C-如何通过直接输入数字和字符（符号）字符串来创建计算器？

Question

INPUT 1+2-3+4 OUTPUT = 4

INPUT 1+2+3+4 OUTPUT = 10

INPUT -1-2-3-4 OUTPUT = -10

Here's my attempt but i gives wrong result

Code fragment:

for(i=0;i<strlen(res)+1;i++){
    if(res[i]=='-'||res[i]=='+'||res[i]=='\0'){
        num[z]='\0';
        dig=atoi(num);
        if(x==0){
            sum=dig;
            x++;
        }
        else{
            if(res[i]=='+')
                sum=sum+dig;
            else if(res[i]=='-')
                sum=sum-dig;
        }
        z=0;

    }
    else{
        num[z]=res[i];
        z++;
}
return sum;

Answer 1

I'd add print statements to the code to see where it's going wrong. But offhand it looks like when you get to the terminating null ( res[i]=='\\0' ) you don't add the final number to sum .

I'm making assumptions about the declarations and initializers of num, x and z and that the missing } is where the indentation implies.

Answer 2

This line references the array "num", but not the correct index:

    dig=atoi(num);

It should be fixed if you changed it to:

    dig=atoi(num[z-1]);

Your solution however doesn't work for calculations with numbers more than one digit long.

Answer 3

But offhand it looks like when you get to the terminating null ( res[i]=='\\0' ) you don't add the final number to sum .

That's true, but not the only error; if a + or - is encountered, the corresponding addition or subtraction is done with the previous instead of the next number. So the input 1+2-3+4 actually computes 1-2+3 , which gives 2 .
You can code your whole loop much simpler using strtol :

    char *beg, *end = res;
    for (sum = 0; dig = strtol(beg = end, &end, 0), end-beg; ) sum += dig;