#include<stdio.h>
int main()
{
      
        
    char s[20];
    int i;
    FILE *fp = fopen("data4.in","r");//data4.in里面存放着一串大写字母
        if(fp  ==NULL)
    {
        printf("error");
        return 0;
    }
    {
        fscanf(fp,"%s",s);   
        for(i=0;s[i]!='\0';i++)
        if(s[i]>='A' && s<='Z')s[i]+=32;//提示warning：comparison between pointer and integer
    printf("%c\t",s[i]);
    printf("\n");
    fclose(fp);
    return 0;
    }

}

知道了，多谢了

多谢高手指点啦多谢