Computer Science 112a

C and Data Structures
Fall 1995
Midterm examination
October 19, 1995

Name:

112a-ax account:
The exam is open book: you may consult your notes, books, even computer 
programs that you have written before. However you may not consult with 
your colleagues. All the work should be yours.

The exam has two parts. Each should take you roughly one hour, but if you 
finish earlier on part 1 you can start part 2 right away.  Provide the answers 
for Part 1 questions in the empty spaces on the exam sheet. Part 2 involves 
writing a running C program.  You will logon to grendel and type in, compile 
and debug  three short C programs.Name the programs according to the 
instructions and submit them using submit Midterm.

There may be more work than you can do  in two hours. Do not panic: do first 
the problems that you consider to be the easiest, then allow time for the most 
difficult  ones. Show all your work: you may get partial credit. Add extra sheets 
if necessary.

Part 1. (50 points)
1. (7 points) You are given the following C program:

#define MAX 10

int array[MAX];
main()
{ int i;
for (i=0; i< MAX; i++)     /* 1*/
{
array[i]= i;
printf("array[%d] = %d\n",i,array[i]);
}

}

Replace the for loop /* 1 */ with a while loop, so that the program will still be 
doing the same thing.

















2.  You are given the following C program:

#include<stdio.h>

int k = 2;
int j = 6;

main()
{
 int i=3;
 int j;

j = funcA(i);
printf("%d",j);

j=funcB(k);
printf("%d",j);

}


int funcA(int j)
{
 printf("%d",j);
 return(2*j);
}


int funcB(int j)
{
  if (j == 1) return(1);
  return(j*funcB(j-1));
}


(3 points) For each occurrence of the j variable, write next to it whether that 
is referring to: 1. the global variable j; 2. a parameter to some function; 3. a 
local variable j (and explain: local to what function?)



(3 points) What is funcA computing? (one sentence suffices!).




(3 points) What are the values printed by the  printf's of this program?






3.(6 points)  Write the postfix form for the following arithmetic expression:

(((a+b)*(c-d))/((a/c)-(b+c)))








4. (6 points) What is the value of the following arithmetic expression written 
in postfix form (assumming that we only use one-digit decimal integers):

12+3*23+-





5. (10 points) Describe the structure of a C program that uses two stacks: a stack 
of pointers to characters and a stack of float numbers.  Give the appropriate 
declarations of global variables and the prototypes of the functions to 
manipulate the two stacks. Do not include the code of the functions.


































6.(6 points) Show the sequence of stack contents as the infix expression 
(x+y-z) * u is transformed into the postfix form. For this description use the 
following table, where Input  shows the current symbol to be read from the 
input stream, Stack  shows the contents of the stack at that step and  Output 
shows what has been output so far.

Input
Stack
Output
































































7. (6 points) Show the contents of the evaluation stack as you evaluate the 
postfix expression  1 2 + 4* 3 2 - +. Use the following table. Each row 
corresponds to a step during the evaluation process. Input  shows the current 
item beeing read from the postfix expression. Stack  gives the contents of the 
stack at that step. You may not need to use all of the rows., but if you need 
more, use extra an sheet.


Input
Stack










































































Part 2.  (50 points)

8. (15 points) Describe a C program that will take as input a sequence of 
decimal digits 0,1,2,...,9  (e.g. 23433233004) and will count the number of 
occurences of the each digit. In the previous example, 2 occurs twice, 3 four 
times, etc. The input stream is terminated by a special character of your choice 
(e.g a dot) and blancs and newlines are ignored.

Name your program count.c. Before submitting it, insert a comment in it 
tellingwhat is its status: compiles or not, runs correctly or not.





9. (15 points) Write a C program which reads two strings of characters s1 and 
s2 into two arrays of characters allocated at run time via malloc. The 
porgram will prompt the user asking for the size of each string, then 
allocate memory for it, then read it. Afterwards, it will concatenate the 
two strings: create a new string  s consisting of s1 followed by s2. You will 
have to also allocate space for s before concatenation.

Name your program concat.c . Before submitting indicate with comments 
whether your program compiles or not, executes correctly or not. 

10. (20 points) Write a function:

int substring(char *s, char*t)

which returns 0 if s points to a string contained in the string pointed to by t, 
-1 otherwise. 

For example:

substring("ell", "Hello") returns 0
substring("Hll", "Hello") returns -1.

For this part, provide only the pseudo-code for the function. You may not 
have time to write  and type in a whole program executing this function. 
(If you do, that is extra  credit). You will also get extra credit  if you get to 
type in a file substring.c containing the C code for the function (and if 
that is correct!). I will insert your function into a main program and test it.