Midterm 1

Midterm 1#

Bash shell#

Consider the following directory structure on a computer:

where the / at the top of the tree is the root directory, bin/, home/, and opt/ are subdirectories beneath /, and so forth.

Your username on this machine is student and the location of your home directory is marked.
1. Give the full (absolute) path to the file analysis.txt.
  
  solution
  
  /home/student/data/draft/analysis.txt
  
  Recall that absolute paths always begin with the root of the filesystem, /.
2. If you are in the directory final/, give the relative path from there to the file analysis.txt.
  
  solution
  
  From final/, we need to go up and directory and then down into draft, so the relative path is ../draft/analysis.txt.
3. If you are in the other/ directory, give a way to change directory to your home directory.
  
  solution
  
  Any of cd, cd .., or cd ~ would work.
4. If you are unsure of where you are in the filesystem, what command can tell you your location?
  
  solution
  
  You would use pwd.
You are in your home directory on portal. Give the sequence of commands to (1) create a directory called tests/, (2) change directory into tests, and (3) open a file there with the text editor called README.
solution
```
mkdir tests
cd tests
nano README
```
You could use emacs instead of nano–those are the 2 editors we’ve focused on.
ls in your directory and see the following files:
```
report-A1.pdf  report-A1.txt  report-A2.pdf  report-A2.txt  report-A3.pdf  report-A3.txt
report-B1.pdf  report-B1.txt  report-B2.pdf  report-B2.txt  report-B3.pdf  report-B3.txt
report-C1.pdf  report-C1.txt  report-C2.pdf  report-C2.txt  report-C3.pdf  report-C3.txt
```
1. Using wildcards, how do you select all the PDF files?
  
  solution
  
  You could just do *.pdf or if you wanted to be more specific, report-*.pdf.
2. Using wildcards, how do you select all of the files that have an A in them?
  
  solution
  
  There are several ways, including *A?.* and report-A?.*.
You created a file called topics.txt. An ls shows the following for your file:
```
-rw-------. 1 user user       71 Feb 23  2026  topics.txt
```
You are using a machine in your research group, and there is a directory on that machine called /home/common/ that anyone on the machine can access.

Give the commands to (1) move topics.txt into /home/common, and (2) to allow anyone on the computer to be able to read-from and write-to the file.
solution

To move the file:
```
mv topics.txt /home/common
```
Then (assuming you are in /home/common), to update the permissions, you could do:
```
chmod a+rw topics.txt
```
or do it in two steps:
```
chmod a+r topics.txt
chmod a+w topics.txt
```
You could also use the numeric variation.
On the portal server, in a directory named files/ under your home directory, you have a file called myscript.sh. Give the scp command that will copy that file from portal to your current directory on your computer (e.g., one of the desktops in the MathLab).
solution

You want to pull the file from portal to your local machine, so you would do:
```
scp username@portal.mathlab.stonybrook.edu:~/files/myscript.sh .
```
You could also write out the directory, as /home/username/files/myscript.sh instead of using ~.
You created a script called process.sh and already set it up with the permissions to be executable. When run (it takes no arguments), it outputs a lot to the screen, so you want to redirect its output to a file named output.txt.

What is the full command to run the script and store the output in output.txt?
solution
```
./process.sh > output.txt
```

C++ basics#

In class, we’ve looked at a basic “Hello, World!” program a number of times. Write a C++ program that outputs “Hello, World”.
solution
```
#include <iostream>

int main() {
    std::cout << "Hello, World" << std::endl;
}
```
To compile a file named hello.cpp to produce an executable hello, what is the full command that you use?
solution
```
g++ -o hello hello.cpp
```
Consider the following two lines:
```
int a{-1.5};
int b = 2 * a + 1;
```
What is the value held by b after these lines are run?

solution

-1.5 cannot be represented as an int, so it is converted by truncation, resulting in -1, and then 2 * (-1) + 1 give -1.
Consider the following two lines:
```
double x{2.0};
double y = x + 4 * x / 2;
```
What is the value held by y after these lines are run?

solution

Multiplication and division have highest precedence, so this is 2.0 + (4 * 2.0 / 2) or 6.0.
You want to write a C++ program that computes \(x^{3/2}\) using double-precision floating point and std::pow(). Recall that std::pow(x, y) means \(x^y\). Give the 2 lines of C++ code that (1) initialize x to a value of 10, and (2) compute \(x^{3/2}\), assigning the value to a new variable y.
solution
```
double x{10.0};
double y = std::pow(x, 3.0/2.0);
```
Note that for the power, we need to use floating point, and not 3/2.
The C++ math library includes a function cos(x) that will take the cosine of the input \(x\). You want to use this in your code. Assume you have access to the value of \(\pi\) as the variable pi.
1. What header do you need to #include?
  
  solution
  
  cmath
2. What is line(s) of code needed to take the cosine of \(45^\circ\) and store the result in a variable called s? Use a datatype that gives the most precision.
  solution
  
  The key to remember is that the trig functions want the angle in radians. So we can do:
  double s = std::cos(45.0 * pi / 180);

Midterm 1

Contents

Midterm 1#

Bash shell#

C++ basics#