Prelim 1 Solutions

Question 1 (30 points). Trace the sequence of assignments to variables made by the program segment given below when it is run with input data 5. Repeated assignments of the same value to a variable should be repeated in the table. You may not need all of the table columns that have been provided.

Assume the variable in has been declared such that expression in.readInt() inputs one integer value.

int p = in.readInt();

int d = 0;

int n = 0;

while ( p > 1 ) {

d = 2;

while ( ( p % d) != 0 ) d = d + 1;

if ( d == p ) n = n + 1;

p = p - 1;

}

System.out.println();

Solution

p	?	5								4		3				2			1
d	?		0		2	3	4	5			2		2	3			2
n	?			0					1						2			3

Question 2 (35 points). Complete the program segment according to the following input/output specification.

Input. The input of your program consists of two or more positive integers terminated by -1 -1. The positive integers specify one or more runs, where a run is a sequence that begins and ends with the same value. In particular, the first integer of the first run (if there is at least one run) is given by the first integer of the run is repeated. The first integer of the second run (if there is a second run) is given by the next integer, and so on.

Here is some sample input consisting of three runs of length 5, 4, and 2, respectively.

10 50 50 20 10

50 60 75 50

50 50

-1 -1

Notice that the runs are as short as possible, e.g., we do NOT say that the above input consists of two runs of lengths 5 and 6, respectively. Assume the input is well-formed, i.e., no run is left open (e.g., 10 -1).

Output. Output the number of runs and the average length run ( rounded to the nearest whole number ). For the sample input given above, the output would be:

Number of runs: 3

Average run length: 4

Solution

TokenReader in = new TokenReader( System.in );

int first = in.readInt(); // first integer of the current run.

int next = in.readInt(); // next integer of the current run.

int length = 2; // length of the current run so far.

int sum = 0; // total length of completed runs so far.

int nRuns = 0; // number of completed runs so far.

while ( first != -1 && next != -1 ){

if ( next != first )

length++;

else {

nRuns++;

sum += length;

first = in.readInt();

length = 2;

}

next = in.readInt();

}

System.out.println("Number of runs: " + nRuns);

System.out.println("Av. run length: " +

+ Math.round( (double)sum/nRuns));

Question 3 (35 points). Complete the program segment according to the following input/output specification.

Input. The input consists of a non-negative integer n followed by n additional non-negative integers. For example

4 5 3 0 2

Output. Output n lines of stars (*'s), where the number of *'s in a line is given by the corresponding integer in the input. For example, for the input given, the output would be:

*****

***

<a blank line>

*

Solution 1

TokenReader in = new TokenReader( System.in );

int n = in.readInt(); // number of lines of output requested.

// other declarations

int stars;

// statements

while ( n > 0 ) {

stars = in.readInt();

while ( stars > 0 ) {

System.out.print("*");

stars--;

}

System.out.println();

n--;

}

Solution 2

TokenReader in = new TokenReader( System.in );

int n = in.readInt(); // number of lines of output requested.

// other declarations

int line = 1;

int j, stars;

// statements

while ( line <=n ) {

stars = in.readInt();

j = 1;

while ( j <= stars ) {

System.out.print("*");

j++;

}

System.out.println();

line++;

}