编译原理，用变量名决定程序功能

Question

如何用上下文无关文法实现这个功能？
f=open("1.txt","r")
for line in f:
for byte in f:
按行读取，或是按字节读取，由变量名决定！

Answer 1

for line in f:
char str[1024];
fgets(str,1024,f);

for byte in f:
char c;
c=fgetc(f);