements: computations; arithmetic operations {+, -, *, /, %}7. Repeat computation using a “while loop”8. Condition for repetition9. Output results using “printf”10. Semicolon (;) as a terminator for statements

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Variable types

int: integer; for representing whole numbersfloat : floating point or real numbers; for

representing fractional numbers, vary large and very small numbers (32 bits)

double: double precision real number; double the size of float (64 bits)

char: single ASCII (American Standard Code of Information Interchange) character

long: longer integer

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Variable names

A sequence of characters used for identifying an entity/item used in the program

Example: partNumvoltage portNummyNameECUNum

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Assignment statement

Assignment operator =SyntaxVariable = expresion;partNum = 84560;double temp = 89.5;int age = 78;

pay = salary + bonus;

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Arithmetic operators

Addition +Subtraction –Multiplication *Division /Modulus %Precedence of operators:

*, /, % + -

Left to right associavityOverride precedence using ( )

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Arithmetic expression

celsius = 5 * (fahr-32) / 9;3 + 5 – 10 * 2 / 3 % 4=3 + 5 -20 /3 % 4= 3 + 5 – 6% 4= 3+5 – 2= 8 – 2= 6

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Iteration/repetition

While loopSyntax:Initialize condition; while (condition){ statements; update condition; }Execution semantics

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If..else: selection

Often we need to make choices in the execution path.

If ..else statement if (sensedTemp > refTemp) //cool the roomelse if (sensedTemp < refTemp) // heat the room

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Multi-way selection

Case or switch statement:switch (grade){ case ‘A’ : printf (“Very good \n”); break; case ‘B’ : printf (“Good\n”); break; case ‘C’ : printf(“not bad\n”); break; case ‘F’: printf(“Bad\n”); default: printf(“Grade out of range \n”);}

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Putting it all together

Lets solve the problem below using C.Consider the number game shown in the next

few slides.

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The Number Game (1)

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9 11 13 15

17 19 21 23

25 27 29 31

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The Number Game (2)

2 3 6 7

10 11 14 15

18 19 22 23

26 27 30 31

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The Number Game (4)

4 5 6 7

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20 21 22 23

28 29 30 31

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The Number Game (8)

8 9 10 11

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24 25 26 27

28 29 30 31

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The Number Game (16)

16 17 18 19

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24 25 26 27

28 29 30 31

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Analysis

What is theory /concept behind this game?How did I arrive at the number you guessed?How can I automate this process?What is the data and what is the algorithm?How can we convey these to a computing machine?While a computer talks binary, we humans write

programs in languages such as Java, C#, C++, Basic etc.

Binary numbers (1’s and 0’s) is the number system used by the computer systems.

We humans use decimal number system that has 10 distinct symbols (0,1,2,3,4,5,6,7,8,9)

Your task: Write a C program to computerize this game.

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1 3 5 7

9 11 13 15

17 19 21 23

25 27 29 31

2 3 6 7

10 11 14 15

18 19 22 23

26 27 30 31

16 17 18 19

20 21 22 23

24 25 26 27

28 29 30 31

4 5 6 7

12 13 14 15

20 21 22 23

28 29 30 31

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12 13 14 15

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CH.4 IN KERNIGHAN AND RITCHIE C TEXTBOOK

C Language: Functions

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Topics

Purpose of functionsFunction designFunction definitionFunction call

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Functions

Functions are modular units that perform a specific operation

It provides the ability to divide the program into coherent modules

Functions can be parameterized providing a general solution that can be customized with specific parameters

Functions offers a method for spreading the code around many files, thus providing a method for organization of code (into libraries, say)

Once defined, a function can be called from anywhere accessible and any number of times resulting in reusability, standardization and uniform application of operations

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Function Design

Lets say you want to turn Fahrenheit to Celsius converter into a function rather than dumping all the code in the main function.

fcel

Value in F

Converted value in C

celf

Value in C

Converted value in F

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Function Definition

1. Define the prototypeType function name (parameter type, parameter type…);

2. Define the headerReturn type function name (param type name, param type name…)

3. Define the body of the function{ local variable statements one or more return statements}Example:float fcel (float);

float fcel (float fah){ float celsius = (5.0/9.0) * (fah - 32.0); return celsius;}

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Function Call

It is not enough defining the function: it needs to be activated.

This is done by calling the function.Calling a function involves specifying its

name and actual parameter values. If there is a return value the call needs to be assigned to a variable.

Examplefloat fahren = 35.0;float celsius = fcel(fahren);

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Complete Example

Lets write another function for Celsius to Farenheit.

float celf (float celsi){ float fahr = celsi * 9.0/5.0 + 32.0; return fahr;}

Call: float fa = celf (35.0);

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Demos

Lets add an input statement to the functions and complete the example.

Scanf is the input statement. You provide the format(type) as well as the location (address) with name the input will be loaded into.

Example: scanf(“%f”, &celsi);scanf(“%f”, &fahr);

celsi

fahr

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Summary

We studied basics of a function design.We learned function definition and function

call.Standard IO using printf and scanf was also

illustrated.Parameter passing by value and reference

was introduced. We will discuss this in detail later.

There is lot more C language: pointers, memory management (allocation, deallocation, etc.) memory leaks, struct, separate compilation, makefile… ( we will discuss all these.)