为什么提示错误，请帮分析下
#include <stdio.h>
/* 考虑一下哪个输出该用无参函数哪个输出该用有参函数呢？ */
int number(int n)
{
printf("小明在慕课网上学习%d课\n",n);
return 0;
}
int learing()
{
printf("%s\n","小明在慕课网上学习");
return 0;
}
int main()
{
learning();
number(5);
return 0;
}
warning: no newline at end of file
/tmp/cc2kEazG.o: In function `main':
hello.c:(.text+0x43): undefined reference to `learning'
collect2: ld returned 1 exit status
