Introduction to C - Part 3

Limits and Memory for various Data Types

The sizes and value limits for various data types can printed from <ctype.h> header file and it is done in the file here

Custom printing and output formatting

printf is prints out whatever format we provide. This helps us use custom formatting.

For example, Consider the below code, that gets two numbers and prints their sum.

// Program to find sum of two numbers
#include<stdio.h>

int main(){
    int num1, num2, sum;

    printf("Enter first number ");
    scanf("%d", &num1);

    printf("Enter second number ");
    scanf("%d", &num2);

    sum = num1 + num2;
    printf("Sum of the numbers %d and %d is %d\n", num1, num2, sum);

    return 0;
}

What you get in your screen, on execution, providing the input, is:

Enter first number 10
Enter second number 20
Sum of the numbers 10 and 20 is 30

Note the last line of output, you have the output formatted so that it is easy to read. This way is the basic output formatting

You can also format the output of computed values stored in variables. These are done by modifying the format specifier as stated below:

• A minus symbol (-) sign tells left alignment

• A number after % specifies the minimum field width. If string is less than the width, it will be filled with spaces

• A period (.) is used to separate field width and precision

The below example uses an string (char array) of name str. More details on them will be discussed later.

1. String Formatting:

// string formatting
#include <stdio.h>

int main() {
    char str[] = "Hello World";

    printf("%s\n", str);

    // shift to the right 20 characters including the string
    printf("%20s\n", str);

    printf("%-20s\n", str); // left align

    // shift to the right 20 characters including the string, and print string
    // up to 5 character
    printf("%20.5s\n", str);

    // left align and print string up to 5 character
    printf("%-20.5s\n", str);

    return 0;
}

Output

Hello World
         Hello World
Hello World
               Hello
Hello

2. Float formatting:

// float formatting
#include <stdio.h>

int main() {
    float f = 12.666667;

    printf("%f\n", f);

    // setting width
    printf("%10f\n", f);

    // setting width and aligning left
    printf("%-10f\n", f);

    // setting width and padding with zeroes
    printf("%015f\n", f);

    // setting precision
    printf("%.2f\n", f);

    // setting width and precision
    printf("%10.2f\n", f);

    // setting width and precision
    printf("%10.2f\n", f);

    // setting width and precision with padding zeroes
    printf("%015.7f\n", f);

    return 0;
}

Output

12.666667
 12.666667
12.666667
00000012.666667
12.67
     12.67
     12.67
0000012.6666670

3. Integer formatting:

// integer formatting
#include <stdio.h>

int main() {
    int a = 126;

    printf("%d\n", a);

    // setting width
    printf("%5d\n", a);

    // setting width and aligning to the left
    printf("%-5d\n", a);

    // setting width and padding with zeroes
    printf("%05d\n", a);

    return 0;
}

Output

126
  126
126
00126

Variables - Storage Classes

A storage class defines the scope (visibility) and life-time of variables and/or functions within a C Program. They precede the type that they modify. We have four different storage classes in a C program −

1. auto
2. register
3. static
4. extern

1. auto:

The auto storage class is the default storage class for all local variables.

int mount;
auto int month;

2. register:

The register storage class is used to define local variables that should be stored in a register instead of RAM. This means that the variable has a maximum size equal to the register size (usually one word) and can’t have the unary ‘&‘ operator applied to it (as it does not have a memory location).

register int miles

3. static:

The static storage class instructs the compiler to keep a local variable in existence during the life-time of the program instead of creating and destroying it each time it comes into and goes out of scope. Therefore, making local variables static allows them to maintain their values between function calls.

static int x = 5;

The static modifier may also be applied to global variables to restrict the variable’s scope to the file in which it is declared.

4. extern:

The extern storage class is used to give a reference of a global variable or a function that is visible to ALL the program files.

When ‘extern‘ is used, the variable cannot be initialized however, it points the variable name at a storage location that has been previously defined.

The extern modifier is most commonly used when there are two or more files sharing the same global variables or functions as explained below.

First File: main.c

#include <stdio.h>

int count ;
extern void write_extern();

void main() {
    count = 5;
    write_extern();
}

Second File: support.c

#include <stdio.h>

extern int count;

void write_extern(void) {
    printf("count is %d\n", count);
}

Here, extern is being used to declare count in the second file, where as it has its definition in the first file, main.c.