Week 1 - C&C++ basics

A basic C program

#include <stdio.h>

int main(){
  puts("Hello World!\n");
  return 0;
}

#include in C resembles import in Java or Python, except that the imported material is a header file; what this means will be discussed later.

The angle brackets <> tell C to search for the stdio.h file in the compiler’s standard libraries.

Using quotes "" is legal but tells C that the file should be in the same directory.

C is not object oriented, and in C++ object orientation is not mandatory as it is in Java.

You do not have to define classes (in C you cannot) and your main “method” must not be in a class (and is thus a function, not a method).

The syntax for defining a function is the same as a method in Java but without the access specifier (public/private etc.) since these don’t apply to functions.

The main function is the main program. It returns an int, which is returned to the operating system.

puts is the C function to output a string. (It’s part of stdio, that’s why we imported it.) \n at the end of the string represents the byte sent to the OS to tell it to move to the next line.

The same in c++

#include <iostream>
using std::cout;
using std::endl;

int main() {
 cout << “Hello World!” << endl;
 return 0;
}

C++ differences

• The .h can be left off the import. Traditionally built-in C++ libraries have no file extension.
• Instead of using the stdio library, we use the iostream library and stream output operators.
• These are a C++ addition that simplifies output but also makes the syntax more abstract.
• This is a good example of the difference in style between C and C++.

A simple loop

#include <stdio.h>
#include <stdint.h>

int main() {
  for (uint8_t x=0; x<10; x++) {
    printf(“%i\n”,x);
  }
  return 0;
}

Looking at the for loop:

  for (uint8_t x=0; x<10; x++) {  
    printf(“%i\n”,x);            
  }

As mentoned before, the for syntax in C and C++ is identical to Java. (Java copied it from C++!)
The printf (print formatted) function is used to print out strings containing numbers or other material.
Also note the type declaration is unusual.

Type declarations in C

• In the original C, the types used were the same as in Java – int, float, etc.

• However, unlike Java, C compiles right down to machine code – and different computers have different abilities to handle numbers in machine code.

• This caused confusion with int meaning different things and having different properties based on the machine being compiled on. As a result, in 1999 the types were respecified to allow the programmer to specify exactly how the computer should store the number.

Integer standard types

Type	Memory	Min	Max
int8_t	1 byte	-128	127
uint8_t	1 byte	0	255
int16_t	2 bytes	-32768	32767
uint16_t	2 bytes	0	65535
int32_t	4 bytes	-2147483648	2147483647
uint32_t	4 bytes	0	4294967295
int64_t	8 bytes	-9223372036854775808	9223372036854775807
uint64_t	8 bytes	0	18446744073709551615

• Modern processors can handle up to 32-bit numbers in single machine code instructions. Many can handle 64-bit numbers.
• However, smaller embedded systems may only be able to deal with 16-bit or even 8-bit numbers directly in their machine code.
• Managing larger numbers on these systems requires the compiler to include extra code to transfer carry, etc, information between parts of the number.

Relative types

(U)Int_leastX_t: declares the variable must have at least X bits but may have more. This is used to allow for systems with machine code that may no longer handle smaller numbers.

(U)Int_fastX_t: declares the variable must have at least X bits and should be “fast” on the platform being used. The precise meaning of this is not defined but compilers are supposed to select a type native to the CPU’s code. ---Rarely used.

Printf

In Python:
print("You have",dogs,"dogs and",cats,"cats.")
In C:
printf("You have %i dogs and %icats.\n",dogs,cats);

• The first parameter (called the format string) gives the overall structure of what you want to print.
• You add “slots” into it with %i (or other markers we’ll see later), then fill them in.

The loop in C++

#include <iostream>
using namespace std;

int main() {
  for (uint8_t x=0; x<10; x++) {
    cout << +x << endl;
  }
return 0;
}

The code is mostly the same as in C except we are using the iostream facilities C++ provides. This is cleaner than printf:

cout << "You have " << dogs << " dogs and " << cats << " cats." << endl;

---

#include <iostream>
using namespace std;

int main() {
  for (uint8_t x=0; x<10; x++) {
  cout << +x << endl;
  }
return 0;
}

However, note the + before x. This is because, without the specific information printf gives, C++ has to assume what format to use to print the contents of the variable. In the case of an 8-bit integer, by default, it assumes wrong! (It’s ok for larger integers though..)

---

A test, please run this line and see the result:

#include <iostream>
using namespace std;

int main() {
for (uint8_t x=0; x<2; x++) {
  cout << +x << endl;
  cout<<-x<<endl;
  cout<<x<<endl;
  }

cout<<sizeof(int)<<endl;
cout<<sizeof(uint16_t );

return 0;
}

See the output

The output is:

0
0
null
1
-1
null
4
2

Input

int main() {
  int32_t age;
  puts(“How old are you?”);
  scanf(“%i”,&age);
  printf(“You typed %i.\n”,age);
  if (age > 18) {
    puts(“You can vote.”);
  }
return 0;
}

Input in C

• scanf is the input equivalent of printf.
• Normally modern programs do not use scanf because it is bad at handling errors (eg, if the user did not type a number). We will learn to do input better later. Use scanf for the moment.
• Note the & sign before age in scanf. This means that we are passing a pointer to age. This is necessary because we want scanf to change the value stored in variable age. We’ll go much more into pointers later on.
• We declare the variable as int32_t because this is what the %i specifier in scanf requires, even though it would be better to use a smaller type for an age.

Input in C++

int main() {
  uint16_t age;
  cout << “How old are you?”;
  cin >> age;
  cout << “You typed” << age <<
  endl;
  if (age > 18) {
    cout << “You can vote.”;
  }
return 0;
}

Input in C and C++

Again at this stage, the code is similar except we use C++’s IO streaming functions.

cin >> is used for input. Again, this is not ideal at handling errors, although it is better than scanf. But use it for the moment.

Note there is no & sign before age in the input statement. We still need the value of age to change, so we are still actually passing a pointer. But C++ deals with this for us in the background so we don’t need to add the explicit &.

This can be helpful, but it can also be awkward, as learning the difference is vital to learning C and C++.

Conversion Specifiers in C

Conversion	Output Specification
%a	Floating-point number, hexadecimal (16进制) digits and p-notation
%c	Single character.
%d	Signed decimal integer.
%e	Floating-point number, e-notation.
%f	Floating-point number, decimal notation.
%i	Signed decimal integer (same as %d).
%o	Unsigned octal integer.
%p	A pointer.
%s	Character string.
%u	Unsigned decimal integer.
%x	Unsigned hexadecimal integer, using hex digits 0–F.
%%	Prints a percent sign.