String to integer, string to integer

String to integer, string to integer

Question: convert a string to an integer. For example, input "123" and output 123.

Solution:

A 0-9 integer number is obtained using the Ascii value corresponding to each 0-9 digit minus 48. Multiply the integer number represented by the first I-1 digit by the number 10 plus the number indicated by the I-digit, you can get the converted integer. Note that the string can contain 1st characters, which can be + or -. When the string is 1st characters is-, the integer number represented by the first I-1 is multiplied by 10 minus the number represented by the I bit, you can get the converted integer. In this case, the preceding number is 0. For example, input "-0123" and output-123. Another thing to note is overflow: the integer converted from a string cannot exceed the range where the integer can represent numbers.

Int strToInt (char str []) {int max = (1 <30)-1 + (1 <30); // The maximum number that a 32-bit system integer can represent, the reason for not using (1 <31)-1 is that integer overflow occurs when 1 <31, although only one warning int min = (-1 <31) is reported ); // The minimum integer value that can be expressed by a 32-bit system. int number = 0; // number indicates the integer that the string is converted to. When the string cannot be converted, will prompt and return 0if (str = NULL) // when the string is NULL {printf ("this string is empty and cannot be converted to an integer"); return 0 ;} int n = 0; while (str [n]! = '\ 0') // indicates whether illegal characters exist in the traversal string {if (n = 0) // The 1st characters in the string can be 0-9 digits or + or-{if (str [n]> = 48 & str [n] <= 57) | str [n] = '+' | str [n] = '-') {} else {printf ("this string cannot be converted to an integer "); return 0 ;}} else // string except 1st characters, all other characters must be 0-9 {if (str [n]> 57 | str [n] <48) {printf ("this string cannot be converted to an integer "); return 0 ;}} n ++;} if (str [0] = '\ 0') // when the string is empty, {printf ("this string is empty, cannot be converted to integer "); return 0;} else if (str [0] = '+') // when the first character of the string is + {int I = 1; while (str [I]! = '\ 0') {// determines whether the converted integer will overflow if (number> max/10) {printf ("converting this string to an integer will overflow, cannot be converted to integer "); return 0;} else if (number = max/10) {if (str [I]-48> max % 10) {printf ("this string converted to an integer will overflow and cannot be converted to an integer"); return 0 ;}number = 10 * number + (str [I]-48 ); I ++;} return number;} else if (str [0] = '-') {int I = 1; while (str [I]! = '\ 0') {// determines whether the converted integer will overflow if (number <min/10) {printf ("this string converted to an integer will overflow, cannot be converted to integer "); return 0;} else if (number = min/10) {if (-(str [I]-48) <min % 10) {printf ("this string converted to an integer will overflow and cannot be converted to an integer"); return 0 ;}number = 10 * number-(str [I]-48 ); I ++;} return number;} else {int I = 0; while (str [I]! = '\ 0') {// determines whether the converted integer will overflow if (number> max/10) {printf ("converting this string to an integer will overflow, cannot be converted to integer "); return 0;} else if (number = max/10) {if (str [I]-48> max % 10) {printf ("this string converted to an integer will overflow and cannot be converted to an integer"); return 0 ;}number = 10 * number + (str [I]-48 ); I ++;} return number ;}}