When I tried to implement reverse Poland, after receiving input in while(scanf("%s",s)!=EOF), the conditional branch begins in if(s[0]=='+'). Why do I keep the index of s[0] and array 0 fixed?
I imagine that s[0] is followed by s[1], and then s[2], and eventually s[n] ends if s[n] is EOF. What's the difference?
source code
#include<bits/stdc++.h>
using namespace std;
int top, S[1000];
void push(intx){
S[++top] = x;
}
int pop(){
top--;
return S[top+1];
}
int main() {
inta,b;
top = 0;
chars [100];
while(scanf("%s",s)!=EOF){
if(s[0]=='+'){
a = pop();
b = pop();
push(a+b);
} else if(s[0]=='-'){
b = pop();
a = pop();
push(a-b);
} else if(s[0]=='*'){
a = pop();
b = pop();
push(a*b);
} else{
push(atoi(s));
}
}
printf("%d\n", pop());
return 0;
}
Also,
int main(){
inta,b;
top = 0;
chars;
while(scanf("%c",s)!=EOF){
if(s=='+'){
a = pop();
b = pop();
push(a+b);
} else if(s=='-'){
b = pop();
a = pop();
push(a-b);
} else if(s=='*'){
a = pop();
b = pop();
push(a*b);
} else{
push(atoi(s));
}
}
printf("%d\n", pop());
return 0;
}
Also, if you change chars[100] to chars, while(scanf("%s",s)!=EOF) to while(scanf("%c",s)!=EOF) and if(s[0]=='+') to if(s=='+') as above, it doesn't work, but I don't know why.
The number you enter, such as 1+1 , is limited to one digit.
The %s specifier for scanf is a blank character (half-width space or new line) separated by a string to the standard input.
(It is not typed in bulk into chars[100])
For example, if there is an input of "1112+",
The first s contains "11"
For the second s,
12
"+"
Each contains
In the case of each symbol, there is no problem even if you only look at the first character for each input. I think you specified s[0].
By the way, isn't it probably an example from a book?
If you have something like an input example, it might be easy to understand if you check the operation while inserting a debugger or printf.
(The following is an example.)
int main(){
inta,b;
top = 0;
chars [100];
int cnt = 1;
while(scanf("%s",s)!=EOF){
printf("%dth time:%s\n", cnt,s);
if(s[0]=='+'){
a = pop();
b = pop();
push(a+b);
} else if(s[0]=='-'){
b = pop();
a = pop();
push(a-b);
} else if(s[0]=='*'){
a = pop();
b = pop();
push(a*b);
} else{
push(atoi(s));
}
++cnt;
}
printf("%d\n", pop());
return 0;
}
*Additional questions
There seems to be a problem with the following three points.
(I wrote the correction code at the end of the sentence, so please check it while comparing it.)
1. You did not specify a pointer for the second argument of scanf
scanf must be a pointer to the stored object.
We have changed from array type to element type (char[100]→char), so in this case, we need to modify it to take a pointer.
2. No pointer to string is specified for atoi
Atoi is a function that converts strings into int-type numbers, so you cannot convert characters
If you want the characters to be numeric, you must write
Interesting characters - '0'
(The language standard indirectly guarantees that this operation will work properly on any platform.)
Example:
chars='3'
int value = s - '0';
3. Enter new line characters and blank characters as numeric values
Unlike %s, they don't skip blank characters, so
When blank spaces or line breaks are entered, they are recognized as numeric values.
while(scanf("%c",s)!=EOF) {←1. When blank is entered
if(s=='+'){←2.+ not
a = pop();
b = pop();
push(a+b);
} else if(s=='-'){←3.-not even
b = pop();
a = pop();
push(a-b);
} else if(s=='*'){←4.* not even
a = pop();
b = pop();
push(a*b);
} else {←5. It's not a number, but it's entered as a number.
push(atoi(s));
}
}
I corrected the above problem and rewritten it a little.
#include<bits/stdc++.h>
using namespace std;
int top, S[1000];
void push(intx){
S[++top] = x;
}
int pop(){
top--;
return S[top+1];
}
int main() {
inta,b;
top = 0;
chars;
while(scanf("%c", &s)!=EOF) {//1.scanf second argument to pointer
if(s=='+'){
a = pop();
b = pop();
push(a+b);
} else if(s=='-'){
b = pop();
a = pop();
push(a-b);
} else if(s=='*'){
a = pop();
b = pop();
push(a*b);
} else if(isspace(s)) {//3. Next without doing anything when new line or blank characters are in it
/* do nothing*/
} else{
Corrected to use s-'0' as push(s-'0');//2.atoi is not available
}
}
printf("%d\n", pop());
return 0;
}
Check against the answers above and the original source code
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