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Computer Interview Questions Answers

C Interview Questions Answers

Question - 31 : - What is an argument ? differentiate between formal arguments and actual arguments?

Answer - 31 : - An argument is an entity used to pass the data from calling function to the called function. Formal arguments are the arguments available in the function definition. They are preceded by their own data types. Actual arguments are available in the function call.

Question - 32 : - What will be printed as the result of the operation below: main() { int x=20,y=35; x=y++ + x++; y= ++y + ++x; printf(“%d%d\n”,x,y) ; }

Answer - 32 : - 5794

Question - 33 : - Can a variable be both const and volatile?

Answer - 33 : - Yes. The const modifier means that this code cannot change the value of the variable, but that does not mean that the value cannot be changed by means outside this code. For instance, in the example in FAQ 8, the timer structure was accessed through a volatile const pointer. The function itself did not change the value of the timer, so it was declared const. However, the value was changed by hardware on the computer, so it was declared volatile. If a variable is both const and volatile, the two modifiers can appear in either order.

Question - 34 : - How do you print an address?

Answer - 34 : - The safest way is to use printf() (or fprintf() or sprintf()) with the %P specification. That prints a void pointer (void*). Different compilers might print a pointer with different formats. Your compiler will pick a format that’s right for your environment. If you have some other kind of pointer (not a void*) and you want to be very safe, cast the pointer to a void*: printf( %Pn, (void*) buffer );

Question - 35 : - What is the difference between strings and character arrays?

Answer - 35 : - A major difference is: string will have static storage duration, whereas as a character array will not, unless it is explicity specified by using the static keyword. Actually, a string is a character array with following properties: * the multibyte character sequence, to which we generally call string, is used to initialize an array of static storage duration. The size of this array is just sufficient to contain these characters plus the terminating NUL character. * it not specified what happens if this array, i.e., string, is modified. * Two strings of same value[1] may share same memory area. For example, in the following declarations: char *s1 = “Calvin and Hobbes”; char *s2 = “Calvin and Hobbes”; the strings pointed by s1 and s2 may reside in the same memory location. But, it is not true for the following: char ca1[] = “Calvin and Hobbes”; char ca2[] = “Calvin and Hobbes”; [1] The value of a string is the sequence of the values of the contained characters, in order.

Question - 36 : - How do I write code that reads data at memory location specified by segment and offset?

Answer - 36 : - Use peekb( ) function. This function returns byte(s) read from specific segment and offset locations in memory. The following program illustrates use of this function. In this program from VDU memory we have read characters and its attributes of the first row. The information stored in file is then further read and displayed using peek( ) function. #include <stdio.h> #include <dos.h> main( ) { char far *scr = 0xB8000000 ; FILE *fp ; int offset ; char ch ; if ( ( fp = fopen ( "scr.dat", "wb" ) ) == NULL ) { printf ( "\nUnable to open file" ) ; exit( ) ; } // reads and writes to file for ( offset = 0 ; offset < 160 ; offset++ ) fprintf ( fp, "%c", peekb ( scr, offset ) ) ; fclose ( fp ) ; if ( ( fp = fopen ( "scr.dat", "rb" ) ) == NULL ) { printf ( "\nUnable to open file" ) ; exit( ) ; } // reads and writes to file for ( offset = 0 ; offset < 160 ; offset++ ) { fscanf ( fp, "%c", &ch ) ; printf ( "%c", ch ) ; } fclose ( fp ) ; }

Question - 37 : - What is hashing?

Answer - 37 : - To hash means to grind up, and that’s essentially what hashing is all about. The heart of a hashing algorithm is a hash function that takes your nice, neat data and grinds it into some random-looking integer. The idea behind hashing is that some data either has no inherent ordering (such as images) or is expensive to compare (such as images). If the data has no inherent ordering, you can’t perform comparison searches. If the data is expensive to compare, the number of comparisons used even by a binary search might be too many. So instead of looking at the data themselves, you’ll condense (hash) the data to an integer (its hash value) and keep all the data with the same hash value in the same place. This task is carried out by using the hash value as an index into an array. To search for an item, you simply hash it and look at all the data whose hash values match that of the data you’re looking for. This technique greatly lessens the number of items you have to look at. If the parameters are set up with care and enough storage is available for the hash table, the number of comparisons needed to find an item can be made arbitrarily close to one. One aspect that affects the efficiency of a hashing implementation is the hash function itself. It should ideally distribute data randomly throughout the entire hash table, to reduce the likelihood of collisions. Collisions occur when two different keys have the same hash value. There are two ways to resolve this problem. In open addressing, the collision is resolved by the choosing of another position in the hash table for the element inserted later. When the hash table is searched, if the entry is not found at its hashed position in the table, the search continues checking until either the element is found or an empty position in the table is found The second method of resolving a hash collision is called chaining. In this method, a bucket or linked list holds all the elements whose keys hash to the same value. When the hash table is searched, the list must be searched linearly.

Question - 38 : - What will print out? What will print out? main() { char *p1=“name”; char *p2; p2=(char*)malloc(20); memset (p2, 0, 20); while(*p2++ = *p1++); printf(“%s\n”,p2); } The pointer p2 value is also increasing with p1 . *p2++ = *p1++ means copy value of *p1 to *p2 , then increment both addresses (p1,p2) by one , so that they can point to next address . So

Answer - 38 : - empty string.

Question - 39 : - What is the heap?

Answer - 39 : - The heap is where malloc(), calloc(), and realloc() get memory. Getting memory from the heap is much slower than getting it from the stack. On the other hand, the heap is much more flexible than the stack. Memory can be allocated at any time and deallocated in any order. Such memory isn’t deallocated automatically; you have to call free(). Recursive data structures are almost always implemented with memory from the heap. Strings often come from there too, especially strings that could be very long at runtime. If you can keep data in a local variable (and allocate it from the stack), your code will run faster than if you put the data on the heap. Sometimes you can use a better algorithm if you use the heap faster, or more robust, or more flexible. It’s a tradeoff. If memory is allocated from the heap, it’s available until the program ends. That’s great if you remember to deallocate it when you’re done. If you forget, it’s a problem. A memory leak is some allocated memory that’s no longer needed but isn’t deallocated. If you have a memory leak inside a loop, you can use up all the memory on the heap and not be able to get any more. (When that happens, the allocation functions return a null pointer.) In some environments, if a program doesn’t deallocate everything it allocated, memory stays unavailable even after the program ends.

Question - 40 : - How do you redirect a standard stream?

Answer - 40 : - Most operating systems, including DOS, provide a means to redirect program input and output to and from different devices. This means that rather than your program output (stdout) going to the screen; it can be redirected to a file or printer port. Similarly, your program’s input (stdin) can come from a file rather than the keyboard. In DOS, this task is accomplished using the redirection characters, < and >. For example, if you wanted a program named PRINTIT.EXE to receive its input (stdin) from a file named STRINGS.TXT, you would enter the following command at the DOS prompt: C:> PRINTIT <STRINGS.TXT Notice that the name of the executable file always comes first. The less-than sign (<) tells DOS to take the strings contained in STRINGS.TXT and use them as input for the PRINTIT program. The following example would redirect the program’s output to the prn device, usually the printer attached on LPT1: C :> REDIR > PRN Alternatively, you might want to redirect the program’s output to a file, as the following example shows: C :> REDIR > REDIR.OUT In this example, all output that would have normally appeared on-screen will be written to the file REDIR.OUT. Redirection of standard streams does not always have to occur at the operating system. You can redirect a standard stream from within your program by using the standard C library function named freopen(). For example, if you wanted to redirect the stdout standard stream within your program to a file named OUTPUT.TXT, you would implement the freopen() function as shown here: ... freopen(output.txt, w, stdout); ... Now, every output statement (printf(), puts(), putch(), and so on) in your program will appear in the file OUTPUT.TXT.

Computer Contributors

Rajeev Katiyar
Yes Baroda

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