Summary of 59101

General structure of a C program:

/*    include statements */
#include <stdio.h>
#include <string.h>

/*    define statements */
#define MAX 2000

/*    declarations of functions and global variables */
void subfunc(void);
int i,j;
char c;
float x,y;
char s[80];

/*    main program */
main() {
    statements...
}

/*    functions used by main */
void subfunc(void) {
    statements...
}

Comments:
    Can occur anywhere that whitespace is allowed. Start with /* end with */
Indentation rule:
    Indent one further tab stop when a { is encountered

    Remove one tab stop from the indentation when a } is encountered

Functions:
    Functions are used to structure programs. They must be declared before being used. void functions have no arguments and do not return a value.

Variables:
    Are used to store data.
    Reading a variable does not affect its contents.
    Writing to a variable overwrites any previous values.
    We must store a value in a variable before we can read it.

    Variables must be declared before they are used. Names must start with a letter, subsequent characters can be letters, numbers or the underscore. They can be any length. Use lower case.

    Those declared at the top of the file are GLOBAL - all functions can use them.
    Those declared inside functions are LOCAL to a function and can only be used by it.

    There are three simple types of variables:
        char
        int
        float

    Strings are created by using arrays of chars, individual characters within strings can be accessed by using the notation s[i] where i is zero for the first element of the string.
    Text strings are enclosed in double quotes. Characters are enclosed in single quotes.
    Special characters can be entered using a \ - \n gives a new line \\ a backslash.
    The last element in a valid string is the NUL character - \0
Assignment statements:
    Are used to assign values to variables.
    i = 3;
    x = 4.5;
    c = 'a';
    strcpy(s,"a string"); /*must use functions for strings*/
Expressions:
    Assignments can also include expressions involving the operators + - * / %.
    Where the % operator forms the remainder after division. Integer division always truncates.
    Use round brackets to force the order of evaluation.
    i = (i*j) + 3;
    c = s[3];
    i = c - 48; /*converts a char digit to a number*/
    x = 4.5*i/1.2e2;
    strcpy(s,t);
    Shortcut operators are ++, += -= etc
        i++;
        j += k;
Output:
    The standard library routine printf is used for output. printf has at least one argument. The first argument is a text string, which may contain format specifers.
    When printf encounters a format specifier it prints the next argument in that form.
        %d    for integers
        %c    for characters
        %f    for real numbers
        %s    for strings
        %%    to output one % sign

    Format specifers can contain an optional field-width. This specifies the minimum number of characters to be used on output. A negative field-width will make the output left justified.

    An optional precision can be used to specify either the maximum field-width (for strings), whether leading zeroes should be printed (for integers) or the number of decimal places (for reals).

    A variable field-width can be created by using an *. When printf encounters an *, it takes the value of the next argument as the field-width.

    printf("%12.4f %-8s %c %*d\n",x,buffer,c,fw,i);
Input:
    gets for complete lines of text.            gets(s1);
    getchar/getch for characters.        a = getchar();
    scanf for numbers and words            scanf("%d %s",&i,s2);
    gets discards the newline character. getchar/getch/scanf do not.
    scanf uses the same format specifiers as printf, use & for simple types.
    
Loops:
    There are two different loop structures: pre-test and post-test.
    Pre-test loops are implemented in C using the while and for loops.
    Post-test loops (which are rarely used) use the do while loop.
    while (test) {
        statements...
    }

    for (initialisation;test;end of loop increment) {
        statements...
    }

    do {
        statements...
    } while (test);

    test uses logical operators:        < <= ==(equals) !=(not equals) >= >
                            &&(and) ||(or) !(not)
    i = 1;
    while (i<10) {
        printf("the value of i is %d\n",i);
        i++;
    }

    for (i=0;i<10;i++) {
        printf("the value of i is %d\n",i);
    }
Conditional statements:
    The if and switch statements allow the program to chose between several options.
    if (test) {
        statements...
    }

 
    if (test) {
        statements...
    }
    else {
        statements...
    }

 
 
    if (test) {
        statements...
    }
    else if (test) {
        statements...
    }
    else {
        statements...
    }

    switch (int or char) {
    case number or character:
        statements...
        break;
    case number or character:
        statements...
        break;
    .
    .
    default:
        statements...
    }

String functions:
    Assignment statements and tests only work for simple types.
    Functions available in the standard library must be used for strings:
    strcpy(s1,s2);
            copies string s2 into string s1
    strlen(s1);
            returns the length of string s1
    strcmp(s1,s2);
            compares s1with s2 alphabetically.
                s1< s2     returns a negative value
                s1== s2     returns zero
                s1> s2     returns a positive value

    strcat(s1,s2);
            adds string s2 onto the end of s1. s2 is not affected

    strncpy, strncmp and strncat are similar to above but with a third argument - the maximum number of characters in the string that the function should handle.
Functions:
can return values, and have arguments or parameters:

eg:

int fct1(int i,int j);    /*returns an int, two int parameters*/
char fct2(char c);    /*returns a char, 1 char parameter */
void fct3(char *s);    /*no return value, 1 char string */
actual parameters are the values used as arguments when the function is called.

formal parameters are the variables used in the function, which are initialised with the value of the actual parameter.

There must be the same number of formal and actual parameters.
Parameters must also be of compatible type.

Arguments which are simple types, have their values copied into the corresponding formal parameter. For arrays, a pointer to the first element of the array is copied instead.

The return statement can be used anywhere within a function.
Its value (optionally enclosed in ()’s) is the returned value of the function.

Global variables are declared at the beginning of the program. If they are not initialised, they default to the value 0 - however it is good practice to explicitly initialise them.

Local variables are declared within a function. They are not automatically initialised.

A program may force its own termination inside any function by use of the exit function.

Arrays:
can be of any type eg:

int m[100];
float f[10];
char s[80];

To access any particular element of an array we use eg:

    m[4] = 3;
    f[i] = f[i+1];

Individual elements of an array, which are simple types, can be acted on using the standard C operators. Actions on whole arrays may only be performed via function calls.

Only a pointer is copied to a function when an array is used as an argument.

Arrays have many uses including: storing many similar data items, counting up categories and looking up items (table look-up).

Loops and arrays are used extensively. Typical actions on arrays are: finding maximum/minimum values, averaging, summing values and sorting.
The selection sort is one of many ways of sorting data.

Arrays may have more than one dimension:

    int mm[10][50];
    char sarray[25][80];
    float x[4][5][6];

declares:




Individual elements can be accessed:

    sarray[10][4]

will access the 5’th character of the 11’th string in the array.

sarray        is an array of strings
sarray[i]    is a string
sarray[i][j]    is a character within a string

 
Initialising variables:
variables may be initialised when they are declared:

char c = ‘A’;
int i = 10;
char s[80] = “hello world”;
int m[5] = {4,7,5,9,1};
float x[2][3] = {
    {1.2,4.0,5.6},
    {2.2,0.0,1.3}
};
char s[3][80] = {
    “string one”,
    “string two”,
    “string three”
};
 
 

Files:
Can be accessed either by I/O redirection:

    test <input.dat >output.dat

keyboard input will now be from a file input.dat and output will go to the file output.dat
or by using library routines:

All file I/O routines use a FILE pointer variable.

 

FILE *f;

f = fopen(filename,mode);
/* opens a file for either reading or writing*/

fprintf(f,”%s”,s);
/* writes to a file - similar to printf*/

fgets(string,size,f);
fscanf(f,”%d”,&i);
/* read from a file*/

fclose(f);
/*closes off the file*/

fopen may fail if you try to open a non-existent file for reading.
In this case it will return the value NULL.

fscanf and fgets will fail if they try to read past the end of a file.
fscanf will return the value EOF, fgets returns the NULL pointer.

sprintf and sscanf work like fprintf and fscanf, except the first argument is a character string, where the data will be either read from or written to.

 
Structures:
Allow the programmer to group similar data items together.

struct person {
    char surname[80];
    char forename[80];
    int sex;
    int age;
};

declares a structure called person. Variables may now be declared with this structure:

struct person x;

The individual elements within the structure are called fields and can be accessed using the dot operator

x.age = 26;
strcpy(x.surname,”Kay”);

New types:
can be defined by the programmer:

typedef struct person person_type;
 
person_type x;

Pointers:
are variables that hold the address of other variables.

To declare a pointer:

int *p;
char *s;
stuct person *q;

Addresses of other variables can be found with the & operator:

    p = &i;
    s = &filename[0];    /*or s = filename*/

The item that the pointer points to can be accessed using the * operator (or -> for structs)

    *p = 6;
    *s = ‘A’;
    q->age = 26;

If a function wants to alter the value of one of its parameters, then a pointer to that variable must be used.

    void fct1(int *i) {
        *i = 6;
    }

Arithmetical operators can be used on pointers

    p++;

will make p point to the next item.
This is particularly useful for writing efficient code that handles arrays of items.

Pointers can also be assigned the address of unused memory - dynamic memory.
malloc is a function that will obtain unused memory from the operating system.

    p = malloc(128);

This memory can be returned to the system using the free function.

Recursion:
A function may call itself (direct recursion) or may call itself via a call to another function (indirect recursion).

A recursive function must always have a route through it that does not involve any more recursive calls.

Local variables in a recursive routine will be created at each call.
There will only be one instance of a global variable in a program.

The recursive call should, in some sense, be nearer to the solution of the problem

Care should be taken with deeply nested recursive calls. Memory requirements for each call can easily mount up. The program will fail when all of memory has been used.