How can I multiply numbers in a string by grouping them according to intervals?-CodePudding

I want to get a result of dividing a string of numbers by a specific interval and multiplying it. I know how to change one character into a number.

Here's the code

#include<stdio.h>

int main(void) {
    char str[] = "123456";
    int a, b, c;
    a = str[0] - '0';
    b = 23; // str[1] ~ str[2]
    c = 456; // str[3] ~ str[5]
    printf("%c * %c%c * %c%c%c = %d", str[0], str[1], str[2], str[3], str[4], str[5], a * b * c);
}

I didn't know how to convert characters into numbers, so I stored values directly for the variables.

Here's the output: 1 * 23 * 456 = 10488

Here's the desired result:

In str, change the number corresponding to index 0 to int type.
In str, change the number corresponding to index 1 to 2 to int type.
In str, change the number corresponding to index 3 to 5 to int type.
Multiply the changed numbers.

This question is related. However, I am curious about how the user arbitrarily divides and multiplies the sections rather than dividing them according to spaces.

CodePudding user response：

to convert two digit string into number you need to multiple first digit by 10 and add second digit, the same for 3 digit, something like this:

#include<stdio.h>

// converts number stored between start/end indexes into integer
int parse(char* str, int start, int end) {
    int result = 0;
    for (int i=start; i<=end; i  ) {
        result = result*10   (str[i] - '0');
    }
    return result;
}

int main(void) {
    char str[] = "123456";
    int a, b, c;
    a = parse(str, 0, 0);
    b = parse(str, 1, 2);
    c = parse(str, 3, 5);
    printf("%d * %d * %d = %d", a, b, c, a * b * c);
}

CodePudding user response：

"I didn't know how to convert characters into numbers"

Based on your example algorithm enumerated in steps 1, 2, 3 & 4, without including any user input instructions, here is an implementation that extracts the 3 integers from the source string by using string parsing, copying and string to number conversion.

int main(void)
{   
    char str[] = "123456";
    char num1[10]={0};//note {0} initializes all memory locations in buffer to NULL
    char num2[10]={0};
    char num3[10]={0};
    int n1, n2, n3;
    char *tmp;
    
    //1. In str, change the number corresponding to index 0 to int type
    strncpy(num1, str, 1);
    n1 = atoi(num1);
    //2. In str, change the number corresponding to index 1 to 2 to int type.
    tmp = strchr(str, '2');
    strncpy(num2, tmp, 2);
    n2 = atoi(num2);
    //3. In str, change the number corresponding to index 3 to 5 to int type.
    tmp = strchr(str, '4');
    strncpy(num3, tmp, 3);
    n3 = atoi(num3);

    //4. Multiply the changed numbers.
    int result = n1*n2*n3;
    printf("%d", result);
    
    return 0;
}

Outputs;