Lecture 17. Strings II. CptS 121 Summer 2016 Armen Abnousi

Transcription

1 Lecture 17 Strings II CptS 121 Summer 2016 Armen Abnousi

2 String concatenation Sometimes we want to put two strings next to each other to make one longer string. firstname = John lastname = Kennedy JohnKennedy We can do this using two functions strcat and strncat

3 strcat char* result = strcat(string1, string2) Both string1 and string2 should be strings (i.e. have \0 in them) It modifies the string1 to contain the joined string and also returns a pointer to it. #define SIZE 14 char str1[size] = John ; char str2[size] = Kennedy ; str1: J o h n \0????????? str2: K e n n e d y \0??????

4 strcat char* result = strcat(string1, string2) Both string1 and string2 should be strings (i.e. have \0 in them) It modifies the string1 to contain the joined string and also returns a pointer to it. #define SIZE 15 char str1[size] = John ; char str2[size] = Kennedy ; str1: J o h n \0????????? str2: K e n n e d y \0?????? char* result = strcat(str1, str2); str1: J o h n K e n n e d y \0?? char* result Note that strcpy inserts a \0 at the end, making a string

5 strcat #define SIZE 14 char str1[size] = John ; char str2[size] = Kennedy ; str1: J o h n \0????????? str2: K e n n e d y \0?????? char* result = strcat(str1, str2); str1: J o h n K e n n e d y \0?? char* result What will happen if str1 is not large enough to contain the concatenated string? Will continue writing in the adjacent memory cells, possibly making other variables values to change, or give run-time error. (i.e. trashing the memory) solution? strncat

6 strncat char* result = strncat(string1, string2, integer) Strncat is similar to strcat except it takes one last argument (int count) Last argument tells C how many characters from the string2 should be concatenated into string1. Use strlen() to know the length of string1, subtracting this length from its size (memory cells available to it) you can find how many characters you can afford concatenating. Remember you need to concatenate one less character, so that the last cell can be used for \0. Strncat copies \0 by itself.

7 strncat #define SIZE 14 char str1[size] = Jacqueline; char str2[size] = Kennedy ; str1: J a c q u e l i n e \0??? str2: K e n n e d y \0?????? char* result = strncat(str1, str2, SIZE strlen(str1) - 1); str1: J a c q u e l i n e K e n \0 char* result = 3

8 Notes Remember when manipulating an array, ask yourself: Does the output parameter have enough space for the result? Does the created string have the null terminator? Note the distinction between character c and string c! (If a function receives a string you cannot give char to it!)? c?? c \0 char myc char* myc Do not use strcat or strncat to add a char to the end of a string. (you can use array element assignment instead)

9 gets Blank space is a valid character in a string. scanf/fscanf separate the input on whitespaces, not allowing the blank spaces to be present in a string: scanf( %s, &str); /*input John Kennedy */ printf( %s, str); /*prints only John */ Use gets to read a complete line from the (console input). #define LINE_SIZE 25 char line[line_size]; gets(line); The input should be less than 25 characters, leaving a space for \0 gets reads a whole line (until a \n ) and puts a \0 at the end. It does not store the \n. If the input line is longer than the array size it will overwrite adjacent memories (can cause runtime error).

10 fgets To read a complete line from file input use fgets When reaching end-of-file, fgets returns a NULL pointer! #define LINE_SIZE char line[line_size]; fgets(line, LINE_SIZE, filepointer); + This will store at most LINE_SIZE 1 characters from the input file or until it reaches a \n, whichever comes early. + Will always set the last character as \0. (No overflow risk) -- If \n is read before the character limit is reached, it stores the \n and then the \0

11 fgets To read a complete line from file input use fgets #define LINE_SIZE 10 char line[line_size]; fgets(line, LINE_SIZE, filepointer); + This will store at most LINE_SIZE 1 characters from the input file or until it reaches a \n, whichever comes early. + Will always set the last character as \0. (No overflow risk) -- If \n is read before the character limit is reached, it stores the \n and then the \0 fgets(line, LINE_SIZE, filepointer); (file: this is the first line in the file) line: t h i s i s t \0

12 fgets To read a complete line from file input use fgets #define LINE_SIZE 10 char line[line_size]; fgets(line, LINE_SIZE, filepointer); + This will store at most LINE_SIZE 1 characters from the input file or until it reaches a \n, whichever comes early. + Will always set the last character as \0. (No overflow risk) -- If \n is read before the character limit is reached, it stores the \n and then the \0 fgets(line, LINE_SIZE, filepointer); (file: a line<new line>) line: a l i n e \n \0??

13 fgets To read a complete line from file input use fgets #define LINE_SIZE 10 char line[line_size]; fgets(line, LINE_SIZE, filepointer); + This will store at most LINE_SIZE 1 characters from the input file or until it reaches a \n, whichever comes early. + Will always set the last character as \0. (No overflow risk) -- If \n is read before the character limit is reached, it stores the \n and then the \0 fgets(line, LINE_SIZE, filepointer); (file: a line<new line>) line: a l i n e \n \0?? You can check and delete the new line character manually: if ( line [ strlen( line ) 1 ] == \n ) line [ strlen( line ) 1 ] = \0 ;

14 fgets - example Write a program that reads a file input and generates a copy of it, that is double spaced and includes line numbers.

15 #include <stdio.h> #define LINE_LEN 40 fgets - example int main() char line[line_len]; int status; int counter = 0; FILE* in = fopen("file.txt", "r"); FILE* out = fopen("out.txt", "w"); Might want to check if opened successfully for (status = fgets(line, LINE_LEN, in); status!= NULL; status = fgets(line, LINE_LEN, in)) fclose(out); fclose(in); return 0; counter++; if (line[strlen(line) - 1] == '\n') line[strlen(line) - 1] = '\0'; fprintf(out, "%3d%s\n\n", counter, line); Check the status of fgets with NULL rather than EOF If the last char in line (before \0 ) is the new line char, remove it.

16 String Comparison Often we want to compare strings Comparison for equality of two string variables Comparison to decide inequality (e.g. in sorting) strcmp (string1, string2) Strcmp returns: 0 if two strings are equal >0 (a positive integer) if string1 is larger than string2 <0 (a negative integer) if string1 is smaller than string2 strcmp( apple, apple ) = 0 strcmp( apple, app ) > 0 strcmp( apple, apply ) < 0 Use strcmp for all-capital or all-lower-case strings. It might give different results on different computers for mixed case strings!

17 Example sorting strings Modify your program for selection sort to sort a list of strings

18 Selection Sort for an array of strings naïve method void selectionsortstrings(char list[][word_size], int listsize) int fill = 0, minindex = 0, temp = 0; for (fill = 0; fill < listsize - 1; fill++) /*last fill is listsize 2*/ minindex = findminindex(list, fill, listsize); /*gives the index of the minimum element of the list in the portion starting from fill to the end of the list*/ if (minindex!= fill) /* if list[fill] is already the min, no swap needed */ strcpy(temp, list[fill]); /* temp = list[fill]; */ strcpy(list[fill] = list[minindex]; /* list[fill] = list[minindex]; */ strcpy(list[minindex], temp); /* list[minindex] = temp; */

19 Selection Sort for an array of strings naïve method /* finds the index of the minimum element in the portion of the array that starts in index beginindex */ int findminindex(char list[][word_size], int beginindex, int listsize) int minindex = beginindex, index = 0; for (index = beginindex + 1; index < listsize; index++) if (strcmp(list[index], list[minindex]) < 0) minindex = index; return minindex;

20 A Better way to implement selection sort for strings? How efficient is the selection sort algorithm for strings that we just saw? In each iteration there are 3 strcpy commands. Each strcpy copies a string into another string Every string is copied character by character if there are 2 strings of length 5 and we want to swap them, we need to copy the second string in temp (5 copies), copy the first string in the second (5 copies) and copy the temp in the first (5 copies). (15 copies for one swap) We have n-1 iterations in selection sort!

21 A Better way to implement selection sort for strings? Remember that arrays in C are implemented using pointers. We can declare an array of pointers (each pointing to the beginning of a string) Instead of using strcpy to move one string (array of characters) into another array of characters: We can move the pointers

22 A Better way to implement selection sort for strings? Instead of using strcpy to move one string (array of characters) into another array of characters: We can use an array of pointers to strings to show us our desired order What we gain? Faster sorting! Instead of swapping strings character by character in each iteration, we only swap two pointers to the strings in each iteration. Declaring an array of pointers: char* pointerarray[]; (this is equivalent to: (char* pointer)[];) (*picture from the textbook - page 481)

23 Selection sort for strings - improved void selectionsortstringsimproved(char* list[], int listsize) int fill = 0, minindex = 0; char* temp = NULL; for (fill = 0; fill < listsize - 1; fill++) Note: you cannot have list[][] here. It will be an array of strings but now it is an array of pointers minindex = findminindex(list, fill, listsize); if (minindex!= fill) temp = list[ fill ]; /* strcpy(temp, list[fill]); */ list[ fill ] = list[ minindex ]; /* strcpy(list[fill] =list[minindex]; */ list[ minindex ] = temp; /* strcpy(list[minindex], temp); */ You can create a new array of pointers to different strings in the caller function and use that to do the sorting. Then you will have both the original ordering and the sorted list, without making a copy of the list of strings (but only making a copy of the beginning addresses of strings)

24 Selection sort for strings - improved int findminindex(char* list[], int beginindex, int listsize) int minindex = beginindex, index = 0; for (index = beginindex + 1; index < listsize; index++) if (strcmp(list[index], list[minindex]) < 0) minindex = index; return minindex;

25 Selection sort for strings - improved int main() char a[][word_size] = "this", "is", "my", "list" ; char* wp[word_count]; for (int i = 0; i < WORD_COUNT; i++) wp[i] = a[i]; selectionsortstringsimproved(wp, 4); /* now wp contains the sorted set of pointers to words */ /* a contains the original array of words; not sorted */ return 0;

26 Character manipulation using ctype library #include <ctype.h> and use functions: isalpha(char) isdigit(char) islower(char) isupper(char) ispunct(char) isspace(char) tolower(char) toupper(char) Return a non-zero for true or zero for false Return the lower/upper case value if they exist, Otherwise return the input character back

27 Example Write a function that converts two strings to all lowercase letters and then compares the resulting two strings using strcmp

28 Example - Lowercase comparison void tolowerstring(char str[]) int i; for (i = 0; i< strlen(str); i++) str[i] = tolower(str[i]); int compareloweredstrings(char str1[], char str2[]) return strcmp(tolowerstring(str1), tolowerstring(str2));

29 String-number conversion printf/scanf use placeholders to convert string input to numerical data type or a numerical value to string output sprintf can be used similar to printf to convert numerical data types to string but letting you to store the result in a string! The resulting string will be stored in the first argument of sprintf Second and next arguments are similar to arguments in printf sprintf(str, I m converting pi to string: pi = %lf, 3.14); printf( %s, str); /* prints I m converting pi to string: pi = 3.14 */ The first argument of sprintf needs to have enough space or it might overflow!

30 String-number conversion printf/scanf use placeholders to convert string input to numerical data type or a numerical value to string output sscanf can be used similar to scanf. to convert a string to other data types. First argument is the string that we want to convert, second and next arguments resemble arguments in scanf. char date[] = wed, jun 15, 2016 ; char day[5], mon[5]; int daynumber, year; sscanf(date, %s%s%d%d, day, mon, daynumber, year); printf( %s, mon); /*prints jun*/ If you are using a %s to convert a extract a string from a longer string, be careful about the size of the destination. It might overflow, just like scanf with %s.

31 Example Write a function for conversion between these two date formats: mm/dd/yyyy and mon dd, yyyy 12/20/1993 and Dec 20, 1993

32 References J.R. Hanly & E.B. Koffman, Problem Solving and Program Design in C (8thed.), Pearson, 2016 Andy O Fallon s lecture notes for CptS121 (