为什么我不能在IF-Else中声明char数组？

Question

I am trying to give the size of a Char array inside if-else depending upon the number of lines in the file. But when I try to use it afterwards, it gives error: "array undeclared"

FILE *f=fopen("G:\\workspaceC\\small1.txt","r");

while((c=fgetc(f))!=EOF)
{
    if(c=='\n')
        no_of_lines++;
}
printf("no_of_lines:  %d",no_of_lines);
int fclose(FILE *f);

if(no_of_lines<10){
    char b[30];
}
else if(no_of_lines>10 && no_of_lines<15){
    char b[60];
}
else{
    char b[106];
}

 for(z=0;z<size;z++)
    {
        if(c==b[z])  ///////Here it gives error: "b undeclared"
        {
            flag=1;
            break;
        }
    }

Answer 1

Array declared within a if block will become local within the block. In technical terms its scope is limited to the if block only. That is why you get the error. Move the declaration outside the block.

There are more problems though:

1. You read the whole file first to decide the number of lines and throw away all the data you read.
2. Where are you initializing the array contents? You will end up with problems because of that.

Therefore better thing is to use file API to find the file size first, dynamically allocate space for that, read the content into the dynamically allocated space and process the data appropriately.

Answer 2

You have declared the array b in the if block. So, it is only visible to if block. And, it cannot be visible outside the if block.

If you change the code as follow, you will achieve what you want.

int size;
if(no_of_lines<10){
    size = 30;
}
else if(no_of_lines>10 && no_of_lines<15){
    size = 60;
}
else{
    size = 106;
}
char b[size];

Answer 3

----> In the IF-ELSE statement, allocate memory instead of declaring the array.

1. char *arr
2. malloc((sizeof(char)*30/60/106)

Answer 4

You declared your array in a block code {}. Regardless whether this block is in if statement or not, you can access this array only in this block.

If you want to dynamically change the size of the array, declare on a pointer at the beginning of the function and then dynamically allocate memory for it (eg, using malloc). However, it seems that in you case, you can just declare on the array and give it the maximal size you will need.