[Courses] C Programming For Absolute Beginners, Lesson 2: Fun With Printf, Scanf, Puts, and Variables

[Emily Gonyer]

Hey Carla, I did lesson 1 and have been busy and am just getting to lesson
2, but all of a sudden, after I compile the program, when I go to run them
my terminal just says 'command not found'... what am I doing wrong?

Emily

On Mon, Feb 20, 2012 at 11:26 PM, Carla Schroder <carla at bratgrrl.com> wrote:

> =========
> C Programming For Absolute Beginners, Lesson 2: Fun With Printf, Scanf,
> Puts,
> and Variables
> =========
>
> Allrighty then, in Lesson One we learned some basics of using gcc, the
> simplest structure of a C program, and some Linux tips for coders. Today
> we're
> going to continue with our simple example program and build on it, and
> learn
> how to make an interactive program that asks for user input and then does
> something with it.
>
> Please remember there are no stupid questions-- the whole point of this is
> to
> learn, so ask!
>
> ## Fun With Printf ##
>
> Printf is a fundamental C function, and it does all kinds of cool and
> useful
> things, so let's make it do stuff. We'll take our Lesson 1 example program
> and
> play some printf tricks with it. This example does multiple-line output:
>
> /*
>   printfex (printf example); a simple example
>   of how printf works
> */
>
> #include <stdio.h>
> void main()
> {
>  printf( "Hello, and welcome to the Beginning C Course!\n" );
>  printf( "Look, we can do a second line.\n" );
>  printf( "And a third line.\n");
>  printf( "And a fourth.\n");
> }
>
> Remember to not use reserved words or existing command names for your
> source
> files and programs, or *fun confusion will result. (*May not be actually
> fun.)
>
> ****
> Side note: A syntax-highlighting text editor will save you a lot of typos.
> For
> example, I never remember where to put the line break, \n, and my editor
> helps
> me with this by telling me when it's in the wrong place by changing the
> color.
> ****
>
> Then compile and run it:
>
> $ gcc -o printfex printfex.c
>
> I copy my executables into a separate directory that I have put in my PATH;
> please review part 1 to learn how to do this.
>
> $ printfex
> Hello, and welcome to the Beginning C Course!
> Look, we can do a second line.
> And a third line.
> And a fourth.
>
> You can also do it this way:
>
> #include <stdio.h>
> int main()
> {
>  printf( "Hello, and welcome to the Beginning C Course!\n" "Look, we can
> do a
> second line.\n" "And a third line.\n" "And a fourth.\n");
>
>  return 0;
> }
>
> Or like this:
>
> #include <stdio.h>
> int main()
> {
>  printf( "Hello, and welcome to the Beginning C Course!\n"
>  "Look, we can do a second line.\n"
>  "And a third line.\n"
>  "And a fourth.\n");
>
>  return 0;
> }
>
> We don't even have to use printf, but can use the puts function instead,
> which
> automatically inserts a line break:
>
> #include <stdio.h>
> int main()
> {
>  puts( "Hello, and welcome to the Beginning C Course!" );
>  puts( "Look, we can do a second line." );
>  puts( "And a third line." );
>  puts( "And a fourth.");
>
>  return 0;
> }
>
> puts does one thing: print output followed by a line break. Let's make an
> interactive program to demonstrate puts:
>
> /*
>   age; an example program showing
>   the difference between puts and printf
> */
>
> #include <stdio.h>
>
> int main()
> {
>    int age;
>
>    puts( "Please enter your age: " );
>    scanf( "%d", &age );
>    printf( "Wow, you are %d years old.\n", age);
>
>    return 0;
> }
>
> The output of this is nicely-formatted:
>
> $ age
> Please enter your age:
> 80
> Wow, you are 80 years old.
>
> There are a number of cool things happening in this tiny program: it asks
> for
> input, we enter something, and then it uses it correctly in a sentence. The
> scanf function scans and formats input, and printf formats output.  %d is a
> placeholder, and it means scan an integer as a signed decimal number. A
> signed
> integer can be either a negative or positive number; an unsigned integer is
> only positive.
>
> scanf( "%d", &age ); means "read the user input and place that value into
> the
> variable age." You must use the & symbol in front of your variable or it
> won't
> work. If you want to jump ahead this is your introduction to using
> pointers,
> which is pointing to a specific location on memory. Pointers is a weird and
> complicated subject to explain, so for now I'm going to leave it at "don't
> forget the & symbol when you're playing with scanf." (Anyone who wants to
> chime in with their own explanation of pointers is welcome!)
>
> scanf has already assigned the value of the user input to our age
> variable, so
> all printf has to do is display it.
>
> Some other commonly-used scanf and printf placeholders are:
>
>    %f -- float
>    %c -- single character
>    %s -- character strings
>    %i -- signed integer, or hexadecimal when preceded by 0x and octal when
> preceded by 0
>
> scanf is pretty simple and doesn't handle input errors very well. You can
> see
> for yourself by typing wrong input on purpose, such as letters and
> punctuation
> marks. In real life you'll use other functions, for example gets and fgets,
> which we will cover later in this course.
>
> This example shows how to handle multiple variables with printf:
>
> /* addition; an example of adding and displaying user input */
>
> #include <stdio.h>
>
> int main()
> {
>    int a, b, c;
>
>    puts( "Please enter any number up to three digits: " );
>    scanf( "%d", &a );
>    puts( "Please enter another number up to three digits: " );
>    scanf( "%d", &b );
>    c = a + b;
>    printf("%d + %d = %d\n", a, b, c);
>
>    return 0;
> }
>
> When you run it it looks like this:
>
> $ addition
> Please enter any number up to three digits:
> 34
> Please enter another number up to three digits:
> 567
> 34 + 567 = 601
>
> Note how the order of each variable is managed-- if you change the order
> of a,
> b, c on the last printf line then you change the results.
>
> I'm afraid I must stop here; we'll be back on track next Sunday with a nice
> long lesson.
>
> Homework: Modify the addition program to echo the user input as it is
> entered,
> so it looks something like this:
>
> "Please enter any number up to three digits:
> 34
> You entered the number 34. Now please enter another number:"
>
> You can also play around with the inputs and do different math operations.
>
> Discussion: Perhaps someone could offer some thoughts on puts vs. printf,
> and
> what are some real-world functions to use in place of scanf?
>
> --
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> Carla Schroder
> ace Linux nerd
> author of Linux Cookbook,
> Linux Networking Cookbook,
> Book of Audacity
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
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