String Processing Function Implementation

string function in c

strcpy

  • 使用一个临时变量保存串的首地址，然后最后返回这个地址
  • 然后在最后判断是否遇到’\0’来结束复制
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char *strcpy1(char *des, const char*src){
  char *address = des;
  while((*des++ = *src++) != '\0');
  return address;
}
int main(){
  char *des;
  char *src = "aaa";
  char *result;
  result = strcpy1(des, src);
  cout<<result;
  return 0;
}

strlen

  • 一直累加直到判断是否遇到’\0’来结束计数
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int strlen1(const char*src){
  int count = 0;
  while((*src++) != '\0')
      count++;
  return count;
}
int main(){
  char *src = "aaa";
  int result;
  result = strlen1(src);
  cout<<result;
  return 0;
}

strcat

  • 把指针移到最后，然后把b字符串的内容复制到a字符串最后
  • 但是要注意a字符串要有足够的空间来支持b字符串内容的大小
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char *strcat1(char *des, const char*src){
  char *address = des;
  while(*address)
      address++;

  while((*address++ = *src++) !='\0');
  return des;
}
int main(){
  char des[10] = "";
  char *src = "aaa";
  char *result;
  result = strcat1(des, src);
  printf("%s", result);
  return 0;
}

strcmp

  • 如果字符串一样，那么返回0
  • 如果a>b那么返回正数，否则返回负数
  • 比较方法是用asc码来比较，然后最后返回相减的结果
    • 从左到右比较，知道出现不一样的字符或者出现’\0’为止
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int strcmp1(const char *s1, const char*s2){
  while(*s1 == *s2){
      if(*s1 == '\0'){
          return 0;
      }
      ++s1;
      ++s2;
  }
  return *s1 - *s2;
}
int main(){
  char *des = "aaa";
  char *src = "aaa";
  int result;
  result = strcmp1(des, src);
  if(result == 0)
      cout<<"the string are the same";
  else
      cout<<"the string are not the same";
  return 0;
}
c++
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