String functions --- detailed description and implementation of the atof () function (full version), --- detailed description of the atof Function

String functions --- detailed description and implementation of the atof () function (full version), --- detailed description of the atof Function

Atof () function

Atof (): double atof (const char * str );

Function: converts a string to a floating point number.

Str: string to be converted.

Return Value: double value returned for each function. This value is generated by parsing the input character as a number. If the input cannot be converted to a value of this type, the return value is 0.0.

Function Description: atof () scans the nptr parameter string and skips the leading space character until conversion starts when a number or positive or negative sign is encountered, the conversion ends only when a non-number or string ends ('\ 0') and returns the result. The str string can contain positive and negative numbers, decimal points, or E (e) to represent the exponential part.

# Include <iostream> using namespace std; double atof_my (const char * str) {double s = 0.0; double d = 10.0; int jishu = 0; bool falg = false; while (* str = '') {str ++;} if (* str = '-') // records positive and negative numbers {falg = true; str ++ ;} if (! (* Str> = '0' & * str <= '9') // If the start is not a number, the system returns 0.0 return s; while (* str> = '0' & * str <= '9' & * str! = '. ') // Calculate the integer before the decimal point {s = s * 10.0 + * str-'0'; str ++;} if (* str = '. ') // str ++; while (* str> = '0' & * str <= '9 ') // calculate the fractional part {s = s + (* str-'0')/d; d * = 10.0; str ++ ;} if (* str = 'E' | * str = 'E') // consider scientific notation {str ++; if (* str = '+ ') {str ++; while (* str> = '0' & * str <= '9') {jishu = jishu * 10 + * str-'0 '; str ++;} while (jishu> 0) {s * = 10; jishu -- ;}} if (* str = '-') {str ++; while (* str> = '0' & * str <= '9') {jishu = jishu * 10 + * str-'0'; str ++ ;} while (jishu> 0) {s/= 10; jishu --;}}} Return s * (falg? -1.0: 1.0);} int main () {char * s1 = "123.456567567e-10"; char * a1 = "123.456567567e-10 "; char * s2 = "1234567.235e + 10"; char * a2 = "1234567.235e + 10"; char * s3 = "123.45656 \ 07567e-10"; char * a3 = "123.45656 \ 07567e-10 "; double sum_1 = atof_my (s1); double sum1 = atof (a1); double sum_2 = atof_my (s2); double sum2 = atof (a2); double sum_3 = atof_my (s3 ); // The End double sum3 = atof (a3); cout <"atof_my:" <sum_1 <endl; cout <"atof: "<sum1 <endl; cout <" atof_my: "<sum_2 <endl; cout <" atof: "<sum2 <endl; cout <"atof_my:" <sum_3 <endl; cout <"atof:" <sum3 <endl; system ("pause"); return 0 ;}

Running result comparison diagram:

In this example, the atof function is used to convert the numeric type into a numerical value change program.

Double atof (const char * s)
{
Double val, power;
Int sign, I;
// Remove spaces from the string
For (I = 0; isspace (s [I]) & s [I]! = 0; ++ I)
;
// Judge Positive and Negative Numbers and point I to the next position
Sign = (s [I] = '-')? -1:1;
If (s [I] = '+' | s [I] = '-')
I ++;
// Retrieve the integer part of the Floating Point Number
For (val = 0.0; isdigit (s [I]) & s [I]! = 0; ++ I)
Val = 10 * val + s [I]-'0 ';
// Decimal point
If (s [I] = '.')
I ++; // the last position
// Obtain the number after the decimal point
For (power = 1.0; isdigit (s [I]) & s [I]! = 0; ++ I)
{
Val = 10 * val + s [I]-'0 ';
Power * = 10; // number of digits in decimal places
}
Return sign * val/power; // calculate the floating point number.
}
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What is the usage of the atof function?

Welcome
Function Name: atof
Function: converts a string to a floating point number.
Source: array to floating point numbers
Usage: double atof (const char * nptr );
Program example:
# Include <stdlib. h>
# Include <stdio. h>
Int main ()
{
Float f;
Char * str = "12345.67 ";
F = atof (str );
Printf ("string = % s float = % f \ n", str, f );
Return 0;
}