Lecture3:IntroductionToCProgramming < Embedded

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---+ A bit of C programming
<h2 align="center">Lecture 3</h2> <h2 align="center">Uli Raich</h2> <h2 align="center">First Semester 2017/2018</h2>

---++ 
---+ Programming Languages

To make a language a programming language it needs to implement:
   * Assignments
   * Conditional statements
   * loops
---++ 
---+ C libraries

C uses a large number of code libraries and you can create C libraries yourself.

These libraries may use special data types, which are defined in include files

Before using the library functions

_#include &lt;stdio.h&gt;_

or

_#include “myOwnIncludeFile.h”_

---++ 
---+ The C main program

As a first example people usually write the

“Hello World” program

<img alt="hello.png" height="95" src="%ATTACHURL%/hello.png" title="hello.png" width="254" />

<br />Let us try to compile and execute this program

First we start the editor, we type and save the program,

then we compile it using the gcc compiler and finally we execute it.

---++ 
---+ C data types
   * C has a number of data types:
   * char, short, int, long, float, double
   * unsigned char, unsigned short, unsigned int
   * can be extended to boolean (in C99 you can #include &lt;stdbool.h&gt;
   * No strings! But a pointer to a zero terminated chain of characters
   * struct
   * union
   * enum
And you can define your own data types with typedef

---+ Assignments

We modify the program to do some calculation:

<br /><img alt="assignement.png" height="165" src="%ATTACHURL%/assignement.png" title="assignement.png" width="489" />

I call the program assignments.c and I compile it with

gcc -o assignments assigments.c

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---+ Type casting

Why do we get this strange result for the division? and how can we correct this?

Yes, the reason is that div has got the wrong (namely integer) type

To correct we must first convert a and b to doubles (or floats) before

doing the calculation and div must be a double as well.

<img alt="typeCast.png" height="112" src="%ATTACHURL%/typeCast.png" title="typeCast.png" width="525" />

---++ 
---+ Printing formatted output

You have seen that I use the function printf to output

the results of the calculation.

The format string “%d” tells the system to output the

result as a decimal number.

In an exercise this afternoon, where you will implement a

simple calculator and we will use the format %10.4f

There are many additional number formats to output

   * strings,<br /><br />
   * decimal,<br /><br />
   * octal and hex numbers …
Look up the man page for printf for an explanation.

Instead of printing on the screen you can also convert the

number to a string using sprintf.

or print to a file using fprintf

(the file must be opened first, see later)

The equivalent call for input also exists and is called scanf.

---++ 
---+ Conditions: the if statement

Conditions can be tested with if

if (a &lt; b)

            printf(“a is smaller than b\n”);

else

           printf(“a is bigger or equal to b\n”);

---++ 
---+ Calculating the Fibonacci numbers

The Fibonacci numbers:

0 1 1 2 3 5 8 13 21 34 …

or xn = xn-1+xn-2

How can we write a program to calculate up to 12 such numbers?

This can easily be done in a for loop

---+ The for loop

<img alt="fibonacci.png" height="201" src="%ATTACHURL%/fibonacci.png" title="fibonacci.png" width="218" />

---+ The while loop

Can we also do calculations as long as the Fibonacci number

does not exceed a certain value?

<img alt="while.png" height="241" src="%ATTACHURL%/while.png" title="while.png" width="211" />

---+ Pointers

We can define variables which do not contain the value but

the address of where the value is stored in memory:

<br />char a=5; is the value

<br />char *myText=”Hello World!”;

myText points to the place in memory where Hello World is stored.

---+ Pointer Example
---++ <img alt="pointerExample.png" height="198" src="%ATTACHURL%/pointerExample.png" title="pointerExample.png" width="544" />
---+ Printing to a file

Instead of printing to stdout with printf you can also print to a file with fprintf:

<img alt="file.png" height="386" src="%ATTACHURL%/file.png" title="file.png" width="704" />

---++ 
---+ Command line arguments

The main routine has 2 parameters, which we did not use

yet as well as a return code.

int main(int argc, char ** argv) or

int main(int argc, char *argv[ ]);

int argc is the number of arguments passed

char **argv is a pointer to a list of null terminated C strings.

---++ 
---+ Command line arguments example

<img alt="commandLineExample.png" height="157" src="%ATTACHURL%/commandLineExample.png" title="commandLineExample.png" width="360" />

Please note that the argv values are zero terminated strings even

if you give a number! If you expect a number then you have to convert

the string into a number with atoi or atof

---++ 
---+ Conditional statements
   * if, else if, else
   * switch (tag)
   * Combination of assignment and condition: the ? operator.
---++ 
---+ Numeric command line arguments and if conditions

<img alt="numericCommandLine.png" height="452" src="%ATTACHURL%/numericCommandLine.png" title="numericCommandLine.png" width="639" />
---+ The switch statement

Imagine a simple calculator taking 3 parameters:

   * 2 numbers
   * An mathematical operator: +,-,*,/
We do not want to have too many if statements to find out which operation must be carried out

switch (operator) {<br />case ‘+’:<br />         do addition;<br />         break;<br />case ‘-’:<br />         do subtraction;<br />         break;<br />default:<br />         error;<br />}

---++ 
---+ Switch example

<img alt="switch-1.png" height="291" src="%ATTACHURL%/switch-1.png" title="switch-1.png" width="513" />

<img alt="switch-2.png" height="329" src="%ATTACHURL%/switch-2.png" title="switch-2.png" width="493" />

---++ 
---+ Composite variables

We can have a series of values of the same type in an array:

<br />int myArray[10]; the size of the array is fixed!

An array is in reality a pointer to a series of values of the same type.

Or we can define structures where values of different types can be combined

In addition we have enumerations with a certain fixed range of values.

---++ 
---+ Composite data type example

<img alt="structs.png" height="404" src="%ATTACHURL%/structs.png" title="structs.png" width="500" />
---++ 
---+ The ? operator

If (a &lt; b)

    c = a

else

    c = b;

Can be written in a single statement:

c = (a&lt;b) ? a : b;

<img alt="questionResv2.png" height="49" src="%ATTACHURL%/questionResv2.png" title="questionResv2.png" width="720" />
---++ 
---+ The ? operator example

<img alt="question.png" height="206" src="%ATTACHURL%/question.png" title="question.png" width="378" />

---++ 
---+ Functions

Up to now the size of our programs did not exceed one page.

Bigger problems must be broken down into smaller, manageable pieces using functions

Example: the operations of our calculator:

double add(double num1, double num2) {

     return (num1+num2);

}

The other operations look similar.

These functions may go into the same file as main but also into separate files.

Then they are compiled separately and linked to the main routine

to form an executable program.

---++ 
---+ Include files

How does the main program know the name of the function and its parameters?

Define these in an include file to be added to the calling program:

<img alt="include-1.png" height="115" src="%ATTACHURL%/include-1.png" title="include-1.png" width="202" />

and the call itself:

result = add( 5.2, 4.8);

---++ 
---+ Contents of the include file

This is how the add.h include file could look like:

/*<br /> include file for add.c function<br />*/

double add(double,double);

---++ 
---+ Libraries

A big number of libraries are available for use with C

and you can write you own libraries to extend the set.

There are two types of libraries: static libraries and dynamic ones.

The static libraries are named lib name of the library .a

Example libm.a is a library that contains a large set of

mathematical functions

Its dynamic brother is called libm.so

When linking statically the library is added to the executable making

it substantially bigger but independent of library versions

The dynamic library is loaded once into memory and can be used by

several executables. The executable is therefore much smaller but there is a risk

it will not work if it was linked to a wrong library version, the library

is not available or cannot be found.

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---+ Linking libraries

Let’s say your program sine.c uses the sine function.

The you must link it to libm

In the static case:

gcc -static -o sine sine.o -lm

In the dynamic case you skip the -static option

gcc -o sine sine.c -lm

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-- %USERSIG{UliRaich - 2017-09-07}%

---++ Comments

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png	while.png	r1	manage	16.1 K	2017-09-15 - 08:18	UnknownUser

Topic revision: r9 - 2017-11-03 - uli