1. Brief Introduction
In C language, the conversion between integers and strings has widely used extension functions (non-standard library), and you can also try simple implementations yourself.
2. Integer to String
1. Extension function itoa
itoa (meaning integer to alphanumeric) is a function that converts an integer to a string.
Under Windows environment, in the <stdlib.h> header file:
char* itoa(int value,char*string,int radix);//value: 要转换的整数，string: 转换后的字符串,radix: 转换进制数，如2,8,10,16 进制等。
Function source code:
char* itoa(int num,char* str,int radix)
{
char index[]="0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";//Index table
unsigned unum;//Store the absolute value of the integer to be converted, the integer to be converted may be negative
int i=0,j,k;//i is used to indicate the corresponding position in the string. After conversion, i is actually the length of the string; the order after conversion is reversed. When there is a sign, k is used to indicate the starting position for adjusting the order; j is used for swapping during the order adjustment.
//Get the absolute value of the integer to be converted
if(radix==10&&num<0)//To convert to a decimal number and it is negative
{
unum=(unsigned)-num;//Assign the absolute value of num to unum
str[i++]='-';//Set '-' at the beginning of the string, and increment the index by 1
}
else unum=(unsigned)num;//If num is positive, assign directly to unum
//Conversion part, note that the result is in reverse order
do
{
str[i++]=index[unum%(unsigned)radix];//Take the last digit of unum, set it to the corresponding position of str, and increment the index by 1
unum/=radix;//Remove the last digit from unum
}while(unum);//Exit the loop until unum is 0
str[i]='\0';//Add the '\0' character at the end of the string, C strings end with '\0'.
//Reverse the order
if(str[0]=='-') k=1;//If it is negative, the sign does not need to be adjusted, start adjusting from after the sign
else k=0;//If not negative, all need to be adjusted
char temp;//Temporary variable, used when swapping two values
for(j=k;j<=(i-1)/2;j++)//Swap symmetrically from head to tail, i is actually the length of the string, the maximum index is length minus 1
{
temp=str[j];//Assign the head to the temporary variable
str[j]=str[i-1+k-j];//Assign the tail to the head
str[i-1+k-j]=temp;//Assign the value of the temporary variable (actually the previous head value) to the tail
}
return str;//Return the converted string
}
Example program:
Example
#include <stdlib.h>
#include <stdio.h>
int main()
{
int number1 = 123456;
int number2 = -123456;
char string[16] = {0};
itoa(number1,string,10);
printf("Number: %d, converted string: %s\n",number1,string);
itoa(number2,string,10);
printf("Number: %d, converted string: %s\n",number2,string);
return 0;
}
Effect screenshot:
2. Simple implementation by yourself
Implementation code:
Example
#include <stdio.h>
char* Int2String(int num,char *str);//Function declaration
int main()
{
int number1 = 123456;
int number2 = -123456;
char string[16] = {0};
Int2String(number1,string);
printf("Number: %d, converted string: %s\n",number1,string);
Int2String(number2,string);
printf("Number: %d, converted string: %s\n",number2,string);
return 0;
}
char* Int2String(int num,char *str)//Decimal
{
int i = 0;//Indicate filling str
if(num<0)//If num is negative, make num positive
{
num = -num;
str[i++] = '-';
}
//Conversion
do
{
str[i++] = num%10+48;//Take the lowest digit of num. The ASCII codes of characters 0~9 are 48~57; in simple terms, number 0 + 48 = 48, the ASCII code corresponds to character '0'
num /= 10;//Remove the lowest digit
}while(num);//If num is not 0, continue the loop
str[i] = '\0';
//Determine the starting position for adjustment
int j = 0;
if(str[0]=='-')//If there is a minus sign, the minus sign does not need to be adjusted
{
j = 1;//Start adjusting from the second position
++i;//Because of the minus sign, the symmetry axis of the swap also needs to move back 1 position
}
//Symmetric swap
for(;j<i/2;j++)
{
//Swap the values at both ends symmetrically. In fact, it saves the intermediate variable to swap a+b: a=a+b; b=a-b; a=a-b;
str[j] = str[j] + str[i-1-j];
str[i-1-j] = str[j] - str[i-1-j];
str[j] = str[j] - str[i-1-j];
}
return str;//Return the converted value
}
Effect screenshot:
3. String to Integer
1. Extension function
atoi (meaning alphanumeric to integer) is a function that converts a string to an integer.
Under Windows environment, in the <stdlib.h> header file.
int atoi(const char *nptr); // String to integer function, nptr: the string to be converted
Function source code
int atoi(const char *nptr)
{
return (int)atol(nptr);
}
long atol(const char *nptr)
{
int c; /*The current character to be converted (convert characters one by one to numbers)*/
long total; /*Current conversion result*/
int sign; /*Flag whether the conversion result has a minus sign*/
/*Skip spaces, spaces are not converted*/
while ( isspace((int)(unsigned char)*nptr) )
++nptr;
c = (int)(unsigned char)*nptr++;//Get a character ready for conversion
sign = c; /*Save sign indicator*/
if (c == '-' || c == '+')
c = (int)(unsigned char)*nptr++; /*Skip '+' and '-' signs, do not convert*/
total = 0;//Set conversion result to 0
while (isdigit(c)) {//If the character is a digit
total = 10 * total + (c - '0'); /*//Convert the character to the corresponding digit according to the ASCII code, and multiply by 10 to accumulate to the result*/
c = (int)(unsigned char)*nptr++; /*//Get the next character*/
}
//Return the result with or without a minus sign according to the sign indicator
if (sign == '-')
return -total;
else
return total;
}
Example program:
Example
#include<stdio.h>
#include<stdlib.h>
int main()
{
printf("String\"123456\"Convert to number: %d\n",atoi("123456"));
printf("String\"-123456\"Convert to number: %d\n",atoi("-123456"));
return 0;
}
2. Simple implementation by yourself
Implementation source code:
Example
#include<stdio.h>
int String2Int(char *str);//Function declaration
int main()
{
printf("String\"123456\"Convert to number: %d\n",String2Int("123456"));
printf("String\"-123456\"Convert to number: %d\n",String2Int("-123456"));
return 0;
}
int String2Int(char *str)//String to number
{
char flag = '+';//Indicate whether the result has a sign
long res = 0;
if(*str=='-')//The string has a minus sign
{
++str;//Point to the next character
flag = '-';//Set the flag to negative
}
//Convert character by character and accumulate to the result res
while(*str>=48 && *str<=57)//Only convert if it is a digit. ASCII codes of digits 0~9: 48~57
{
res = 10*res+ *str++-48;//The ASCII code of character '0' is 48, 48-48=0, which just converts to digit 0
}
if(flag == '-')//Handle the case of a negative number
{
res = -res;
}
return (int)res;
}
Screenshot:
4. Using sprintf() function and sscanf() function
Integer to string
Test code:
Example
#include <stdio.h>
char* Int2String(int num,char *str);//Function declaration
int main()
{
int number1 = 123456;
int number2 = -123456;
char string[16] = {0};
Int2String(number1,string);
printf("Number: %d, converted string: %s\n",number1,string);
Int2String(number2,string);
printf("Number: %d, converted string: %s\n",number2,string);
return 0;
}
char* Int2String(int num,char *str)
{
sprintf(str, "%d", num);
return str;
}
Running result:
String to integer
Test code:
Example
#include<stdio.h>
int String2Int(char *str);//Function declaration
int main()
{
printf("String\"123456\"Convert to number: %d\n",String2Int("123456"));
printf("String\"-123456\"Convert to number: %d\n",String2Int("-123456"));
return 0;
}
int String2Int(char *str)//String to number
{
char flag = '+';//Indicate whether the result has a sign
long res = 0;
if(*str=='-')//The string has a minus sign
{
++str;//Point to the next character
flag = '-';//Set the flag to negative
}
sscanf(str, "%ld", &res);
if(flag == '-')
{
res = -res;
}
return (int)res;
}
Running result:
5. Appendix: ASCII code table (partial)
Author: Genven_Liang
Original article: https://blog.csdn.net/nanfeibuyi/article/details/80811498