Intro to C Part 3: © Walter Milner 2005: Slide 1 Introduction to ANSI C Part Three Strings Strings in structs File handling Linked list example

Intro to C Part 3: © Walter Milner 2005: Slide 1

Introduction to ANSI CIntroduction to ANSI CPart ThreePart Three

Strings Strings in structs File handling Linked list example


Recall – strings stored in memory as character codes terminated by 0

So “ABC” stored in 4 locations - maybe: 65, 66, 67, 0 Character code might be ASCII, or UNICODE or others –

not defined in C

StringsStrings


Strings as char arraysStrings as char arrays

For example,

char myString[4];myString[0]='A'; // use ’ for charsmyString[1]='B';myString[2]='C';myString[3]=0;printf("%s\n",myString);

What would happen if we replaced the 4th assignment by:myString[3]=65;


Strings as pointer to charStrings as pointer to char

Recall an array is the same as a pointer to the start of a block of memory

Equivalent to last example:

char * myString;myString= (char *) malloc(4);*myString='A';*(myString+1)='B';*(myString+2)='C';*(myString+3)=0;printf("%s\n",myString);


Input and output of stringsInput and output of strings

char * myString;myString= (char *) malloc(10);scanf("%s",myString);printf("%s\n",myString);

NB see health warning on next slide


Exercise on string I/OExercise on string I/O Run the following program, but type in more than 10

characters:

#include <stdio.h>

int main(){int x = 5;char myString[10];scanf("%s",myString);printf("%s\n",myString);printf("%i\n",x);return 0;

}Explain what happens


One fix to string input One fix to string input problemproblem

char myString[10];

scanf("%10s",myString);


Typical char * processing: Typical char * processing: count the capital letterscount the capital letters

int capCount = 0;char * myString = "Born in the USA";char * where;where = myString;while (*where!=0){

if (*where>='A' && *where<='Z')capCount++;

where++;}printf("%i\n",capCount);return 0;


String exerciseString exercise

Write code like the last example to count the spaces in a string


String constantsString constants

For example

char * myString = “What a fine day!”;At compile time the assembler – Puts aside enough memory to hold the stringPuts appropriate data there (“What a fine..Generates code to make myString point to that

memory


string functionsstring functions These are in <string.h> strcpy(a,b) - copy string b to a,

including 0 - return a strcat(a,b) - concatenate string b to a strcmp(a,b) - compare - <0 if a<b, 0 if

a==b, >0 if a > b strchr(s,c) - return pointer to first

occurence of char c in string s strlen(s) - return length of string s plus others


Using string functionsUsing string functions

char * s1 = "hello";char * s2;int i;char * where;s2=calloc(80, sizeof(char)); // get some spacestrcpy(s2, s1); // copy hello into iti = strcmp(s2, s1); // i becomes 0strcat(s2,s1); // s2 becomes hellohelloi = strlen(s2); // i is 10where = strchr(s2, 'l'); // *where is llohello


String exerciseString exercise

1. Write a program to test strlen, strcpy, strcat and strcmp 2. Write your own version of strlen


String constants againString constants again

In many implementations, string constants are in ‘non-modifiable memory’

to prevent you doing something likechar * s = “Hello”;strcat(s, “Goodbye”);

Correct version -char * s;s = calloc(256, sizeof(char));strcpy(s,"Hello");strcat(s,"Goodbye");printf("%s\n",s);


Tricky string processingTricky string processing

// count Capitals in a string again..int capCount = 0;char * myString; char * where;myString=malloc(50);strcpy(myString,"Born in the USA");where=myString;while (*where++)

if (*(where-1)>='A' && *(where-1)<='Z')capCount++;

printf("%i\n",capCount);


Structs again – Structs again – typical codetypical code

struct employee{

int payrollnumber;int deptCode;

};

int main(){

struct employee employee1;employee1.payrollnumber=38;printf("%i",employee1.payrollnumber);return 0;

}


Struct with string fieldStruct with string fieldstruct employee{

char * name;int payrollnumber;

};

void showdetails(struct employee someone){

printf("Name = %s\n",someone.name);printf("Payroll number = %i\n",someone.payrollnumber);return;

}

int main(){

struct employee john;john.name = "John Brown";john.payrollnumber = 123;showdetails(john);return 0;

}


struct employee{

char * name;int deptCode;

};int main(){

struct employee * employeePtr;employeePtr = calloc(1, sizeof(struct

employee));employeePtr->deptCode=38;employeePtr->name=malloc(50);strcpy(employeePtr->name,"John Brown");printf("%s",employeePtr->name);return 0;

}

Structs with strings and Structs with strings and dynamic memorydynamic memory


Struct exerciseStruct exercise

Modify previous code to have an array of 100 employees. Give data to each one.


FilesFiles

‘Normal’ file is stored on disc (was on tape) or other non-volatile medium

C also uses files called stdin and stdout, which default to keyboard and screen

So input and output is also file-handling Functions protoyped in stdio.h


File handlingFile handling

Access is through a pointer to a file declared likeFILE * fp;

File handling is done in a sandwich:

open the file

read or write it

close it


Writing to file 1Writing to file 1

FILE * myFile;

myFile = fopen("test1.dat", "wb");

fwrite("Hello",5,1,myFile);

fclose(myFile);

External filename

Mode – write binary – deletes existing file

Pointer to data, items to write, size each item, filepointer


Writing integer data to fileWriting integer data to fileFILE * myFile;

int * data = calloc(10, sizeof(int));

int i;

for (i=0; i<10; i++)

*(data+i)=i;

myFile = fopen("test1.dat", "wb");

fwrite(data,10,sizeof(int),myFile);

fclose(myFile);


Reading integer dataReading integer dataFILE * myFile;

int * data = calloc(10, sizeof(int));

int i;

myFile = fopen("test1.dat", "rb");

fread(data,10,sizeof(int),myFile);

for (i=0; i<10; i++)

printf("%i ",*(data+i));

fclose(myFile);


File read write exerciseFile read write exercise

Define a struct (use the employee one) Give a struct variable some data Write it into a disc file Read the disc file back and display it


Reading text filesReading text filesFILE * myFile;

char * myBuffer=malloc(80);

myFile = fopen("test2.dat", "r");

while (1)

{

fgets(myBuffer, 80, myFile);

if (feof(myFile)) break;

printf("%s",myBuffer);

}

fclose(myFile);


Text file exerciseText file exercise

Adapt the last example to write a program which counts the lines in a text file.


File open modes -File open modes -"r" -- to open an existing text file for reading "w" -- to create a text file or to open and truncate an existing text file, for writing "a" -- to create a text file or to open an existing text file, for writing. The file-position indicator is positioned at the end of the file before each write "rb" -- to open an existing binary file for reading "wb" -- to create a binary file or to open and truncate an existing binary file, for writing "ab" -- to create a binary file or to open an existing binary file, for writing. The file-position indicator is positioned at the end of the file (possibly after arbitrary null byte padding) before each write "r+" -- to open an existing text file for reading and writing "w+" -- to create a text file or to open and truncate an existing text file, for reading and writing "a+" -- to create a text file or to open an existing text file, for reading and writing. The file-position indicator is positioned at the end of the file before each write "r+b" or "rb+" -- to open an existing binary file for reading and writing "w+b" or "wb+" -- to create a binary file or to open and truncate an existing binary file, for reading and writing "a+b" or "ab+" -- to create a binary file or to open an existing binary file, for reading and writing. The file-position indicator is positioned at the end of the file (possibly after arbitrary null byte padding) before each write


Using strings againUsing strings again Task - take a string like “Have a fine day” and split it

into separate words, one word in each string Where to put the word strings - what data structure? Have an array of pointers to the strings How many elements? First count the words Then get space for the array Then copy each word somewhere, and point each array

element to it

Array of pointers to char

H a v e \0a \0f i n e \0d a y \0


String exampleString example

char * s1 = "Have a fine day";int i, wordcount=1;char *where, *start, *end;char * * pointerstowords; // note the char * *//count the number of wordsfor (where=s1; *where; where++) if ( (*where) == ' ') wordcount++;// make space for the array of pointerspointerstowords = calloc(wordcount, sizeof(char *));// put all but last word into strings pointed to by arraystart=s1;for (i=0;i<wordcount-1; i++) { // find first space after start end=strchr(start,' '); // make pointer element point to enough memory pointerstowords[i]=calloc(end-start+2, sizeof(char)); // copy correct number of characters strncpy(*(pointerstowords+i), start, end-start+1); // put 0 at end *(pointerstowords[i]+(end-start)+1)='\0'; // next time, start at letter after the SPACE start=end+1; }// for last word.. point to enough memory..pointerstowords[wordcount-1]=calloc(strlen(start)+1, sizeof(char));// copy the wordstrcpy(*(pointerstowords+wordcount-1), start);*(*(pointerstowords+wordcount-1)+strlen(start)+1)='\0';// put 0 at end


Linked listsLinked lists

A linked list is a data structure made of a set of nodes, each containing a pointer to the next node

34 21 39

Head pointer

As the program runs, calloc and dispose are used to add and remove nodesIn general, the nodes will not be next to each other in memory


Linked list implementedLinked list implementedstruct node{ int key; struct node * next;};

int main(){ struct node * head; struct node * newnode;

head = calloc(1, sizeof(struct node)); head->key = 123; head->next = NULL;

newnode = calloc(1, sizeof(struct node)); head->next = newnode; newnode->key = 321; newnode->next = NULL; return 0;}

123 321

head newnode

Documents

Intro to C Part 3: © Walter Milner 2005: Slide 1 Introduction to ANSI C Part Three Strings Strings in structs File handling Linked list example