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02_c-quickstart-printf.md

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name: main

.aim[Systems: The Basics, printf]

<style> .aim {font-size: .75em} .remark-inline-code { background-color: lightgray; border-radius: 3px; padding-left: 2px; padding-right: 2px; } h4 {font-size: 1.5em} </style>
--- template: main

C Basics

  • C program files are mostly a series of function definitions. As C is not an OOP language, there is no class-like structural overhead.

  • Like in Java, main is a special function, it is the only function that runs at program execution. Any functions called from within main will also be run.

  • C is a statically typed language, meaning identifiers (variables and functions) must be given a type when they are declared.

    • The types are similar to those in Java, with a few notable exceptions.
    • For the moment, use int for integers and double for floating point values.

  • main should have a return type of int. This can be used later on to check if a program executed successfully. By convention, a return value of 0 means everything went as planned.

template: main

Writing, compiling, and running

  • By convention, C source files should have a .c file extension (i.e. dylan.c).

  • The C compiler we will be using is gcc (the Gnu C Compiler)

    • usage: $ gcc dylan.c
      • This will create a standalone executable file.
      • The default name for the output file is a.out
      • There is no preferred extension for c executable files.
      • You can provide your own output file name with the -o flag.
        • usage: $ gcc -o dj dylan.c

  • Compiled C programs are natively executable, to run them just type ./program (i.e. $ ./a.out or $ ./dj

    • The ./ is only needed because you probably compiled the file in a folder outside your PATH environment variable.

printf

  • printf is the function normally used in C to print to standard out.

  • usage: printf( string, arg0, arg1, ...)

    • Sends string to standard out.
  • The first argument must be a literal string enclosed by ".

--

  • string can contain special placeholder characters that are used to insert other values into the output.

--

  • If placeholder characters are used, then they will be replaced by the arguments following the string when printf is executed.

--

  • The value arguments can be either variables or literal values.
    • example: printf(“these are numbers: %d %lf\n”, 3, 845.273); would display: these are numbers: 3 845.273

printf Formatting Characters

Type Placeholder
int %d
long %ld
float %f*
double %lf*
char %c
string %s
pointer %p

* %0.xf or %0.xlf will print x significant digits after the floating point

printf will attempt to interpret any value with the provided formatting character, even if they do not match.

Variables and Types

  • All C primitives are numeric. The only differences are floating point vs. integer and size of variable in memory.

  • Size can be platform dependent

    • sizeof(type) can be used to find the size in bytes (stdlib.h)
Type Size (bytes) Range
char 1 -128 --> 127
short 2 -32,768 —-> 32,767
int 4 -231 --> 231-1
long 8 -263 --> 263-1

-- |float | 4 | 7 digits of precision | |double | 8 | 14 digits of precision |

--

  • Variables can be declared as unsigned. Unsigned variables do not use a bit to store the sign of the number, making the lower bound 0 and increasing the upper bound.

--

  • All variables must be declared before being used.

--

  • Variables are not given initial values.