C语言 字符串转换成int、long和double型

#include <stdio.h>
#include <stdlib.h>
#define LENGTH 128

int&nbsp;main(void){
  char&nbsp;str[LENGTH];
  
  puts(&quot;请输入字符串：&quot;);
  scanf(&quot;%s&quot;,&nbsp;str);
  
  printf(&quot;转换为int型后为&nbsp;%d。\n&quot;,&nbsp;atoi(str));
  
  printf(&quot;转换为long型后为&nbsp;%ld。\n&quot;,&nbsp;atol(str));
  
  printf(&quot;转换为double型后为&nbsp;%lf。\n&quot;,&nbsp;atof(str));
  
  return&nbsp;0;
}

运行结果：

使用函数库：#include <stdlib.h>

atoi 函数实现：

int atoi(const char *nptr){
  int flag = 1;
  int result = 0;
  
  if(nptr == NULL)
    return 0;
  while(*nptr == ' ' || *nptr == '\t')
    nptr++;
  
  if(*nptr == '-'){
    flag = -1;
    nptr++;
  }
  
  while(*nptr != '\0'){
    if(*nptr >= 0 && *nptr <= '9'){
      result = result*10 + (*nptr - '0');
    } else {
      break;
    }
    nptr++;
  }
  
  return result * flag;
}

atol 函数实现：

int atol(const char *nptr){
  int flag = 1;
  long result = 0;
  
  if(nptr == NULL)
    return 0;
  while(*nptr == ' ' || *nptr == '\t')
    nptr++;
  
  if(*nptr == '-'){
    flag = -1;
    nptr++;
  }
  
  while(*nptr != '\0'){
    if(*nptr >= 0 && *nptr <= '9'){
      result = result*10 + (*nptr - '0');
    } else {
      break;
    }
    nptr++;
  }
  
  return result * flag;
}

atof 函数实现：

#include <stdio.h>
#define LENGTH 128
typedef&nbsp;enum{false,true}&nbsp;bool;

double&nbsp;atof(const&nbsp;char*&nbsp;nptr){
  double&nbsp;result&nbsp;=&nbsp;0.0;
  double&nbsp;d&nbsp;=&nbsp;10.0;
  int&nbsp;count&nbsp;=&nbsp;0;
  
  if(nptr&nbsp;==&nbsp;NULL){
    return&nbsp;0;
  }
    
  while(*nptr&nbsp;==&nbsp;&#39;&nbsp;&#39;&nbsp;||&nbsp;*nptr&nbsp;==&nbsp;&#39;\t&#39;){
    nptr++;
  }
    
  bool&nbsp;flag&nbsp;=&nbsp;false;
  
  while(*nptr&nbsp;==&nbsp;&#39;-&#39;){&nbsp;&nbsp;
  
    flag&nbsp;=&nbsp;true;
    nptr++;
  }
  
  if(!(*nptr&nbsp;&gt;=&nbsp;&#39;0&#39;&nbsp;&amp;&amp;&nbsp;*nptr&nbsp;&lt;=&nbsp;&#39;9&#39;)){&nbsp;&nbsp;
    return&nbsp;result;
  }    

  while(*nptr&nbsp;&gt;=&nbsp;&#39;0&#39;&nbsp;&amp;&amp;&nbsp;*nptr&nbsp;&lt;=&nbsp;&#39;9&#39;){&nbsp;&nbsp;
  
    result&nbsp;=&nbsp;result&nbsp;*&nbsp;10&nbsp;+&nbsp;(*nptr&nbsp;-&nbsp;&#39;0&#39;);
    nptr++;
  }
  
  if(*nptr&nbsp;==&nbsp;&#39;.&#39;){&nbsp;&nbsp;
  &nbsp;&nbsp;&nbsp;&nbsp;nptr++;
  }
  
  while(*nptr&nbsp;&gt;=&nbsp;&#39;0&#39;&nbsp;&amp;&amp;&nbsp;*nptr&nbsp;&lt;=&nbsp;&#39;9&#39;){&nbsp;&nbsp;
    result&nbsp;=&nbsp;result&nbsp;+&nbsp;(*nptr&nbsp;-&nbsp;&#39;0&#39;)&nbsp;/&nbsp;d;
    d&nbsp;*=&nbsp;10.0;
    nptr++;
  }
  
  result&nbsp;=&nbsp;result&nbsp;*&nbsp;(flag&nbsp;?&nbsp;-1.0&nbsp;:&nbsp;1.0);
  
  if(*nptr&nbsp;==&nbsp;&#39;e&#39;&nbsp;||&nbsp;*nptr&nbsp;==&nbsp;&#39;E&#39;){&nbsp;&nbsp;

    flag&nbsp;=&nbsp;(*++nptr&nbsp;==&nbsp;&#39;-&#39;)&nbsp;?&nbsp;true&nbsp;:&nbsp;false;
    if(*nptr&nbsp;==&nbsp;&#39;+&#39;&nbsp;||&nbsp;*nptr&nbsp;==&nbsp;&#39;-&#39;){
      nptr++;
    }
      
    while(*nptr&nbsp;&gt;=&nbsp;&#39;0&#39;&nbsp;&amp;&amp;&nbsp;*nptr&nbsp;&lt;=&nbsp;&#39;9&#39;){
      count&nbsp;=&nbsp;count*10&nbsp;+&nbsp;(*nptr&nbsp;-&nbsp;&#39;0&#39;);
      nptr++;
    }
    
    if(flag&nbsp;==&nbsp;true)&nbsp;{&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
      while(count&nbsp;&gt;&nbsp;0){
        result&nbsp;=&nbsp;result&nbsp;/&nbsp;10;
        count--;
      }
    }
    
    if(flag&nbsp;==&nbsp;false){&nbsp;&nbsp;
      while(count&nbsp;&gt;&nbsp;0){
        result&nbsp;=&nbsp;result&nbsp;*&nbsp;10;
        count--;
      }
    }
  }
&nbsp;
&nbsp;&nbsp;&nbsp;&nbsp;return&nbsp;result;  
}

int&nbsp;main(void){
  char&nbsp;str[LENGTH];
  
  puts(&quot;请输入字符串：&quot;);
  scanf(&quot;%s&quot;,&nbsp;str);
  
  printf(&quot;转换为double型后为&nbsp;%f。\n&quot;,&nbsp;atof(str));
  
  return&nbsp;0;
}