数据类型转换函数strtod | atoi | atol

strtod  > Convert string to double

atoi   > Convert string to int

atol > Convert string to long int

 

 

1、strtod

double strtod ( const char * str, char ** endptr );

Convert string to double

Parses the C string str interpreting its content as a floating point number and returns its value as a double . If endptr is not a null pointer, the function also sets the value pointed by endptr to point to the first character after the number.

The function first discards as many whitespace characters as necessary until the first non-whitespace character is found. Then, starting from this character, takes as many characters as possible that are valid following a syntax resembling that of floating point literals, and interprets them as a numerical value. A pointer to the rest of the string after the last valid character is stored in the object pointed by endptr .

A valid floating point number for strtod is formed by a succession of:

• An optional plus or minus sign
• A sequence of digits, optionally containing a decimal-point character
• An optional exponent part, which itself consists on an 'e' or 'E' character followed by an optional sign and a sequence of digits.

If the first sequence of non-whitespace characters in str does not form a valid floating-point number as just defined, or if no such sequence exists because either str is empty or contains only whitespace characters, no conversion is performed.

Parameters

str

C string beginning with the representation of a floating-point number.

endptr

Reference to an already allocated object of type char* , whose value is set by the function to the next character in str after the numerical value.
This parameter can also be a null pointer, in which case it is not used.

 

Return Value

On success, the function returns the converted floating point number as a double value.
If no valid conversion could be performed, a zero value (0.0 ) is returned.
If the correct value is out of the range of representable values, a positive or negative HUGE_VAL is returned, and the global variable errno is set to ERANGE .
If the correct value would cause underflow, zero is returned and errno is set to ERANGE .

Example

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/* strtod example */ #include <stdio.h> #include <stdlib.h> int main () { char szOrbits[] = "365.24 29.53" ; char * pEnd; double d1, d2; d1 = strtod (szOrbits,&pEnd); d2 = strtod (pEnd,NULL); printf ("The moon completes %.2lf orbits per Earth year./n" , d1/d2); return 0; }

Output:

The moon completes 12.37 orbits per Earth year.


2、atoi

int atoi ( const char * str );

Convert string to integer

Parses the C string str interpreting its content as an integral number, which is returned as an int value.

The function first discards as many whitespace characters as necessary until the first non-whitespace character is found. Then, starting from this character, takes an optional initial plus or minus sign followed by as many numerical digits as possible, and interprets them as a numerical value.

The string can contain additional characters after those that form the integral number, which are ignored and have no effect on the behavior of this function.

If the first sequence of non-whitespace characters in str is not a valid integral number, or if no such sequence exists because either str is empty or it contains only whitespace characters, no conversion is performed.

Parameters

str

C string beginning with the representation of an integral number.

 

Return Value

On success, the function returns the converted integral number as an int value.
If no valid conversion could be performed, a zero value is returned.
If the correct value is out of the range of representable values, INT_MAX or INT_MIN is returned.

Example

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/* atoi example */ #include <stdio.h> #include <stdlib.h> int main () { int i; char szInput [256]; printf ("Enter a number: " ); fgets ( szInput, 256, stdin ); i = atoi (szInput); printf ("The value entered is %d. The double is %d./n" ,i,i*2); return 0; }

Output:

Enter a number: 73 The value entered is 73. The double is 146.

3、atol

long int atol ( const char * str );

Convert string to long integer

Parses the C string str interpreting its content as an integral number, which is returned as a long int value.

The function first discards as many whitespace characters as necessary until the first non-whitespace character is found. Then, starting from this character, takes an optional initial plus or minus sign followed by as many numerical digits as possible, and interprets them as a numerical value.

The string can contain additional characters after those that form the integral number, which are ignored and have no effect on the behavior of this function.

If the first sequence of non-whitespace characters in str is not a valid integral number, or if no such sequence exists because either str is empty or it contains only whitespace characters, no conversion is performed.

Parameters

str

C string containing the representation of an integral number.

 

Return Value

On success, the function returns the converted integral number as a long int value.
If no valid conversion could be performed, a zero value is returned.
If the correct value is out of the range of representable values, LONG_MAX or LONG_MIN is returned.

Example

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/* atol example */ #include <stdio.h> #include <stdlib.h> int main () { long int li; char szInput [256]; printf ("Enter a long number: " ); gets ( szInput ); li = atol (szInput); printf ("The value entered is %d. The double is %d./n" ,li,li*2); return 0; }

Output:

Enter a number: 567283 The value entered is 567283. The double is 1134566.