Hello, world!

Welcome to CISC187 and welcome to this book.

This book is intended for students taking CISC187 at SD Mesa. You should already have a solid understanding of the material presented in the vast majority of first semester C and C++ courses.

This entire course focuses on writing code. Even this book provides opportunities for you to write, compile, and evaluate code on may pages. Often the interactive code is on a separate tab, for example:

A C++ construct that may be new to you is the range-based for loop. When your goal is to iterate over all the elements in a container or array, a range-for loop is easier to write and understand.

for (auto value: data)           // for each value in data
{
  std::cout << "value is " << value << '\n';
}

Introduced in C++11, a range-for loop extracts values from containers one at a time and assigns them to a variable.

A complete example program using the code explained on the Example tab.

// compiled with: -std=c++11 #include <iostream> int main () { int data[] = { 1, 2, 3, 5, 8 }; for (auto value: data) // for each value in data { std::cout << "value is " << value << '\n'; } return 0; }

Feel free to modify this code and re-run it to see what happens when you change it.

The remaining section on this page list some things I expect everyone to know on the first day of class. Review this material and if anything looks unfamiliar then read the linked content and ask questions.

Source files and header files

One of the primary goals of this course is to begin creating programs more complex than those written previously. One of the core skills required when writing large programs is to split different parts of the program source into separate files.

You may have only had to do this a few times previously, but you should know by now:

• What are the differences between source and header files?

  • Why do they exist?

  • What are header guards?

  • What is #pragma once?

  See preprocessor/include for more information.

Compilation vs. linking

• What happens during compilation? Linking?

• How to use function main(), argc, and argv

• cout and the meaning of statements like:

#include <iostream>
#include <stdio.h>

int main() {
  std::cout << "Hello C++!" << std::endl;
  puts("Hello C!");
  printf("Hello Alice!\n");
  printf("Hello %s!\n", "Bob");
}

You may not have seen printf and puts before. They are output functions C++ inherits from C. Normally, in C++ we use stream I/O functions and classes, but the old C functions are still there if you need them.

Built-in types, variables and operations

You should already be familiar with declaring fundamental types (int, char, double, unsigned, etc.). You should also know that other fixed width integer types exist (int16_t, uint64_t, etc.) even if you haven’t used them very much. You should also be familiar with the basic math operations and operators (+, -, =, ==, etc.). Including the shortcut operators (++, +=, etc.). We will be expanding our knowledge of operators and operations extensively during this course.

You should know the difference between declaring, initializing, and assigning a value to a variable. It is (sometimes) valid to assign variables of one type to those of a different type, for example, double x = 12; assigns the integer 12 to the double x. This is a widening conversion and is always safe. The opposite of a widening conversion is a narrowing conversion. A narrowing conversion frequently involves the loss of information. Most compilers will warn about narrowing conversions even in cases where they are allowed.

Keep in mind that a common source of error in programs is unintentional narrowing conversions that occur during math operations. For example:

What is the output, given the following?

double value = 3 / 2;

#include <iostream> int main() { double value = 3 / 2; std::cout << "The value is: " << value << ".\n"; }

Fix this program so that the correct value is displayed.

You should know how to explicitly cast fundamental types from one type to another. Most people should be familiar with the static_cast form:

auto almost_pi = static_cast<int>(3.14159);

Some people may have also learned the C-style cast:

auto almost_pi = (int)3.14159;

These two casts are roughly equivalent, but the first is preferred. We discuss why later in the book. We will be learning other ways to explicitly cast that are a bit more consistent with C++11’s more uniform initialization syntax.

Finally, you should know the basic keywords of the language, at least those common to both C and C++, and legal identifier names for functions and variables.

User-defined types

Although you may not have done any object oriented programming yourself, you probably have used objects, even if you weren’t aware of it. The C++ standard provides many classes. Two of the oldest classes handle stream formatted input and output: cin and cout.

You should have already encountered code like:

std::string  name;
std::cout << "Enter your name: ";
std::cin  >> name;
std::cout << "Hello," << name << "!\n";

You may have been taught the basics of string and vector. It is hard to do much (non-embedded) C++ programming without ever using either. A bit like writing a paragraph in English without using the letter ‘e’. Try that sometime!

We will be working with std::string and std::vector often in this course, so if you haven’t used them yet, don’t worry - you will.

File input and output

I expect you to know how to use some form of file input and output, whether it is the C-style printf and scanf, or the C++-style input and output file streams: ofstream and ifstream. Both are serviceable, have their own advantages and disadvantages. This course emphasizes contemporary C++ and encourages the use of C++ generally, but sometimes printf is a perfectly acceptable alternative to cout.

Don’t panic.

While file I/O is not a primary focus of this course, you will be expected to employ basic I/O in labs and projects.

This example uses printf and scanf to interact with the standard console input and output.

#include <cstdio> int main () { char name[20]; scanf("%s", name); printf("Hello, %s!\n", name); }

Try This!

Feel free to change the input to something else to see what happens.

What happens if we enter a name longer tahn our buffer size?

This example uses std::cout and std::cin to interact with the standard console input and output.

#include <iostream> #include <string> int main () { std::string name; std::cin >> name; std::cout << "Hello, " << name << "!\n"; }

Feel free to change the input to something else to see what happens.

Reading from a file to access external data:

#include <cstdio> int main() { // assuming the file 'poem' exists in the current directory FILE* ptr = fopen("poem","r"); if (ptr == NULL) { printf("Unable to open poem."); return 1; } char c; // read the text file one byte (char) at a time while (fscanf(ptr,"%c",&c) == 1) { putchar(c); } return 0; }

Reading from a file to access external data:

#include <fstream> #include <iostream> int main () { // assuming the file 'poem' exists in the current directory std::ifstream is("poem"); char c; // read the text file one byte (char) at a time while (is.get(c)) { std::cout << c; } is.close(); return 0; }

Try This!

Change this program to read from the poem file one line at a time instead of reading single characters at a time.

Hint: change char to std::string and use getline instead of get.

Data file: poem

          Jabberwocky

"Beware the Jabberwock, my son!
  The jaws that bite, the claws that catch!
Beware the Jubjub bird, and shun
  The frumious Bandersnatch!"

He took his vorpal sword in hand:
  Long time the manxome foe he sought --
So rested he by the Tumtum tree,
  And stood awhile in thought.

And, as in uffish thought he stood,
  The Jabberwock, with eyes of flame,
Came whiffling through the tulgey wood,
  And burbled as it came!

One, two! One, two! And through and through
  The vorpal blade went snicker-snack!
He left it dead, and with its head
  He went galumphing back.

And, has thou slain the Jabberwock?
  Come to my arms, my beamish boy!
O frabjous day! 'Callooh! Callay!'
  He chortled in his joy.

              -- Lewis Carroll, 1871

Keep in mind that each of the I/O examples presented are just one way to solve these problems. Each of them could have been written differently and achieved exactly the same goals. For example, reading files one byte at a time is not generally the most efficient file read solution, but it is extremely simple. At this point, we are not concerned with a thorough treatment of input and output, rather we are just reviewing major concepts and showing the differences between C standard I/O and C++ I/O.

Statements and branching

Writing basic statements and conditionally executing code, or executing blocks of code repeatedly, are fundamental skills common to all programming languages.

Everyone should be extremely familiar with writing if, switch, for, and while blocks.

You should have used combinations of statements and branching to perform tasks perhaps as complex as:

• Computing an amortization table

• Computing population growth

• Parsing text

For example, loops and conditionals make it easy to print all the odd numbers between 1 and 100, inclusive.

#include <iostream> using std::cout; int main() { cout << "Odd numbers:\n"; for (int num = 1; num <= 100; ++num) { if (num % 2 != 0) { cout << '\t' << num << '\n'; } } }

Fixing errors in code

You should know the difference between basic types of errors:

• Compile-time errors

• Link-time (linker) errors

• Runtime errors

• Semantic errors

I expect some basic experience using a debugger in whatever programming environment you may have used previously.

If not, refer to the section Debugging.

Note

If any of the material in the preceding sections sounds unfamiliar, then

• Consider working through the week 1 example source code

• Review the material from your first semester text

---

More to Explore

• range-based for loop and loops

• if

• Debugging

• Jeff Atwood’s blog: Code smells

You have attempted of activities on this page