Strings 1 Sharif University of Technology Department of Computer - PowerPoint PPT Presentation

Strings 1 Sharif University of Technology Department of Computer Engineering

Input and Output – Lecture 4 Introduction  Until now  We have seen strings in printf  Our old definition: string is a set of char between “” printf("This is a string\n"); printf("This is %s\n", "a string\n");  Strings:  An array of chars  Terminated by the null char '\0' 2 Sharif University of Technology Department of Computer Engineering

Input and Output – Lecture 4 Strings in C  Since strings are array char str3[] = {'p', 'r', 'o', 'g', 'r', 'a', 'm', '\0'}; char str1[8] = "program"; char str2[] = "program"; 'p' 'r' 'o' 'g' 'r' 'a' 'm' '\0' 3 Sharif University of Technology Department of Computer Engineering

Input and Output – Lecture 4 String • Initializing char array ... – char s[10] = "unix"; /* s[4] is '\0'; */ – char s[ ] = "unix"; /* s has five elements */ 4 Sharif University of Technology Department of Computer Engineering

Input and Output – Lecture 4 Strings are Character Arrays • Strings in C are simply arrays of characters – Example: char s [10]; • This is a ten (10) element array that can hold a character string consisting of  9 characters • This is because C does not know where the end of an array is at run time – By convention, C uses a NULL character '\0' to terminate all strings in its library functions • For example: char str [10] = {'u', 'n', 'I', 'x', '\0'}; • It ’ s the string terminator (not the size of the array) that determines the length of the string 5 Sharif University of Technology Department of Computer Engineering

Input and Output – Lecture 4 Strings • Each character has an integer representation a b c d e … … … … z 97 98 99 100 101 ……………………… 112 A B C D E … … … … Z 65 66 67 68 69 ……………………… 90 0 1 2 3 4 5 6 7 8 9 48 49 50 51 52 53 54 55 56 57 \0 \n 0 10 6 Sharif University of Technology Department of Computer Engineering

Input and Output – Lecture 4 Accessing Individual Characters • The first element of any array in C is at index 0. The second is at index 1, and so on ... char s[10]; s[0] = 'h'; h i ! \0 ? ? ? ? ? ? s[1] = 'i ’ ; s [0] [1] [2] [3] [4] [5] [6] [7] [8] [9] s[2] = '!'; s[3] = '\0'; • This notation can be used in all kinds of statements and expressions in C: – For example: c = s[1]; if (s[0] == '-') … switch (s[1]) ... 7 Sharif University of Technology Department of Computer Engineering

Input and Output – Lecture 4 Strings • Characters can be interpreted as integers char c = ‘ A ’ ; printf( “ %c \n ” ,c); prints A printf( “ %d \n ” ,c); prints 65 Printf( “ %c \n ” ,65); prints A 8 8 Sharif University of Technology Department of Computer Engineering

Reading & Writing Strings Input and Output – Lecture 4  printf can be used to print strings printf("program"); printf("%s", "program");  scanf can be used to read strings char str[200]; scanf("%s", str);  Note: No ampersand(&) when inputting strings into character arrays!  Initial white spaces are ignored  Read until white space (which is replaced by \0)  We must allocate sufficient size 9 Sharif University of Technology Department of Computer Engineering

Input and Output – Lecture 4 Example char str[11]="unix and c"; u n i x a n d c \0 printf("%s", str); str [0] [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] printf("\n"); str[6]='\0'; u n i x a \0 d c \0 printf("%s", str); str [0] [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] printf("\n"); printf("\n"); printf(str); printf("\n"); u n I x a \0 d c \0 str[2]='I'; str [0] [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] printf(str); printf("\n"); 10 Sharif University of Technology Department of Computer Engineering

Input and Output – Lecture 4 Exercise • Write a function to count the number of characters in a string. • Idea: count the number of characters before \0 H e l l o \0 11 Sharif University of Technology Department of Computer Engineering

Input and Output – Lecture 4 Solution int count_letters(char cdata[]) { int i=0; while (cdata[i] != '\0') i = i + 1; return(i); } 12 Sharif University of Technology Department of Computer Engineering

Input and Output – Lecture 4 Exercise • Write a function that prints a string in reverse • Idea: find the end of the string and print the characters backwards. H e l l o \0 Output: olleH 13 Sharif University of Technology Department of Computer Engineering

Input and Output – Lecture 4 Solution void print_reverse(char pdata[]) { int size,position; size = count_letters(pdata); position = size - 1; while (position >= 0) { printf("%c",pdata[position]); position = position -1; } printf("\n"); return; } 14 Sharif University of Technology Department of Computer Engineering

Input and Output – Lecture 4 Exercise • Write a function that compares 2 strings S1 and S2 using lexicographic order. • Idea: compare character by character • Return – a neg value if S1 < S2, H e l l o \0 – 0 if S1 == S2 – a pos value if S1 > S2 H e l o o \0 l < o in lexicographic order 15 Sharif University of Technology Department of Computer Engineering

Input and Output – Lecture 4 Solution (incomplete) int compare(char cdata1[], char cdata2[]) { int i= 0; while (cdata1[i] == cdata2[i]) i = i + 1; return (cdata1[i] - cdata2[i]); } 16 Sharif University of Technology Department of Computer Engineering

Input and Output – Lecture 4 Solution (complete) int compare(char cdata1[], char cdata2[]) { int i= 0; while (cdata1[i] != ‘ \0 ’ && cdata2[i] != ‘ \0 ’ && cdata1[i] == cdata2[i]) i = i + 1; return (cdata1[i] - cdata2[i]); } 17 Sharif University of Technology Department of Computer Engineering

Input and Output – Lecture 4 Exercise: Spell out a number in text using arrays of strings • Write a program that reads a number between 1 and 999 from user and spells out it in English. For example: • 453  Four hundred fifty three • 37  Thirty seven • 204  Two hundred four 18 Sharif University of Technology Department of Computer Engineering

Input and Output – Lecture 4 Reading & Writing Strings (cont ’ d)  puts(str) is very simple version of printf  Can only be used to print strings  Adds ‘ \n ’ to end of string  Prototype of puts is defined in stdio.h – This is more efficient than printf : because your program doesn't need to analyze the format string at run-time. – For example: char sentence[] = "The quick brown fox"; puts(sentence); // printf("The quick brown fox \n "); 19 Sharif University of Technology Department of Computer Engineering

Input and Output – Lecture 4 Reading & Writing Strings (cont ’ d)  Gets (str) can be used to read strings  Gets does not ignore the white spaces •  Read until \n 20 Sharif University of Technology Department of Computer Engineering

Input and Output – Lecture 4 Difference between gets and scanf • gets( ) read a line • scanf("%s", … ) read up to the next space char line[80]; char line[80]; gets(line); scanf("%[ ^\n]s", line); puts(line); printf( “ %s\n", line); 21 Sharif University of Technology Department of Computer Engineering

Input and Output – Lecture 4 String Library  Access to string library by #include <string.h>  Many functions to work with strings  Find the length of string  Compare strings  Copy strings  Search in strings  Concatenating strings 22 Sharif University of Technology Department of Computer Engineering

Input and Output – Lecture 4 Length of String  strlen(str) : Length of string  From start to first occurrence of the null char char str[] = "This is test"; char str1[10]={'a', 'b', '\0', 'c', '\0'};  12 strlen(str) strlen(str1)  2 23 Sharif University of Technology Department of Computer Engineering

Input and Output – Lecture 4 Compare Strings  str1 and str2 are compared as follows  Compare char by char from left to right until str1 and str2 has same chars.  In the first different char  If(char of str1 < char of str2)  str1 < str2  If (both string finish)  str1 = str2  strcmp(str1, str2): compare str1 and str2  If(str1 == str2)  return 0  If(str1 < str2)  return -1  If(str1 > str2)  return 1 24 Sharif University of Technology Department of Computer Engineering

Input and Output – Lecture 4 Compare Strings: Examples char s1[] = "abc"; char s2[] = "abc"; i = strcmp(s1, s2); //i = 0 char s3[] = "abc"; char s4[] = "abx"; i = strcmp(s3, s4); //i = -1 char s5[] = "axc"; char s6[] = "abc"; i = strcmp(s5, s6); //i = 1 char s7[] = "ab"; char s8[] = "abc"; i = strcmp(s7, s8); //i = -1 char s9[] = "abc"; char s10[] = "aBc"; i = strcmp(s9, s10); //i = 1 25 Sharif University of Technology Department of Computer Engineering