C文件操作 —— 在文件中查找关键字并替换

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本文介绍了如何使用C语言在文本文件中查找特定关键字并进行替换。提供了两种方法：1) 当文件每行长度固定时，通过`fseek`直接在原文件中修改；2) 当行长度不固定时，将数据输出到新文件。代码示例详细解释了每个步骤。

2011-03-30 wcdj

问题描述 ：
有一个txt文件，里面的数据是这个样子
980000001 AAE134D000
980000002 0AF2EBCE00
......
980000098 0ABDCEDB2A
......

现在有一个新数据
980000098 5ACBDC5BDC
想替换原来的那一行数据

原问题：http://topic.youkuaiyun.com/u/20110330/16/6e4f6070-d3c6-4a2f-8394-04e3a8cb9a7e.html

方法1 ：如果每一行记录的长度是一样的，则可以通过fseek直接在原文件中修改。
#include <cstdio> #include <cstdlib> #include <cstring> int main() { FILE *in = NULL; if( (in = fopen("test.txt","rt+")) == NULL )// Note "rt+" return -1; int get1, find; char str[16]=""; printf("Please enter number to replace: "); while (rewind(stdin), scanf("%d %s", &find, str) != 2) printf("Please input again: "); char tmp[128] = ""; while (fscanf(in,"%d %*s", &get1) != EOF) { if (get1 == find) { int res = fseek(in, -20L, SEEK_CUR);// sizeof(980000001 AAE134D000) == 20B itoa(find, tmp, 10); strcat(tmp, " "); strcat(tmp, str);// replace your string strcat(tmp, "/n"); fputs(tmp, in); break; } } fclose(in); return 0; } /* 980000001 AAE134D000 980000002 0AF2EBCE00 980000098 0ABDCEDB2A */

修改一下代码，如果每行固定最长为20，下面代码允许后面的字符串长度小于10，但不能大于10，否则会覆盖下面的合法数据。实现的原理是，如果字符串小于10，将后面多余的字符串用空格字符串覆盖。( 测试过程中，发现下面代码仍有问题 )
#include <cstdio> #include <cstdlib> #include <cstring> int main() { FILE *in = NULL; if( (in = fopen("test.txt","rt+")) == NULL )// Note "rt+" return -1; int get1, find; char str[16]=""; printf("Please enter number to replace: "); while (rewind(stdin), scanf("%d %s", &find, str) != 2) printf("Please input again: "); char tmp[128] = ""; int beg = 0, end = 0; int line = 0; bool bfirst = true; int nwrite = 0, nleft; beg = end = ftell(in); while (fscanf(in,"%d %*s", &get1) != EOF) { bfirst ? NULL, bfirst=false : line+=2;// because of "/r/n" beg = end; end = ftell(in); if (get1 == find) { int res1 = fseek(in, -(end-beg-line), SEEK_CUR); int t1 = ftell(in); itoa(find, tmp, 10); strcat(tmp, " "); strcat(tmp, str);// replace your string fputs(tmp, in); int t2 = ftell(in); // clear useless characters nwrite = strlen(tmp); if (nwrite < 20) { nleft = 20 -nwrite; memset(tmp, ' ', nleft); tmp[nleft]='/0'; fputs(tmp, in); } break; } } fclose(in); return 0; } /* 980000001 AAE134D000 980000002 0AF2EBCE00 980000098 0ABDCEDB2A */

方法2 ：如果每一行记录的长度不同，只能输出到另外一个文件。

#include <cstdio> #include <cstdlib> #include <cstring> int main() { FILE *in = NULL, *out = NULL; if( (in = fopen("test.txt","r+")) == NULL ) return -1; if( (out = fopen("test2.txt","a")) == NULL ) return -1; int get1, find; char str[16]=""; printf("Please enter number to replace: "); while (rewind(stdin), scanf("%d %s", &find, str) != 2) printf("Please input again: "); char get2[16] = "", tmp[128] = ""; while (fscanf(in,"%d %s", &get1, &get2) != EOF) { if (get1 == find) { itoa(find, tmp, 10); strcat(tmp, " "); strcat(tmp, str);// replace ur string strcat(tmp, "/n"); fputs(tmp, out); } else { itoa(get1, tmp, 10); strcat(tmp, " "); strcat(tmp, get2); strcat(tmp, "/n"); fputs(tmp, out); } } fclose(in); fclose(out); return 0; } /* 980000001 AAE134D000 980000002 0AF2EBCE00 980000098 0ABDCEDB2A */

参考：
fopen
http://www.cplusplus.com/reference/clibrary/cstdio/fopen/
fseek
http://www.cplusplus.com/reference/clibrary/cstdio/fseek/
fputs
http://www.cplusplus.com/reference/clibrary/cstdio/fputs/
ftell
http://www.cplusplus.com/reference/clibrary/cstdio/ftell/