1. 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

2. Intro to C Part 3: © Walter Milner 2005: Slide 2  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 Strings

3. Intro to C Part 3: © Walter Milner 2005: Slide 3 Strings as char arrays  For example, char myString[4]; myString[0]='A'; // use ’ for chars myString[1]='B'; myString[2]='C'; myString[3]=0; printf("%s\n",myString); What would happen if we replaced the 4 th assignment by: myString[3]=65;

4. Intro to C Part 3: © Walter Milner 2005: Slide 4 Strings 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);

5. Intro to C Part 3: © Walter Milner 2005: Slide 5 Input and output of strings char * myString; myString= (char *) malloc(10); scanf("%s",myString); printf("%s\n",myString); NB see health warning on next slide

6. Intro to C Part 3: © Walter Milner 2005: Slide 6 Exercise on string I/O  Run the following program, but type in more than 10 characters: #include int main() { int x = 5; char myString[10]; scanf("%s",myString); printf("%s\n",myString); printf("%i\n",x); return 0; } Explain what happens

7. Intro to C Part 3: © Walter Milner 2005: Slide 7 One fix to string input problem char myString[10]; scanf("%10s",myString);

8. Intro to C Part 3: © Walter Milner 2005: Slide 8 Typical char * processing: count 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;

9. Intro to C Part 3: © Walter Milner 2005: Slide 9 String exercise  Write code like the last example to count the spaces in a string

10. Intro to C Part 3: © Walter Milner 2005: Slide 10 String constants  For example char * myString = “What a fine day!”; At compile time the assembler – Puts aside enough memory to hold the string Puts appropriate data there (“What a fine.. Generates code to make myString point to that memory

11. Intro to C Part 3: © Walter Milner 2005: Slide 11 string functions  These are in  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  strchr(s,c) - return pointer to first occurence of char c in string s  strlen(s)- return length of string s  plus others

12. Intro to C Part 3: © Walter Milner 2005: Slide 12 Using string functions char * s1 = "hello"; char * s2; int i; char * where; s2=calloc(80, sizeof(char)); // get some space strcpy(s2, s1); // copy hello into it i = strcmp(s2, s1); // i becomes 0 strcat(s2,s1); // s2 becomes hellohello i = strlen(s2); // i is 10 where = strchr(s2, 'l'); // *where is llohello

13. Intro to C Part 3: © Walter Milner 2005: Slide 13 String exercise 1. Write a program to test strlen, strcpy, strcat and strcmp 2. Write your own version of strlen

14. Intro to C Part 3: © Walter Milner 2005: Slide 14 String constants again  In many implementations, string constants are in ‘non- modifiable memory’  to prevent you doing something like char * s = “Hello”; strcat(s, “Goodbye”);  Correct version - char * s; s = calloc(256, sizeof(char)); strcpy(s,"Hello"); strcat(s,"Goodbye"); printf("%s\n",s);

15. Intro to C Part 3: © Walter Milner 2005: Slide 15 Tricky 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);

16. Intro to C Part 3: © Walter Milner 2005: Slide 16 Structs again – typical code struct employee { int payrollnumber; int deptCode; }; int main() { struct employee employee1; employee1.payrollnumber=38; printf("%i",employee1.payrollnumber); return 0; }

17. Intro to C Part 3: © Walter Milner 2005: Slide 17 Struct with string field struct 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; }

18. Intro to C Part 3: © Walter Milner 2005: Slide 18 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 dynamic memory

19. Intro to C Part 3: © Walter Milner 2005: Slide 19 Struct exercise  Modify previous code to have an array of 100 employees. Give data to each one.

20. Intro to C Part 3: © Walter Milner 2005: Slide 20 Files  ‘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

21. Intro to C Part 3: © Walter Milner 2005: Slide 21 File handling  Access is through a pointer to a file declared like FILE * fp;  File handling is done in a sandwich: open the file read or write it close it

22. Intro to C Part 3: © Walter Milner 2005: Slide 22 Writing 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

23. Intro to C Part 3: © Walter Milner 2005: Slide 23 Writing integer data to file FILE * 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);

24. Intro to C Part 3: © Walter Milner 2005: Slide 24 Reading integer data FILE * 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);

25. Intro to C Part 3: © Walter Milner 2005: Slide 25 File 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

26. Intro to C Part 3: © Walter Milner 2005: Slide 26 Reading text files FILE * 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);

27. Intro to C Part 3: © Walter Milner 2005: Slide 27 Text file exercise  Adapt the last example to write a program which counts the lines in a text file.

28. Intro to C Part 3: © Walter Milner 2005: Slide 28 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

29. Intro to C Part 3: © Walter Milner 2005: Slide 29 Using 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

30. Intro to C Part 3: © Walter Milner 2005: Slide 30 String example char * s1 = "Have a fine day"; int i, wordcount=1; char *where, *start, *end; char * * pointerstowords; // note the char * * //count the number of words for (where=s1; *where; where++) if ( (*where) == ' ') wordcount++; // make space for the array of pointers pointerstowords = calloc(wordcount, sizeof(char *)); // put all but last word into strings pointed to by array start=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 word strcpy(*(pointerstowords+wordcount-1), start); *(*(pointerstowords+wordcount-1)+strlen(start)+1)='\0'; // put 0 at end

31. Intro to C Part 3: © Walter Milner 2005: Slide 31 Linked lists  A linked list is a data structure made of a set of nodes, each containing a pointer to the next node 342139 Head pointer As the program runs, calloc and dispose are used to add and remove nodes In general, the nodes will not be next to each other in memory

32. Intro to C Part 3: © Walter Milner 2005: Slide 32 Linked list implemented struct 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