#### String to Number Conversions

String-to-Number Conversions:

At times you will require converting the string representation of a number to a real numeric variable. For illustration, the string "123" can be transformed to a type int variable with the value 123. Three functions can be utilized to transform a string to a number. They are described in the following sections; their prototypes are in STDLIB.H.

The atoi() Function:

The library function atoi() transforms a string to an integer. The prototype is:

int atoi(char *ptr);

The function atoi() transforms the string pointed to by ptr to an integer. Besides digits, the string can have leading white space and a + or -- sign. Conversion begins at the starting of the string and continues till an unconvertible character (for illustration, a letter or punctuation mark) is encountered. The resultant integer is returned to calling program.

When it finds no convertible characters, atoi() returns 0. The table lists shown below illustrate few examples.

=> String-to-number conversions with atoi():

The atol() Function:

The library function atol() works precisely identical to atoi(), apart from that it returns a type long. Function prototype is as:

long atol(char *ptr);

The atof() Function:

The function atof() transforms a string to type double. The prototype is:

double atof(char *str);

The argument str points to string to be transformed. This string can include leading white space and a + or -- character. The number can have the digits 0 via 9, the decimal point, and the exponent indicator E or e. When there are no convertible characters, atof() returns 0. Table below lists some illustrations of using atof().

=> String to number conversions with atof():

Character Test Functions:

The header file CTYPE.H comprises the prototypes for a number of functions which test characters, returning FALSE or TRUE depending on whether the character meets a certain condition. For illustration, is it a letter or a numeral? The isxxxx() functions are really macros, stated in CTYPE.H.

The isxxxx() macros all contain similar prototype:

int isxxxx(int ch);

In preceding line, ch is the character being tested. The return value is TRUE (that is, non-zero) if the condition is met or FALSE (or zero) if it isn't. The table below lists the complete set of isxxxx () macros.

=> The isxxxx() macros:

You can do lots of interesting things with the character-test macros. One case in point is the function get_int(), shown in Listing. This function inputs integer from stdin and returns it as a type int variable. The function skips over leading white space and returns 0 when the first non-space character is not a numeric character.

=> Employing the isxxxx() macros to implement a function which inputs an integer.

/* Using character test macros to create an integer */
/* input function. */
#include <stdio.h>
#include <ctype.h>

int get_int(void);
main()
{
int x;
x =  get_int();
printf("You entered %d.\n", x);
}
int get_int(void)
{
int ch, i, sign = 1;

while ( isspace(ch = getchar()) );
if (ch != `-' && ch != `+' && !isdigit(ch) && ch != EOF)
{
ungetc(ch, stdin);
return 0;
}
/* If the first character is a minus sign, set*/
/* sign accordingly. */
if (ch == `-')
sign = -1;
/* If the first character was a plus or minus sign*/
/* get the next character. */
if (ch == `+' || ch == `-')
ch = getchar();
/* Read characters until a nondigit is input. Assign*/
/* values, multiplied by proper power of 10, to i.*/

for (i = 0; isdigit(ch); ch = getchar() )
i = 10 * i + (ch - `0');
/* Make result negative if sign is negative.*/
i *= sign;
/* If EOF was not encountered, a nondigit character*/
/* must have been read in, so unget it.*/
if (ch != EOF)
ungetc(ch, stdin);
/* Return the input value.*/
return i;
}

Output:

-100
You entered -100.
abc3.145
You entered 0.
9 9 9
You entered 9.
2.5
You entered 2.

Mathematical Functions:

The C standard library comprises a variety of functions which perform mathematical operations. The prototypes for mathematical functions are in the header file MATH.H. The math functions return a type double. For trigonometric functions, the angles are stated in radians. Keep in mind, one radian equivalents 57.296 degrees, and a full circle (360 degrees) has 2p radians.

Trigonometric Functions:

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