Some notes on the use of C language strings

String input problem

String input function ：

1. scanf(“%s”, str) Read in a string of characters , With Space 、 enter 、 Tab or EOF Is the separator symbol , Space 、 enter 、 Tab or EOF Will not be read in . The character after the separator will be treated as the next input .
2. gets(str) Read in a line of characters , With Carriage return or EOF Is the separator symbol , Carriage return or EOF Will not be read in .

Out of commission Assignment statement Will a String constant or A character array Directly assigned to a A character array . The character array name is an address constant , It cannot change the value , Just as a numeric array name cannot be assigned . If the following two lines are illegal ：

str1 = "China";
str2 = str1;

use Assignment statement Only one character Give to one Character variables or Character array elements . So a quick implementation is to use strcpy function Put a string Copy Go to another character array .

String copy function ：strcpy(str1, str2) hold character string str2 Copy to character array str1 In the middle ( Include ’\0’). character string str2 The location of can also be a String constant . if str2 Valid characters digit <= str1, Will be partially or completely Cover . if str2 Number of valid characters > str1, May be an error .

String refers to the location copy function ：strncpy(str1, str2, n) The string str2 in front n individual Copy characters to character array str1 In the middle , replace str1 Original front n individual character .

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String length problem

When we initialize the character array , from brackets Whether to write or not Numbers There are two ways to write it ：

1. Numbers are written in brackets
   A. The number is equal to the actual size ： System It will not be followed by '\0', The number of characters equals the size of the array equals the number .

    char str[6] = "abc123";
    char str[6] = {
         "abc123"};
    char str[6] = {
         'a', 'b', 'c', '1', '2', '3'};
   

   B. The number is not equal to the actual size ： The system will be right Uninitialized element set '\0', The number of characters is different from that of numbers , The array size is equal to the number .

   char str[8] = "abc123";                       //"abc123\0\0"
   char str[8] = {
         "abc123"};                     //{"abc123\0\0"}
   char str[8] = {
         'a', 'b', 'c', '1', '2', '3'}; //{'a','b','c','1','2','3','\0','\0'}
   

2. Brackets do not write numbers ： In the first two cases, the system will Automatically add... At the end '\0', At this point, the size of the array is equal to the number of characters Add one . The third case is Different , The length of the array is equal to the number of characters .

   char str[] = "abc123";       //"abc123\0"
   char str[] = {
          "abc123" };   //{"abc123\0"}
   
   char str[] = {
          'a', 'b', 'c', '1', '2', '3' };
   // Therefore, when using this method to define , We usually add... Manually '\0', as follows ：
   //char str[] = { 'a', 'b', 'c', '1', '2', '3', '\0' };
   

And we generally think of the string length , It refers to the number of valid characters in the character array , So... Is not included ‘\0’.
So using sizeof() Calculated Character array length Can not be used as String length Of , So we usually use strlen() function .strlen() function adopt '\0' To determine the end position of the string , And returns the string Number of valid characters , To calculate the String length .

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Other string functions

String concatenation function ：strcat(str1, str2)
Put the string str2 Splicing To string str1 Of Back ( Include '\0'), The result is put in str1 Character array , Therefore, we must ensure that str1 The array size meets the requirements . After the function call Return value yes str1 Character array Address .

String comparison function ：strcmp(str1, str2)
be used for Compare str1 and str2, The comparison rule is to compare two strings From left to right Press... Character by character ASCII code Value size comparison , Until it appears Different characters Or encounter '\0' until . In fact, when using this function, we are more concerned about whether it equal Result , If All characters identical , Two strings are considered equal . Instead, I don't care who is bigger than who , String size doesn't make sense .

1. If two strings are same , return 0.
2. if str1>str2, Returns a positive integer .
3. if str1<str2, Returns a negative integer .

String case conversion function

1. strlwr()： In the string Capitalization The letters are converted to A lowercase letter Letter .
2. strupr()： In the string A lowercase letter The letters are converted to Capitalization Letter .