What will happen if ‘&’ is not put in a ‘scanf’ statement?

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When a variable is defined, the compiler allocates memory for that variable.

int i;  // The compiler will allocate sizeof(int) bytes for i

i defined above is not initialized and have indeterminate value.

To write data to that memory location allocated for i, you need to specify the address of the variable. The statement

scanf("%d", &i);

will write an int data by the user to the memory location allocated for i.

If & is not placed before i, then scanf will try to write the input data to the memory location i instead of &i. Since i contains indeterminate value, there are some possibilities that it may contain a value equivalent to the value of a memory address or it may contain a value which is out of range of memory address.

In either case, the program may behave erratically and will lead to undefined behavior. In that case anything could happen.

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