Lecture 3

Uli Raich

First Semester 2017/2018

• Assignments
• Conditional statements
• loops

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

Before using the library functions

#include <stdio.h>

or

#include “myOwnIncludeFile.h”

“Hello World” program

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 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 <stdbool.h>
• No strings! But a pointer to a zero terminated chain of characters
• struct
• union
• enum

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

gcc -o assignments assigments.c

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.

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,
  
  
• decimal,
  
  
• octal and hex numbers …

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.

if (a < b)

printf(“a is smaller than b\n”);

else

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

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

does not exceed a certain value?

the address of where the value is stored in memory:

char a=5; is the value

char *myText=”Hello World!”;

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

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.

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

• if, else if, else
• switch (tag)
• Combination of assignment and condition: the ? operator.

• 2 numbers
• An mathematical operator: +,-,*,/

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

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.

c = a

else

c = b;

Can be written in a single statement:

c = (a<b) ? a : b;

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.

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

and the call itself:

result = add( 5.2, 4.8);

/*
include file for add.c function
*/

double add(double,double);

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.

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